Power Supply FAQ


[H]F Junkie
Sep 9, 2003
Table of Contents

I. Definition of a Power Supply

II. The Power Supply

III. Power Supply Functions and Signals
  • AC-DC Voltage Conversion
  • Standard Output Voltages
  • Power Good Signal
  • Soft Power (Power On and 5V Standby Signals)
  • Additional Power Signals
IV. Parts of the Power Supply
  • Case and Cover
  • Power Cord and Power Pass-Through
  • Power Switch
  • External Voltage Selector Switch
  • Power Conversion Circuitry
  • Motherboard Power Connectors
  • Drive Power Connectors
  • Power Supply Fan
  • Power Supply Fuse
V. Power Supply Form Factors
  • PC/XT Form Factor
  • AT Form Factor
  • Baby AT Form Factor
  • LPX Form Factor
  • ATX (NLX) Form Factor
  • SFX Form Factor
  • WTX Form Factor
VI. Power Supply Output and Ratings
  • Output Power
  • System Power Requirements
  • Peak vs. Continuous Power
  • Redundant Power Supplies
  • Power Supply Loading
VII. Power Supply Specifications and Certifications
  • Physical Specifications
  • Environmental Specifications
  • Input Voltage Requirements and Tolerances
  • Output Specifications
  • Electrical Characteristics
  • General Quality Specifications
  • Certifications
VIII. Confusing Performance Data On Packaging And Case

IX. Increased Efficiency Saves Energy

X. Stable Voltages

XI. Quiet Operation Thanks To Regulated Fan

XII. Overloading Cables

XIII. Common PSU Questions

XIV. Recommended PSU Manufacturers

UPDATED: April 20th 2004


UPS and External Power Sources
Power Supply Reviews
PSU sleeving guides
Sourge Protectors

If you have any good links or info you want me to add, PM me or if you want to add them yourself feel free.

Credits: Please Note that most all information in this FAQ is derived from reputable resources and all credit for the information supplied should be given to them. Links are provided at the end of each section to show where the information was gathered.


[H]F Junkie
Sep 9, 2003
I. Definition of a Power Supply

A power supply unit (sometimes abbreviated power supply or PSU) is a device that supplies electrical power to a device or group of devices. The term is most commonly applied to units that are integrated with the devices they supply, such as computers and household electronics, and never to devices supplying, conditioning or otherwise supporting an electric utility grid. (For large-scale power supplies, see electricity generation.)

The range of different types of power supply is very broad, since widely differing design criteria affect each application.

As well as the usual requirements of cost, reliability, weight and size, constraints that commonly affect power supplies are the amount of power they can supply, how long they can supply it for without needing some kind of refueling or recharging, how stable their output voltage or current is under varying load conditions, and whether they provide continuous power or pulses.

Common power supply technologies include:
  • Batteries
  • Chemical fuel cells
  • ...and other forms of power storage systems
  • Solar power
  • Conversion of another form of electrical power into the desired form (typically converting 120 or 240 volt alternating current supplied by a utility company (see electricity generation) into low-voltage direct current for electronic devices); see switched-mode power supply, linear regulator, rectifier, inverter (electrical)
  • Generators or alternators (particularly useful in vehicles of all shapes and sizes, where the engine has rotational power to spare, or in semi-portable units containing an internal combustion engine and a generator)

More specialised power supplies might be based upon:
  • Flywheels coupled to generators or alternators
  • Capacitors
  • Compulsators
  • Explosively pumped flux compression generators

II. The Power Supply

Power is supplied to your computer in two stages. First, power is conveyed to the case from your electrical utility to your wall, and through the black power cord to the PC. Then, the internal power supply transforms this standard household electricity into the forms that your computer needs. Most people take electrical power for granted and don't think too much about it. This is also true of the internal power supply, which is usually just considered part of the case and given little attention. (The power supply is not part of the case!) The power from the utility itself is taken for granted and rarely given a second thought--that is, until disaster strikes. It is my hope that by taking the time to explain in more detail how electricity works and how it powers your PC, this subject will be given more attention in the future.

If you are going to use your PC lightly, it is fine not to pay too much attention to power. However, as the old computer saying goes, "garbage in, garbage out". If your motherboard and components are being supplied poor-quality power, you will have problems that you wouldn't have if they received proper, high-quality power. If you plan on using your PC heavily, or if your data is important, or if you are looking for upgradeability in the future, you must pay attention to power! Power issues are responsible for more PC problems than probably any other single source, even though most people don't realize that the power is responsible.

The internal power supply is responsible for converting your standard household power into a form that your computer can use. The power supply is responsible for powering every device in your computer; if it has a problem or is of low quality you may experience many difficulties that you may not realize are actually the fault of the electrical system. This section discusses what makes up the power supply and how it works in detail.

The power supply plays an important role in the following areas of your system:
  • Stability: A high quality power supply with sufficient capacity to meet the demands of your computer will provide years of stable power for your PC. A poor quality or overloaded power supply will cause all sorts of glitches that are particularly insidious, because the problems occur in other, seemingly unrelated, parts of the system. For example, power supplies can cause system crashes, can make hard disks develop bad sectors, or cause software bugs to appear, problems which can be very difficult to trace back to the power supply.
  • Cooling: The power supply contains the main fan that controls the flow of air through the PC case. This fan is obviously a major component in your PC's cooling system.
  • Energy Efficiency: Newer PC power supplies work with your computer's components and software to reduce the amount of power they consume when idle. This can lead to significant savings over older systems.
  • Expandability: The capacity of your power supply is one factor that will determine your ability to add new drives to your system, or upgrade to a more powerful motherboard or processor. Many people don't realize, for example, that a high-speed Athlon CPU and motherboard consume far more power than a similar Pentium-based system, and the power supply needs to be able to provide this power. If you build a new system with a power supply that barely meets your needs, you may have to replace it when you upgrade down the road.

Despite its critical role, the power supply is one of the most ignored and under-studied components in the PC. In fact, some people don't even bother to check out what power supply is included when they purchase a case! Upon these faulty foundations, important PC systems are built. Don't let that happen to you.


III. Power Supply Functions and Signals

The power supply's main function is simple: take the power input to it from the power system of your home or office, and turn it into a form that the PC can use. However, the PC power supply's job isn't as simple as that of a standard power converter, such as the kind you might plug into your car's lighter socket to let you run a TV off your car battery. The PC power supply must provide several different voltages, at different strengths, and must also manage some additional signals that the motherboard uses.


AC-DC Voltage Conversion

The electricity you get from your utility company is in the form of alternating current (AC), while the electricity your PC requires is direct current (DC). Therefore, the primary function of your power supply is to convert your wall AC power into a DC form that your PC can use. In fact, the supply normally provides several different voltage levels, to meet the demands of different components in the machine.

In fact, while almost everything in your home runs off standard AC power, many devices actually use DC internally. Just a few examples of this are telephone answering machines, many types of audio equipment, some kinds of battery chargers, and in the PC world, certain types of printers, external modems and other peripherals. Two indicators that a device actually uses DC inside are: the ability of the device to run on batteries, and the presence of a device outside the unit that powers it. These small "bricks" with one plug for the wall and another for the device are often called AC adapters. AC adaptors are really DC power supplies that converts the AC of the wall into DC for the device.

The difference between this sort of DC power supply and the kind in your PC is the design. AC adapters are linear power supplies. These supplies are cheap and simple to make. The main problem with them is that they are tremendously wasteful; typically, 50% or more of the energy supplied to one of these adapters is wasted as heat. You can feel this readily--just touch the adapter when the unit is working; many become quite hot to the touch. A hot AC adapter means electricity is being wasted. This is an acceptable compromise for small appliances, but unacceptable for a PC power supply.

Instead of the linear design, PCs use switching power supplies. (The full name for this sort of design is actually "constant-voltage, half-bridge forward-converting switching power supply") Explaining in detail how the design works would take many paragraphs and make your eyes gloss over unnecessarily. In a nutshell, the switching power supply uses a transistor switch and a closed feedback loop to produce DC output that is properly regulated regardless of the load on it, with only the amount of AC power required to draw the DC load being taken from the utility.

The main advantage of a switching power supply is that it is far more efficient than a linear design. When you are dealing with hundreds of watts of power, this is a more serious issue than when you are talking about an answering machine. The second advantage is that all the energy wasted in the power supply as heat has to be removed by the PC's cooling system. Therefore, more efficient power supplies produce less heat that the system has to exhaust. The main disadvantage of a switching supply is that it generates high-frequency signals within it as part of its conversion process, which can radiate out of the unit and cause interference to other electronic devices (inside or outside the PC). For this reason, you will always see PC power supplies encased in metal boxes for shielding.

The power factor of a device refers to the ratio of the actual power used by the device to the product of the current and voltage supplied to it. Traditional power supplies have a power factor of about 0.6 to 0.7. The power factor is important especially for determining the sizing of UPSes as well as circuits that supply larger units. Some newer power supplies, especially larger ones for servers, have additional circuitry added to correct the power factor of the supply. These are, unsurprisingly, called power-factor-corrected supplies, and have a power factor of (or near) 1.0. They make UPS sizing either easier or more confusing, depending on how you look at it. Another reason why power-factor correction is being added to some supplies is that low power factor devices, if used in sufficient quantities, cause problems for electrical utilities. In some parts of the world the utility companies are starting to impose surcharges on companies with excessive loads at low power factors, though this is not really a concern for a home PC user.


Standard Output Voltages

PCs use several different voltages to power their various components. The core voltages have mostly remained unchanged over the 20-year history of the PC, though a couple of the less-used voltages have essentially been dropped, and an important new one has been added. The power supply provides each of these voltages, in varying amounts depending on the model, directly from its circuitry.

Most of the power provided by the power supply is in the form of positive voltages, but some is in the form of negative voltages. Negative voltage is a slightly strange concept when used in reference to a DC current. In a nutshell, it just means that the voltage potential is measured from ground to the signal, instead of the signal to ground. It's essentially like turning a battery upside-down: same voltage, the current just goes backwards.

The amount of current provided at each voltage level is important because of its impact on determining the supply's ability to provide sufficient power for your system. That larger issue is discussed in a separate section. Here are the details on the various voltages provided by today's power supplies:
  • -12 V: This voltage is used on some types of serial port circuits, whose amplifier circuits require both -12V and +12V. It is not needed on some newer systems, and even on older ones not very much is used, because the serial ports require little power. Most power supplies provide it for compatibility with older hardware, but usually with a current limit of less than 1 A.
  • -5 V: A now archaic voltage, -5 V was used on some of the earliest PCs for floppy controllers and other circuits used by ISA bus cards. It is usually provided, in small quantity (generally less than 1A), for compatibility with older hardware. Some form factor power supplies such as the SFX no longer bother to supply it (systems using the SFX power supply are intended not to have ISA bus slots).
  • 0 V: Zero volts is the ground of the PC's electrical system, also sometimes called common or (especially in the UK) earth. The ground signals provided by the power supply are used to complete circuits with the other voltages. They provide a plane of reference against which other voltages are measured.
  • +3.3 V: The newest voltage level provided by modern power supplies, it was introduced with the ATX form factor and is now found on the ATX/NLX, SFX and WTX form factors. It is not found in Baby AT or older form factors. Originally, the lowest regular voltage provided by the power supply was +5 V, which was used to provide power to the CPU, memory, and everything else on the motherboard. Starting with the second generation Pentium chips, Intel went to a reduced 3.3 V voltage, in order to reduce power consumption as the chips got faster. This required motherboard manufacturers to put voltage regulators on their boards to change the +5 V to +3.3 V. The regulators produced a great deal of waste heat and having to do this reduction on the motherboard was very inefficient, so now the power supply provides +3.3 V directly. It is used to run most newer CPUs, as well as some types of system memory, AGP video cards, and other circuits.
  • +5 V: On older form factor systems (Baby AT and earlier) , this is the voltage used to run the motherboard, the CPU (directly or indirectly) and the vast majority of other components in the system. On newer systems, many of the components, especially the CPU, have migrated to the lower +3.3 V described above, but the motherboard and many of its components still use +5 V.
  • +12 V: This voltage is used primarily to power disk drive motors. It is also used by fans and other types of cooling devices. It is in most cases not used by the motherboard in a modern PC but is passed on to the system bus slots for any cards that might need it. Of course, drives are connected directly to the power supply through their own connectors.

Power Good Signal

When the power supply first starts up, it takes some time for the components to get "up to speed" and start generating the proper DC voltages that the computer needs to operate. Before this time, if the computer were allowed to try to boot up, strange results could occur since the power might not be at the right voltage. It can take a half-second or longer for the power to stabilize, and this is an eternity to a processor that can run half a billion instructions per second! To prevent the computer from starting up prematurely, the power supply puts out a signal to the motherboard called "Power Good" (or "PowerGood", or "Power OK", or "PWR OK" and so on) after it completes its internal tests and determines that the power is ready for use. Until this signal is sent, the motherboard will refuse to start up the computer.

In addition, the power supply will turn off the Power Good signal if a power surge or glitch causes it to malfunction. It will then turn the signal back on when the power is OK again, which will reset the computer. If you've ever had a brownout where the lights flicker off for a split-second and the computer seems to keep running but resets itself, that's probably what happened. Sometimes a power supply may shut down and seem "blown" after a power problem but will reset itself if the power is turned off for 15 seconds and then turned back on.

The nominal voltage of the Power Good signal is +5 V, but in practice the allowable range is usually up to a full volt above or below that value. All power supplies will generate the Power Good signal, and most will specify the typical time until it is asserted. Some extremely el-cheapo power supplies may "fake" the Power Good signal by just tying it to another +5 V line. Such a system essentially has no Power Good functionality and will cause the motherboard to try to start the system before the power has fully stabilized. Needless to say, this type of power supply is to be avoided. Unfortunately, you cannot tell if your power supply is "faking" things unless you have test equipment. Fortunately, if you buy anything but the lowest-quality supplies you don't really need to worry about this.


Soft Power (Power On and 5V Standby Signals)

Early PCs using the PC/XT, AT, Baby AT and LPX form factors all use a mechanical switch to turn the computer on and off. Newer form factors, starting with the ATX/NLX, and including the SFX and WTX, have changed the way the power supply is turned on and off. Instead of using a physical switch, these systems are turned on by a signal from the motherboard telling the power supply what to do. In turn, the motherboard can be told to change this signal under software control. This is what allows Windows to shut the power down to a PC, or what allows such features as turning a PC on from a button on the keyboard. This feature is called "Soft Power" and the signal that controls the power supply is called "Power On", or alternately, "PS On" or "PS_On".

This feature would seem to create a small "chicken and egg" situation however. How can the motherboard tell the power supply to turn on, electronically, when the motherboard is also off due to not having any power from the supply? The answer is the other "Soft Power" signal, which is called "+5 V Standby" (or "+5VSB", or "5VSB", etc.) This signal is the same output level as the regular +5 V lines from the power supply, but is independent of the other provided voltages and is always on, even when the rest of the power supply is turned off. A small amount of current on this wire is what allows the motherboard to control the power supply when it is off. It also permits other activities that must occur while the PC is off, such as enabling wakeup from sleep mode, or allowing the PC to be turned on when activity is detected on a modem ("Wake on Ring") or network card ("Wake on LAN").

The WTX form factor also includes a similar standby signal for +3.3 V.

Additional Power Signals

Some power supply form factors define additional power signals beyond the standard voltage outputs and the power good and soft power signals. Most of these are signals that can be implemented optionally by the power supply manufacturers, "optional" meaning that they are not required by the form factor for the power supply to meet the specification. In practice, this means that they are left off of most power supplies, especially less expensive models, to save cost. However, they are present on some supplies, so it's useful to understand what they are..

The following additional signals are specified for ATX/NLX systems:
  • +3.3 V Sense: This signal is used to detect the voltage level of the +3.3 V signal being provided to the motherboard. This allows the power supply to "fine tune" the +3.3 V output in the event of excessive voltage drop between the supply and the components that use +3.3 V. This is more needed for +3.3 V than the other signals probably because CPUs use +3.3 V. (Note that the ATX specification makes this signal "pseudo-optional"; it is by default included in the main ATX connector but can be replaced by a wire in the auxiliary ATX connector. See here for more details.)
  • FanC: This is a fan control signal, which allows the motherboard (and hence the system as a whole) to control the speed of the power supply fan. If implemented, when the voltage on this signal is less than 1 volt, the fan is turned off. As the voltage is increased the fan spins faster, and when it is over 10.5 V, the fan is run at full speed. This can be used to shut the fan off if the system is put into a sleep mode, or to allow the fan's speed to be increased or decreased based on the temperature of the system (saving power and reducing unnecessary noise.)
  • FanM: A companion to FanC, this is the fan monitor signal, which allows the motherboard to keep track of the current speed of the power supply fan. Sort of a "power supply fan tachometer" for fans designed to implement it. This could be used to provide a warning to the user if the main cooling fan in the power supply failed.
  • 1394V and 1394R: This pair of signals provides a separate, unregulated voltage circuit for powering IEEE-1394 ("FireWire") peripherals. It is not used by the motherboard.
The SFX form factor defines just one optional signal, called "Fan ON/OFF", which is essentially the same as the "FanC" ATX signal described above.

WTX, reflecting its status as a high-end form factor, includes several additional signals. These include the +3.3 V Sense, FanC and FanM signals described as above for ATX/NLX, as well as these:
  • Sleep: Puts the power supply into sleep mode. This is used for power savings, to power down parts of the power supply. It is used in conjunction with the Power On signal.
  • +3.3 VAUX: This is a standby +3.3 V signal just like the +5 V Standby signal defined for standard ATX Soft Power.
  • +5 V Sense: Just like the +3.3 V Sense signal, but for +5 V.
The WTX form factor also provides special, dedicated grounds (called returns) for its sense lines.



[H]F Junkie
Sep 9, 2003
IV. Parts of the Power Supply

The exact contents of any supply vary depending on both the supply's form factor and its individual design, but most of them have the same general components.

Warning: Power supplies generate high voltages internally and can be dangerous. Unless you have been specifically trained to work inside power supplies, you should not open one. Even with the unit unplugged dangerous electricity can remain stored within its components for some time.

Case and Cover

Every PC power supply comes surrounded by a metal case with a metal cover. The cover is normally secured with four screws and comes up off the top of the case. In many ways, the case of the power supply is to the power supply what the case of the PC is to the PC as a whole. It has several functions.

The case isolates the components inside the power supply from the rest of the PC. This serves to keep harmful electromagnetic interference inside the box, which is important because the switching design used for PC power supplies can otherwise cause emissions that will wreak havoc on other components inside and outside the PC. The case also keeps prying fingers outside the box where they will remain safely non-electrocuted. Power supplies are usually intended to be considered as "black boxes" and not serviced by individual PC owners.

The design of the case and cover are also important because they play a role in cooling the power supply components, and to some extent, the whole PC. Ventilation slots or holes are placed into the case in key locations to allow the power supply fan to provide air flow over critical components.

Warning: In addition to other warnings about not opening the power supply for safety reasons that you will find on this site, another reason is that most companies will void your power supply warranty--and possibly your system warranty if you purchased a retail PC--if you open the power supply. Look for small "warranty void if removed" stickers around the perimeter of the cover.


Power Cord and Power Pass-Through

Virtually all PCs come with a standard black power cord that runs from a receptacle on the power supply to a power outlet in the wall (or preferably, a power protection device or UPS.) This cord is virtually unchanged since the dawn of the personal computer. It has a special keyed shape on the end that plugs into the power supply. All PC power cords are three-pronged.

Warning: Circumventing the ground pin on the power cord, say to use the PC with a two-conductor extension cord, leaves your power supply with no ground connection and is a safety hazard.

Some power supplies, especially older ones, have a "pass-through" connector on the back into which you can plug the monitor's power cord (if it has the right shape). You can also buy inexpensive adapters that will convert a standard outlet plug into the shape needed to go into the back of the computer's supply. When you do this, the monitor is turned on and off using the computer's power switch. I don't believe this feature is employed in the newer form factor power supplies, since they use soft power and not a mechanical switch to turn the PC on and off. It was quite common on PC/XT, AT, and Baby AT systems, and allowed the PC to use only one power outlet instead of two.


Power Switch

Older form factor desktop PC/XT cases had the power switch at the back of the machine, usually on the right side of the case. This switch was actually inside the power supply itself, with a hole cut out in the case so that it could be reached from the outside. Users hated having to reach to the back of the machine to turn it on or off! The positioning of the switch also meant the PC could not be oriented with its right side towards a wall or partition.

Starting with the AT form factor, tower cases changed to a remote, physical toggle power switch that was connected to the power supply using a cable. AT desktop cases retained the old style case in the back of the PC, but clone manufacturers soon began to use a remote switch on these units as well. The switch is normally mounted to the front of the case. Some "slimline" (LPX) systems actually use a mechanical plastic stick (!) that is pushed on by the button on the front of the case, and presses against the real power switch on the power supply itself, in the back of the machine.

The remote switch cable has four leads that run to it (with a fifth ground lead, to ground the power supply to the case, optional) . One pair of these (the brown and blue) run to the power cord on the back of the power supply. The other pair (black and white) run from the switch to the power supply circuitry. When the switch is on, brown connects to black, and blue connects to white, and the power supply is energized. These wires attach to spade connectors on the body of the switch.

Warning: The brown and blue leads to the remote power switch on an AT-style system carry live 110V (or 220V) AC power whenever the power supply is plugged in, even when the power is off! You should not work inside the computer with the power plugged in.

Warning: Switching the pairs of wires from one set of spade connectors to another will cause no problems as long as you exchange black with brown, and white with blue. However, if you accidentally make any other type of change, say, swapping black with blue and white with brown, the results will range from a blown fuse or circuit breaker, to smoke!

Starting with the ATX/NLX form factor, the way the power switch works has been changed altogether. Instead of using a physical toggle switch connected to the power supply, on modern systems the power switch is electronic. It connects to the motherboard, much the way that the reset switch does, using a feature called soft power. So on an ATX system, when you press the power switch, you aren't really turning on the power supply; it is more like sending a "request" to the motherboard to turn the system on. As a result, the switch is a simple affair and the wires carry only low-current DC power, removing the potential risks inherent in the AT-style switch (well, risks if you tamper with the PC when it is plugged in anyway!)

One consequence of the "soft power" method of operating the power supply comes into play if there is a power failure. Imagine that you have a PC running unattended. There's a power outage, and the system shuts down. Several hours later, the power comes back on. With an old-fashioned mechanical power switch, the system would immediately restart, because as soon as the power was restored the power supply would turn on. With an ATX/NLX, SFX or WTX form factor supply however, the power supply would sit there waiting for a "turn on" signal from the motherboard! This is not much of an issue for most personal PCs, but is a big problem for business servers and other machines running without users physically present. To solve this problem, some high-end power supplies include an auto-restart feature that powers up the system immediately when the system detects that the AC power has returned after a power failure.


External Voltage Selector Switch

PC power supplies support 110V input, 220V input or both. Dual-voltage supplies normally have a selector in the back that controls which voltage you are using; obviously, you want to make sure it is set correctly. There are also some supplies that will automatically support either 110V or 220V without a selector switch, but these are often found only on more expensive units.

Power supplies that support dual voltage input are preferable since they are more flexible, although few people transport their PCs overseas (other than laptops, for which this discussion isn't relevant).

Warning: If your power supply does have a 110/220 switch, make sure it is set correctly, or else! This is usually done for you by the manufacturer of the case (which normally contains the power supply), but I have seen cases that were originally intended for one part of the world end up in a different part, with the wrong external voltage set as the default. Running a power supply set to 220 on 110 V power will probably cause it just to not work, but if you set the switch to 110 and run it on 220 V, damage might result.


Power Conversion Circuitry

The "guts" of the power supply is usually a circuit board with various electrical components on it, mounted inside the metal box of the supply. All the cables going into and out of the power supply go to this circuit, including those of the remote power switch, if any.

This circuitry is what is responsible for the work of converting AC to DC within the power supply. It also manages the other power supply functions of course. In newer supplies, many of the features of the power supply are combined into special integrated circuits to reduce space requirements and eliminate manufacturing costs. The circuitry inside the case of the power supply relies on the power supply fan for ventilation and cooling.


Motherboard Power Connectors

One of the most important connections in the PC is that between the power supply and the motherboard. It is through this connection (or set of connections) that the various voltages and other signals are sent between these two important devices. Different form factors use different numbers, types, shapes and sizes of connectors between the power supply and motherboard.

Before we look at the connectors, let's talk a bit about the wires that run between the power supply and the connectors themselves. Pretty much all wires within the PC are made from copper, due to its excellent conductivity, relative low expense, and flexibility. The most important characteristic of a wire is its size, and more specifically, its cross-sectional area. The reason is that the resistance of the wire is inversely proportional to the cross-sectional area of the wire. Thicker wires can carry more current, while the higher resistance of small wires causes heating when they are subjected to a high current, which can be hazardous. Since some wires need to carry more power than others, they are given different thicknesses. In addition, most motherboard connectors have multiple wires for the main voltage levels. This allows for more current, spread out between the different wires.

In the electronics world one standard used for wire thicknesses is American Wire Gauge, or AWG for short. The smaller the AWG number, the larger the wire. These numbers go from 0 (below 0 actually) to 50 and above, but for electronics the most common gauges are between 8 and 24. For motherboard connectors the wires are usually AWG 16, 18, 20 or 22. The table below shows these four sizes and some relevant statistics. You'll notice that the numbers are not linear with the actual size of the wire; AWG 16 wire is almost four times the cross-sectional area of AWG 22 wire.

The other issue of interest to us regarding wires is the color of their insulation. There are standards established for the colors of various wires, to help avoid confusion by those who work with different components and PCs. While not all manufacturers follow these conventions, most do. If they do not, problems can easily occur when a technician sees a black wire, assumes it is a ground (which it usually would be) and then finds out the hard way that it is not.

Below are diagrams that show the configuration of pins for the various connectors used by different form factors between the power supply and motherboard. In each diagram the pins on the power supply connector are shown in their correct orientation. The color of each pin is the color of the wire established as a standard for that pin. Outside the rectangular outline of each connector, next to each pin, is a depiction of the recommended AWG size for the wire going to that pin, and the name of its signal or voltage. Note that the diagrams are not to scale. Note also that they are shown from the perspective of the connector coming from the power supply. For those connectors with two columns of pins, the mating motherboard connector will have its pins in a mirror-image configuration.

Alright, enough with the preamble. Let's look at the connectors, starting with the oldest style. The PC/XT, AT, Baby AT and LPX form factors all use the same pair of 6-wire connectors, usually called "AT Style" connectors. They are typically labeled either "P8" and "P9" (what IBM originally labeled them) or "P1" and "P2". (Actually, the PC/XT form factor omits the +5 V signal on pin #2 of P8, but otherwise is the same.)

The biggest problem with the design IBM used for these power connectors is simply the fact that there are two of them and they are the same size and shape. The connectors are physically keyed so they cannot be inserted backwards, but it is very possible to accidentally swap them. If you do this, you will be putting ground wires where the motherboard expects live power and vice-versa, and the results would be catastrophic. Thus, technicians working with older systems developed the well-known mantra: "black wires together in the middle"!

Starting with the ATX/NLX power supply, Intel did away with the potential P8/P9 risk by making the main connection a single piece, and using only dissimilar shapes on any other connections between the power supply and motherboard. These are called "ATX Style" connectors. For its regular power supply connection, ATX uses a 20-pin connector with a square hole for pin #1 and round holes for the other 19 pins.

In addition, the ATX specification (version 2.03 is the latest) defines an auxiliary 6-wire connector (in a 1x6 configuration) and an optional 6-wire connector (in a 2x3 configuration). The auxiliary is intended for motherboards that require a lot of power to run their components (250 W or more); it consists simply of more, thicker (AWG 16) wires for the +3.3 V and +5 V signals.

The SFX power supply uses a main connector very similar to that of the ATX. The only difference is that pin #18 is omitted, since the SFX specification does not call for a -5 V signal. The SFX optional connector is similar to the ATX one but stripped down; only the Fan ON/OFF signal is provided, on pin #2. There is no auxiliary connector for the SFX supply, which is not intended for use in systems requiring a lot of power.

Finally, the WTX form factor. Since WTX is a design intended for workstations and other high end systems, it has a large number of connections to carry the tremendous amount of current that WTX supplies are capable of providing. WTX power supplies therefore have a completely different motherboard interface. The two primary connectors are the 24-pin "main" connector ("P1") and 22-pin "additional" connector ("P2"). Despite P2's name, it is really required by the design, since all the control signals are on it.

But wait, 40 lines isn't enough; we're not done with WTX yet. In addition to the above, three more connectors are defined. P3 is an eight-pin optional connector (with six pins used) that provides +12 V power to optional power modules or DC-to-DC converters used for additional processors and/or memory within the system. P4 and P5 are six-pin optional connectors used in a similar fashion, to provide additional current for multiple-CPU motherboards or other applications. (Some +12 V power is also provided on P2.) The spec seems to be intentionally flexible (read: vague) regarding how these connectors are to be used.



[H]F Junkie
Sep 9, 2003
Drive Power Connectors

The power supply provides power to internal hard disk, floppy disk, CD/DVD and other drives directly, through four-wire connectors that are designed to attach to the rear of each drive. The four wires provide +5 V and +12 V power, along with two grounds, to the various drives that use them.

The connectors themselves come in two basic styles. The larger size, often called a Molex connector (after the name of one of the big connector companies) is keyed by virtue of the connector itself being "D-shaped", and is used on most internal drives, including hard disk, CD/DVD, Zip and other removable media drives, and the older 5.25" floppy disk drives. The smaller size, typically called a "mini-plug", is used for the newer style of 3.5" floppies. It is also keyed, but in a different way than the larger connector, and actually secures to its mating connector with a retention clip of sorts.

The number of connectors that come with each power supply varies considerably. In general, the bigger the supply, the more devices the manufacturer expects you to run, so the more connectors are included. Totals can range from 3 or 4 connectors to as many as a dozen. Another factor is just general quality; some makers skimp on the connectors to save money, and make you buy adapters to let you run additional drives. These adapters, usually called "Y-splitters" or "Y cables" after their general shape (sort of :^) ) contain one male and two female large-style hard disk connectors, cost around $2-5 in the U.S,. and are available in most electronics or computer stores. They are increasingly needed in modern systems, because not only do newer systems have more drives, they also have more fans and cooling devices, which also often attach using a disk drive power connector. Do remember, however, that adding a Y-splitter doesn't magically increase your power supply's output capacity! It just gives you more connectors.

Warning: It is best to avoid Y-splitters if possible, for a few reasons. First, there have been reports of incorrectly wired Y-splitters. Watch out for them, as they have the potential to damage your equipment. (It's pretty easy to see if the adapter has been wired correctly by inspecting it carefully. Using an ohmmeter to test for correct connectivity is even better.) Second, they are an additional potential source of failure in the system, and are often cheaply made and hard to align and plug in properly. Third, they can further clutter the inside of a busy case. Fourth, every time you share two devices on a single connector, all the power drawn by the two devices has to travel down the same set of wires from the power supply. If you chain three or four drives off the same connector using multiple splitters you may exceed the current rating for the wires and/or connector. See the discussion of wire size and resistance in the section on the motherboard connectors for more details.

Most systems only come with a single mini-plug connector--because most systems come with only a single floppy drive that uses it--but some may come with two. If you need an additional mini-plug connector in a system with only one, or if the first one breaks, you can employ a simple adapter to change a large style into a small style. (They may make them the other way as well, but I am not sure.) You may also on occasion see mini-plugs, or mini-plug adapters, that only use two wires instead of the standard four. They omit pins 1 and 2 because newer floppies may use only the +5 voltage. This won't cause problems in most cases, but beware.

A couple of final thoughts. The power supply fan in your PC runs directly off a connection within the supply itself, but additional fans within the case are growing in popularity, and typically each of these requires a drive connector. They don't draw very much power (but fancier electrothermal coolers can draw quite a bit). Finally, the hot-swappable drives used with RAID in newer server boxes typically do not use standard drive connectors for their drives. They make use of a technology called single connector attachment (SCA). In this scheme, special bays are installed in the system, which take either standard power connectors from the power supply, or a special wiring harness. The drives themselves plug into the bays and draw all their power and signals from a single mated connector, one half on the back of the drive and one half inside the bay. This allows them to be easily removed while the system is still operating.


Power Supply Fan

One of the more important components in the power supply is one that seems tangential to it: the power supply fan. Since the earliest PCs, the power supply fan has been the primary cooling source for the entire PC. Today's PCs of course incorporate additional cooling methods, including auxiliary fans and CPU cooling devices, but the power supply fan remains an important factor in the overall cooling equation. You can find out more on the general subject of system cooling in this section of the System Care Guide.

The fan is traditionally located at the rear of the power supply, and special vents are provided for it in the case of the supply itself to allow for it to exhaust (though the newer ATX form factor changes things; see below). Most fans use +12 V power to operate, despite the fact that the wires that run to them are normally red for the +12 V line, and black for the ground (not yellow and black as you might expect from the wire color standards elsewhere in the PC).

In addition to the regular fan found in the power supply, most newer systems include auxiliary fans for improved air flow and system cooling. These are typically mounted at various venting locations around the outside of the system case. The standard size of a PC cooling fan is about 3.25" or 80 mm square, but they come in other sizes as well.

A very important quality consideration when it comes to PC cooling fans is the quality of construction of their motors, and in particular, the motor bearings. Cheaper fans use sleeve bearings that are much less durable than their ball bearing counterparts. While "sleeve vs. ball" isn't the only dimension upon which to measure cooling fan quality, it is an important one. Sleeve bearing fans can lock up after as little as a year of use, while ball bearing fans typically last many years.

Another quality consideration of a fan is how much air it can move. This is normally measured in cubit feet per minute (CFM). The higher the rating, the more work the fan is accomplishing. Fan speed can be controlled on some systems through the use of the FanC, FanM and/or Fan On/Off signals. Many power supplies also have automatic thermal control of the power supply fan: they reduce or increase its speed based on internal temperature without any intervention required by the rest of the system.

The fan is the component most likely to go first in a power supply. The usual cause of this is dirt that gets into the motor of the fan and gums up the works. The average time until failure is greatly increased if the PC is used in a very dirty or dusty environment, or if the PC is never cleaned. When the fan stops working, overheating of the components within the power supply as well as components in the rest of the PC are likely. A PC that makes use of the optional fan monitoring signal FanM can detect a fan failure and sound an alarm to the user, or shut down the PC. Another way of detecting an overheating condition is through hardware that monitors the internal temperature of the system.

The power supply fan is probably the only component that can be replaced by an end-user (although I still recommend against it for most users since it requires opening the power supply case.) If a technician replaces the fan, an easy way is to use one of the fans shown above, with the drive connector snipped off and the wires spliced into the ones that the old fan used. A possible solution to a bad fan that does not involve opening the power supply is an add-on external fan. These fans plug into the wall directly and are typically heavier-duty and higher-capacity than a standard power supply fan. They are actually designed for improving the cooling of the existing fan even when it is running.

The final issue to consider regarding the power supply fan is the direction in which it circulates air. Older PC/XT, AT, Baby AT and LPX form factor power supplies are designed to exhaust air out the back of the PC. For older machines using older, slower CPUs, this worked fine, but starting with the Intel 486, separate cooling started to become required for the processor. In response, Intel designed the ATX form factor to reverse the flow of air and move the power supply fan to the inside edge of the supply case, with the goal of using the power supply fan to also cool the CPU. Later, when it became obvious that the newest CPUs still needed their own cooling and having the power supply fan blow already-heated air on them wouldn't get the job done, Intel made the fan direction (and location) optional.

One advantage of a power supply fan that blows into the case is that it provides much better control over the air that enters the system. Instead of being drawn in through all the holes and cracks in the case as with a fan that pushes air out the back of the system, the air entering the case all comes in from the power supply fan intake. This intake can be filtered to dramatically reduce the amount of dirt that accumulates within the system.


Power Supply Fuse

Some power supplies come with their own integrated fuse. The fuse is designed to protect the circuits in the power supply from damage should an over-current situation arise. You can read more about fuses on this PC Fundamentals page about basic electrical components. If there is a problem with the electrical system (surge, lightning strike) or internal fault within the power supply, the fuse will blow. It can then be replaced and if it did its job properly, the supply should operate normally.

Unfortunately, many PC power supplies don't have fuses at all. I suppose this is a cost-savings measure but it seems pretty short-sighted to me. Even many power supplies that do have fuses hide them from the user within the power supply case. It's not a good idea to open up the power supply unless you are sure you know what you are doing, so I don't recommend opening the supply to search for a fuse (especially since too many units no longer have them). It's a good idea though to search the back of your system to see if there is a user-replaceable power supply fuse.


V. Power Supply Form Factors

The form factor of the power supply refers to its general shape and dimensions. The form factor of the power supply must match that of the case that it is supposed to go into, and the motherboard it is to power. You may not find too many people discussing form factors as they relate to power supplies--this is because power supplies normally come included in system cases, so people talk about the form factor of the case instead. This is changing as the power supply starts to get more of the attention it really deserves. Also, newer power supply form factors can often work with more than one type of case, and vice-versa.


PC/XT Form Factor

The first PC was of course the IBM PC. Its power supply, and that of its hard-drive-equipped successor, the IBM PC/XT, used the same original form factor. These systems were all desktop units, with the power supply tucked into the rear of the case on the right-hand side, and controlled via an up/down toggle switch. While the PC/XT power supply began as an IBM design, IBM's key decision to keep the PC architecture open allowed "clone" manufacturers to make similar PC boxes and use the same size and shape of power supply for interoperability. In this manner, the first PC form factor "standard" was born.

PC/XT units were sold as desktop boxes only. Equipped with only one or two 5.25" (low-density) floppy disk drives for storage, and having limited expansion possibilities, the original PC came with a very low-powered supply by today's standards: 63.5 W. The XT added the first PC hard disk drive and an appropriately doubled power supply rating: 130 W (still relatively small by today's standards.) These IBM power supplies were physically large for their output--not surprising, since they used much older components and were designed before some power supply functions were combined into integrated circuits. They were also very well-made, another reason for their somewhat large size. They of course are used in PC/XT form factor system cases and with PC/XT motherboards. The PC/XT was the first form factor to use the well-known pair of six-conductor motherboard connectors that were used through the Baby AT and LPX form factors, and the four-conductor disk drive connectors that are still used to this day.

These units are of course not only obsolete today but heading rapidly into "antique" status. You will still find them in use however, usually as dumb terminals for larger minicomputers, or for controlling industrial equipment. This is as much a testament to the quality of these first PC power supplies as anything.


AT Form Factor

In 1984 IBM introduced the IBM PC/AT, "AT" standing for "advanced technology", an abbreviation whose use still survives to this day in some contexts. Very similar in overall physical design to the PC and XT models that preceded it, the power supply in these units was increased in size and changed slightly in shape, establishing it as a distinct form factor. Whereas "clone" manufacturers made a few PC/XT units compatible with the IBM PC and XT, it was with the AT that the PC world really began to explode. Many different manufacturers began creating AT-compatible systems and with them, AT form factor power supplies. The original AT power supply provided 192 W--a respectable figure, especially for the time, which represented a tripling of the original PC's power supply output just three years later. It is used in AT form factor cases and with AT or Baby AT form factor motherboards. It has the same motherboard and drive connectors as the PC/XT form factor.

The AT form factor was the first to introduce tower-style cases and systems to the PC world. The desktop power supply and tower power supply were internally the same. The only difference was in the on/off switch. The desktop used the same red toggle switch as the PC and XT, while the tower introduced the first remote power switch in the PC world. The control wires for the switch were passed through the same hole in the front of the power supply case that was used for the motherboard and drive connector bundle.

The AT form factor is now obsolete of course, though many of these systems remain in use. After only a few years, the AT form factor was quickly dethroned by its smaller successor, the Baby AT form factor, due to the advantages of the latter's smaller size, so far fewer AT systems than Baby AT systems are found today.


Baby AT Form Factor

The Baby AT form factor is so named because it is a smaller version of the original AT form factor. It has the same height and depth, but is about 2" narrower. Since it is "similar but smaller", the Baby AT power supply will fit both in Baby AT form factor cases and in full-size AT cases as well, in both tower and desktop styles. It has the same output motherboard and drive connectors as the AT. Due to this flexibility, and the fact that it was introduced at around the time that PCs began to really grow in popularity, the Baby AT form factor reigned as the most popular design for over a decade--far longer than any other. From around 1985 to 1995, a large percentage of new PCs were Baby ATs (though later on LPX form factor power supplies came to be used in many Baby AT systems.)

The Baby AT power supply was made in both a tower and desktop configuration, like the full-sized AT, and like the full-sized AT these differ only in the type of power switch used. Unlike the full-sized AT, however, in the Baby AT form factor tower versions became much more popular. Even in many desktop systems, tower-style power supplies began to be installed for the simple reason that most users prefer having a power switch on the front of the case and not in the rear.

This form factor has now been replaced in new systems by the ATX and other form factors. However, the huge installed base has given Baby AT momentum and given manufacturers of new components incentive to provide upgrade options for the millions who still use these systems.


LPX Form Factor

One power supply form factor that has given Baby AT a run for its money over the last 15 years has been the LPX form factor. The "LP" in "LPX" stands for "low profile", another name given to these power supplies. They are also often called "slimline" power supplies because LPX cases are often called slimline cases, and "PS/2" power supplies after the famous IBM model. The main goal of this form factor is size reduction. The height in particular of the power supply is significantly reduced, facilitating the design of much smaller, consumer-oriented PCs. The connectors of the LPX form factor power supply are the same as that of the Baby AT and AT.

While never officially specified as a standard, the LPX or "slimline" power supply basically became one anyway. Due to its small size and convenient rectangular shape, these power supplies were put into all sorts of cases; not just LPX cases but Baby AT and even full-sized AT cases. Until the rise of its anointed successor, NLX, LPX systems were made in large quantity, and millions of these power supplies are still in use.


ATX (NLX) Form Factor

At the time of its introduction by Intel in 1995, the ATX form factor was the most significant change in system design since the invention of the PC over a decade earlier. Although it took several years to "catch on", the ATX form factor and its variants are now the standard in a large segment of the marketplace. In addition, the NLX motherboard and case form factor--designed to replace LPX--intentionally use the same power supply because Intel wanted to avoid having another power supply form factor on the market. Therefore, the ATX form factor is sometimes called the "ATX/NLX" form factor.

On the outside, the ATX power supply appears virtually identical to an LPX power supply in terms of its dimensions and component placement. The biggest visible difference between the two is that the power pass-through outlet for the monitor has been removed (primarily because modern monitors always come with their own power cord these days, so the pass-through hasn't been commonly used for some time.)

The inside of the ATX form factor, however, is an entirely different story. The ATX power supply design differs from the previous market standards, the Baby AT and LPX form factors, in several important ways:
  • True Standard: The ATX form factor is a standard, as opposed to the "de facto standards" of prior form factors. You can find detailed specifications about ATX and other newer form factors at the Platform Development Support Web Site. Included there is a document specific to the ATX power supply.
  • +3.3 V Power: ATX systems were the first to include +3.3 V power directly, avoiding the need for voltage regulators to provide it on the motherboard.
  • Soft Power: ATX systems were the ones where the +5 Standby and Power On signals were introduced. These signals are used along with a change to the way the power switch works, as part of the "Soft Power" feature that enables features such as allowing the operating system to turn off the PC.
  • Additional Signals: ATX defines several additional signals used for fan control, IEEE 1394 compatibility, and more.
  • Changed Motherboard Connectors: Breaking with 15 years of tradition created by the PC/XT, AT, Baby AT and LPX form factors, Intel specified new motherboard connectors for the ATX form factor. This was in part due to the additional signals used by the ATX power supply and motherboards. For compatibility, some motherboards include both the new and old style of connector. Read more about the motherboard connectors here.
  • Modified Fan Direction and Placement: One of the goals of the original ATX specification was to change the way the power supply fan worked. At around the time ATX was introduced, cooling fans were becoming the standard for the newer, faster CPUs on the market. Instead of exhausting air out the back of the case as had always been the norm, Intel wanted to use this exhaust air to cool the processor directly, saving the cost of a cooling fan. Therefore, the ATX specification calls for the fan to run in the opposite direction and be placed near the CPU's location on the motherboard, to blow on it for cooling. The other advantage of this method is that it keeps the system cleaner, since air entering the case all comes from one place, and can be filtered if necessary. Unfortunately, while a good idea, this hasn't worked out quite the way Intel hoped. The primary problem is that newer CPUs continue to generate more and more heat as they get faster, and a regular power supply fan doesn't have enough flow to cool them properly. This problem is compounded by the fact that the air blowing on the CPU is warmed by the components in the power supply itself, so it is several degrees above ambient temperature before it ever gets near the CPU. Thus, newer versions of the ATX specification make the fan direction optional. The newest ATX power supplies have gone back to the old style of placing the fan on the back of the power supply and exhausting air to the outside.

Since it has become the industry standard, ATX power supplies are found everywhere. Ostensibly designed to work with ATX cases and ATX (and Mini-ATX) motherboards, ATX power supplies are also used in NLX systems, as mentioned above. They can also be used for microATX motherboards in microATX cases if the case is large enough, because the ATX and SFX main motherboard connectors are essentially the same.


SFX Form Factor

As part of the continuing trend towards smaller and smaller PCs, Intel in 1997 introduced the new microATX form factor, based upon the original ATX form factor. In 1999, Intel produced the FlexATX addendum to the microATX specification, detailing plans for an even smaller motherboard and case standard. Neither of these form factors include specifications for a power supply. Instead, Intel created the SFX power supply form factor, which they may optionally use. The "S" in "SFX" is for "small" of course! microATX and FlexATX systems can also use the ATX power supply, though since miniaturization is the key with these systems, the SFX power supply makes much more sense. You can find detailed specifications on the SFX power supply form factor at the Platform Development Support Web Site.

The SFX specification actually calls for a default configuration, and several options. The "regular" SFX power supply is nominally 100 mm wide, 125 mm deep and 63.5 mm in height. It includes a 60 mm power supply fan for cooling. An optional configuration calls for the placement of a larger fan on the top of the power supply. This fan option is 80 mm and is very often selected by manufacturers as it provides for improved system cooling. It increases the height of the supply by about 10 mm. Another option is for an extra-small power supply with dimensions of only 100 x 125 x 50, and a 40 mm power supply fan. This configuration however requires an additional fan for system cooling, because the small 40 mm fan is only sufficient to cool the power supply itself.

In many ways the SFX form factor could be considered a "little brother" to ATX. It is mostly interchangeable with the ATX power supply. The main SFX motherboard connector is 20 pins, in the same shape and size as the ATX connector, and 19 of the pins are the same as those of ATX. The one difference here is that the SFX power supply specification does not call for providing the -5 V compatibility voltage. The reason is that -5 V is only required for ISA bus compatibility, and since Intel wants to move new systems away from ISA (to PCI and AGP only) it intentionally left -5 V off the specification (presumably to save on the cost of the power supply). Systems that need -5 V and want to use the SFX power supply must generate it on the motherboard. The SFX power supply specification calls for the power supply fan to be internally thermally speed-controlled, but an additional "Fan On/Off" signal is included on the SFX optional motherboard connector. Another issue with ATX exchangeability is that an SFX power supply equipped only with the standard 60 mm fan may have considerable trouble cooling a large ATX system case.

The specified output rating of the SFX power supply is 90 W. This is sufficient to run rather small systems with low-powered CPUs and few peripherals, but makes things a bit tight and leaves little room for expansion. Fortunately, some manufacturers are producing SFX power supplies with much higher output ratings.


WTX Form Factor

If the SFX form factor is the little brother to ATX, WTX isn't quite its big brother. WTX is more like its overgrown third cousin from a distant country. :^) Introduced by Intel (who else) in 1998, and revised in 1999, the WTX form factor is designed specifically for workstations (thus the "W" in "WTX"). WTX defines a standard for motherboards, cases, and power supplies.

To meet the increased needs of the largest regular PC systems, the WTX form factor is totally different from the other PC form factors. It is designed in a modular way from the ground up to allow it to meet the needs of large, multiple-CPU systems now and in the future. The system is segmented physically into different "zones" where different functions are supposed to be incorporated into the system. The motherboard is mounted on a special mounting plate which gives motherboard makers the flexibility to design boards without "hard-coded" mounting hole restrictions. For its part, the power supply has been completely changed to suit the needs of these larger systems. The best way to really understand the WTX form factor is to download and read the specifications that are available at the WTX Home Page.

Unsurprisingly, WTX power supplies are large and powerful. The WTX specification actually includes design guides for three specific sizes of power supply: 460 W, 610 W, and a whopping 800 W, though manufacturers are not limited to those particular numbers. For designs up to about 500 W, a single power supply fan is specified, with overall power supply dimensions of 150 mm width x 230 mm depth x 86 mm height. For larger capacity supplies, a dual-fan configuration is recommended, which increases the width of the package to 224 mm.

The motherboard connectors used for the WTX are completely different from, though similar in concept, those of ATX and SFX. Two large connectors with a total of 46 pins (6 reserved for future use) are the main connection to the WTX motherboard (or set of boards) Several additional connectors are also optional for powering additional CPUs or other devices. The WTX power supply also supplies several extra signals unique to the WTX form factor. WTX supplies are intended to be matched to WTX motherboards and put in WTX form factor cases. They normally include a large number of drive connectors to run a large number of hard disk drives and other devices, or special wiring to accomodate RAID bays.



Feb 19, 2004
Just a comment.

The power factor can never be correct to 1.0. you can get extremly close to 1.0 but never 1.0.

Also the power factor correction is usually done by placing a capacitive load in parallel with the inductive load of the PSU, So that the angle between the real power and reactive power is minimized.

It's a good thing someone made a power supply FAQ. This should be a sticky.


[H]F Junkie
Sep 9, 2003
There is a whole lot more technical stuff that needs to be added in but i just wanted to get the basics out of the way so this thing would at least get posted.


More Human than Human
May 26, 2000
....a tremendous effort. First up - thank you for the post....:cool: Some small points;

One rail you missed (..??..) is the +5.0 Vaux line. This is powered anytime the PSU is plugged in, even if the system is "off". Among other things, it's used by network cards to "stay awake" enough so that a command over the ethernet can "wake up" the machine from a suspended state or even "fully off" state. This is why it's important to acutally switch the supply off (on the back of the PC) or lacking a switch, unplug the PC prior to adding or removing hardware.

Keep in mind that the P4 power connector supplies.....+12 Volts. You mention that +12 runs the drives, but it also typically supplies the switching regulators that power the Vcore on the CPU - typically many amps at very low voltage. It's not intuitive, but when you have to deliver a lot of current, one advantage of a switcher is that it can take a higher voltage and regulate it down and realize very good efficency, as opposed to being fed with +3.3 or even +5. A linear supply, as you mention, would just dump the extra voltage as heat, but a switcher realizes some conversion efficency for high current, low voltage loads.

It's been my experience that the A/D's on most motherboards are very good at measuring relative differences, (like was 3.34 Volts now 3.05 volts) but NOT very good at measuring absolute voltages. For example, my motherboard (see sig) reads the +12 at 11.49 volts. My Fluke 77 multimeter thinks it's 12.07 volts under load (running Prime95 torture test). Guess which one I believe...??...:D I believe the reason is primarily due to not having a stable Vcc reference, rather than damaged connectors, resistance, etc. Keep in mind that in order to measure an absolute voltage, you have to have something to compare the unknown voltage to that's "known". It's tougher than you think to generate temperature stable sub-mV accurate voltages, when you can't trust what you'll have for incoming voltage accuracy. Sure, it's possible, but keep in mind cost is a big issue with mobo design.

Last, to easily test these voltages, you can get to +5 and +12 via an unused drive connector. Most decent PSU's these days also have the six pin "Aux" power connector used by many server motherboards with on board RAID controllers, etc. It's a six pin connector, that offers +3.3, and +5. This way, you can test the three most important Vcc's on your supply,+3.3, +5, and +12 without having to get into a crowded MoBo / power connector with your DVM probe. As you said, the negative Vcc rails are rarely used, if at all.

Great post - thanks again.



Jan 6, 2002
"250-300Watt=Low-end system"

Well, that depends on the quality of the PSU.
I use a 300Watt for....

Barton 2500+
Radeon 9800pro
1gig ram
2 harddrives
2 optical drives
Baybus with 4 fans attached

And its not on the above recommended list, it do weight a ton though, very good PSU.

Ice Czar

Whos who in PSU

This is an effort to track down the manufacturers
that make the brands of PSUs you all know and love (or hate :p)
and several youve never heard of (Asian, European distribution ect)

If you have an additional information to contribute, or know of any factual errors, please PM me or post to the [H]ardcore PSU Info thread (even rumors are welcome)

Note: While several brands might share the same manufacturer, that doesnt mean the PSUs are the same, its quite common that there are major comonent differences between brands, of even built to spec for a brand distributor

Subsidiaries and Brands
(brand names that this manufacturer has sold to or under a seperate company's brand, not neccesarilly exclusively)

FSP Group (China\Taiwan)(Fortron-Source Technology \ SPI Electronics \ Sparkle Power) Sparkle, PC Power & Cooling, Verax, Conrad, AOpen, Powerman, Hi-Q, possibly Zalman and Inwin, (look for an FSP model designation like FSP300-60ATV )

Channel Well Technology (Taiwan)
Channel Well, Antec (will not have a CWT designation), Yeong Yang, Lead Power. ( Look for CWT model designations like CWT-300ATX-12d )

Zippy (Taiwan)
Zippy Emacs, PC Power & Cooling

Heroichi \ HEC Group (Taiwan)
look for a HEC model designation like HEC-300LR-PT
Heroichi, Compucase, old Antecs, with models
starting with "PP"

Enermax (Taiwan)
maybe Casemart and Wavesonic

Designate: MPT

Thermaltake, Compusa, Aspire, Vantec, High Power, Super Power, Enlight, Enhance, Silverstone, Cheiftec and maybe Coolermaster.

Ablecom (Taiwan) Designate: SP
Manufactures exclusive for or is a subsidiary of SuperMicro?

Topower (Taiwan)
Designate: TOP
Vantec Stealth, Tagan, OCZ Powerstream, Toria, TTGI, maybe also Hiperpower and Superflower

Delta (International) Designate: DPS \ GPS
Acer, some OEM supplies


Justy DPX-400A

Key Mouse
Soyo, MaxPower, EverPower

MaPower (Taiwan)

Seventeam (Korea)
Designate: ST

AcBel Polytech (Taiwan, China, Philippines)

Codegen Group (China)

In Win (Taiwan)
Model Designate IW
possibly manufactured by FSP

Etasis (Taiwan) Designate: EPA \ EPR

HiPro (Taiwan\China\Thailand) Designate: HP or HPC

Powmax, Robanton, Raidmax, Ultra-X

Allied, Codegen, L&C, Logic, Foxconn, Mustang, Powerstar,
Eagle, Foxlink, Mercury, Duro, Austin, Turbolink, Real Power.

Brands that are probably seperate companies (as opposed to brands marketed by a manufacturer listed above), and maybe manufacturers in their own right. Information\Corrections encouraged

Antec (USA)
Thermaltake (Taiwan\China)
Vantec (USA)
Conrad (Netherlands)
Casemart (Korea)
Designation (EG) suggests at least Enermax is one supplier


Disclaimer, This list maybe wrong in particulars, I appologize for any errors
Special Thanks to larrymoencurly, SJetski71, Vertigo Acid & computerpro3


[H]F Junkie
Sep 9, 2003
I didn't know it was full Ice Czar lol. Normally i get an e-mail when its full but i just now got one today. I had no idea :p.

Its cleared out now :).

I was wondering why you hadn't answered any of my PM's lol.


[H]F Junkie
Sep 9, 2003
VI. Power Supply Output and Ratings

A power supply's components, operation and form factor are all very important quality and design considerations. When it comes down to it though, the bottom line with any power supply is: how much power will it provide? Though this matter has been greatly over-emphasized in the past--"What kind of power supply do you have?" "Oh, a 250 watt one"--it is certainly quite essential. Each year it becomes more important as PC components get faster, larger and more demanding of power.

This section takes a comprehensive look at issues related to the output power capacity and ratings of the power supply. This includes a detailed answer to the all-important question: "What exactly is the 250 W in that 250 W power supply?" The section also takes a close look at PC power requirements, the importance of allowing for peak power demand, and the matter of power supply loading.


Output Power

When people talk about a power supply's output, they usually say it produces a certain number of watts. While a convenient shorthand, it is unfortunately both vague and imprecise. Buying a power supply solely on the basis of its wattage rating would be like buying a house solely on the basis of its square footage, or a car by how many horsepower its engine produces. In both cases the parameter is an important one, but it doesn't come remotely close to telling the whole story.

Let's start by taking a look at what the output rating means. For purposes of this section, let's consider a "300 W" power supply. What does that number really mean? It is the nominal, total maximum output power of all the voltages that the power supply provides. As described in the power basics section on power, for DC electricity the computation of power is as simple as multiplying its current in Amps, and its voltage in Volts. Of course, power supplies produce several different voltages. That's the first reason why just knowing the total number of watts is insufficient!

If you check the output specifications for the power supply, you will normally see listed all the different voltages that the power supply provides, and the amount of current it can supply for each. (You will also likely see listings for peak output and minimum load.) This is sometimes called the power distribution for the supply. Each voltage produced by the PC is used for a different purpose, which is why it is crucial to check out the individual current ratings for each voltage, and not just use the total wattage of the supply. We can also use the power distribution to calculate the actual total output rating of the supply, and compare it to the marketing numbers--you might be surprised how they differ. How we do this depends a bit on the form factor of the power supply, and in particular, whether or not the supply provides +3.3 V power.

For the PC/XT, AT, Baby AT and LPX form factors, which do not supply +3.3 V power, multiplying each voltage and maximum current value and adding them together should yield the approximate total output power of the supply. Note that the "negative voltages" are added to the total, not subtracted from it

For the ATX/NLX, SFX and WTX form factors, which provide +3.3 V power (as well as +5 V Standby power and potentially others), there is an added complication: there is a maximum rating for each of the +3.3 V and +5 V currents, but also a combined "+3.3 V / +5 V" rating. The power supply will provide up to the combined total on these two voltages, in any combination as long as the individual current ratings are not exceeded.

The power supply will only provide as much current as is needed by the PC. A 300 W power supply isn't always putting out 300 W of power. Most PCs uses significantly less than the maximum rating of their power supply. This is important to remember for UPS sizing purposes.

Some power supply form factor specifications provide guidelines showing typical power distributions for various output rating totals. You can use these to help you get a better idea of the sort of distribution is typical for the form factor at a given size.


System Power Requirements

The goal behind analyzing the power supply's output rating and power distribution is to match it to the needs of your system to ensure it will provide you with the power you need. Unfortunately, this is usually much easier said than done. The key problem in this regard is trying to figure out exactly how much power your system uses. This is not an easy task, and the manufacturers of most systems don't make it any easier.

It's important to remember when picking a power supply that you need to leave room for expansion. Many people purchase or build systems using motherboards that they hope will allow them to upgrade to newer CPUs, or buy large cases with room for lots of drives and other peripherals. However, the power for these devices has to come from the power supply--something many people never consider. Newer processors in particular can be very demanding in terms of their power requirements, especially regarding the total of +3.3 V and +5 V power the supply can provide. If you want to increase your chances of success when upgrading in the future, leave "headroom" in your power supply.

Determining how much power your system needs can be either simple or difficult, depending on whether you want to make a crude estimation or a more exact calculation. Here are some methods you may find useful:
  • The "I Don't Want to Worry About It" Approach: This is what I use and recommend to most users. The idea here is simple: buy something really big, and then you don't have to worry about this issue. And not having to worry about your power supply's capacity can give real peace of mind. Rather than figuring out that your system requires 142.791 watts and then buying a 150 W supply for it, just get a 250 W supply and be done with the matter. For most regular desktop PCs, a 250 W power supply will provide enough power for most anything you can throw at it. For a typical tower PC, a 300 W supply is probably all you will ever need, and the difference in price between a 200 W and a 300 W supply of the same type and manufacture is often surprisingly small!
    On the other hand, if you are planning to build a server with four CPUs and 12 internal SCSI drives, this method is not likely to be sufficient for you. Even dual CPU systems can require prodigious amounts of +5 V and +3.3 V power. Obviously, the exact way that the power is distributed is important for special applications. Check out the different ratings before you buy. If your system has a lot of drives, pay particular attention to the +12 V rating. If it has more than one CPU, or one known to draw a lot of current, pay special attention to the +3.3 V and +5 V numbers.
  • Use an Approximation: Based on the general intended use of the machine and what you foresee requiring for future expansion, approximate the amount of power you will require. This can be difficult to do if you have not worked a great deal with PCs before, because it basically requires you to estimate power requirements based on your general knowledge of the components, and your past experience.
  • Calculate the Requirements: Calculate your requirements from the power use specifications of the components inside your machine. For each voltage level, determine how much current is required by each device, add them up, and get a power supply that can handle the load.This can also be difficult to do, because many devices do not come with complete specifications, and power use is not a spec that is commonly sought by most people. It's easier if you build your own PC, as then you usually get a manual of some sort with each device. However, even there, the manuals often don't say how much power the devices use. I personally don't recommend trying to do this unless you are fairly knowledgeable about PC components, and have a considerable amount of time on your hands.

Peak vs. Continuous Power

When you read the current (or power) rating of a device such as a hard disk, you are usually seeing the manufacturer's claim of how much the device uses during normal, continuous operation. The actual peak draw of the device is at its maximum, however, at startup--not while it is running. The +12 voltage provided by the power supply is used primarily to drive disk drive motors. Because of their design, these motors can take up to double their steady-state current when they are spinning up from rest. If you have 3 or 4 hard disks in your system and all of them start up at the same time, this can be a tremendous demand on your power supply's ability to provide +12 V power.

Fortunately, most power supply companies take this into account and build into the power supply the ability to exceed its normal output for a short period of time during startup. You will usually see this specified as a "peak" rating, often only for the +12 V line, where this is a particular problem. (It is much less of an issue for the +3.3 and +5 voltages, and therefore many power supplies do not specify a peak rating for these voltages.)

Despite this extra capacity, it is still a good idea to not load up your system to the very limit of your power supply's stated power capacity. It is also wise, if possible to employ features that delay the startup of some disk drive motors when the PC is first turned on, so the +12 voltage is not overloaded by everything drawing maximum current at the same time. See this section for more on hard disk power issues and how problems can be avoided at startup.


Redundant Power Supplies

One advanced feature available on high-end machines (especially servers) but also available to the general public for those willing to pay for it, is a redundant power supply. In essence, this is a power supply that actually includes two (or more) units within it, each of which is capable of powering the entire system by itself. If for some reason there is a failure in one of the units, the other one will seamlessly take over to prevent the loss of power to the PC. You can usually even replace the damaged unit without taking the machine down. This is called hot swapping, and is an essential productivity backup for use in servers and other machines used by a number of people.

Obviously, this sort of option isn't for everyone, and these units are not cheap. After all, for starters you have (at least) two full power supplies in there! But if you need the security and can afford it, it may make sense for you. Certainly, this sort of feature is a good complement for an uninterruptible power supply--a UPS provides insurance against the power provided to the system going out, and a redundant supply provides insurance against the power within the system going out.

Redundant power supplies are commonly used in conjunction with RAID arrays in systems requiring a high degree of fault tolerance.


Power Supply Loading

PC power supplies use a technique called switching to generate their DC voltages. Due to the manner in which these sorts of power supplies function, they need to have a load, meaning something that draws power from the supply, in order to function properly. A power supply that is turned on with no load attached will either fail to function or will function improperly. Better-quality supplies will detect a no-load situation and shut down, but cheap ones can be damaged. This is why you should not "test" a power supply by just plugging it in with nothing attached to it.

The amount of load required by a particular power supply is often specified as its minimum load. Just as the total wattage of a power supply doesn't tell you enough about its output, neither would a minimum power requirement--you have to look at the minimum load requirements for each voltage level provided by the power supply. You will sometimes see these minimum current requirements listed as part of the power supply's output specifications. The amount of load required can vary considerably between different form factors, between manufacturers and even between specific designs from the same manufacturer.

In the early days of the PC, power supplies often had considerable load requirements, both for the +5 and +12 voltages. The +5 voltage requirement was easily satisfied by connecting the power supply to the motherboard, but the only devices that draw +12 V consistently are hard disk drives. (Some floppy disk drives also use +12 V, but only when they are actually spinning, which is not the case most of the time.) Thus, people who tried to troubleshoot power supplies without connecting a hard disk drive could be tricked into thinking the supply was bad. Also, systems with no hard disk drives would have a problem starting up unless equipped with a "dummy load". Essentially, this is a large resistor that plugs into a power connector to create a +12 V draw. Of course, this is rather wasteful of electricity, and heats the system needlessly.

Modern power supplies have drastically reduced the degree to which loading is an issue. Most newer power supplies have very small +3.3 V and +5 V load requirements, and many have no minimum at all for +12 V. The lower loading requirements make testing and troubleshooting much easier.



[H]F Junkie
Sep 9, 2003
VII. Power Supply Specifications and Certifications

This section provides a detailed look at the various specifications that accompany power supplies. While the specifications of any PC or PC component can tend towards the cryptic, power supply specifications are particularly bad in this regard. Many of the specifications are related to the electrical characteristics of the supply, and since most people aren't electrical engineers, they don't understand what they are reading. So most just skip them and figure they can't be that important. I think that learning power supply specifications takes some time and effort, but is well worth it if you want to understand what you are buying. After reading this section, you can be the judge.

The specifications are sorted into groups to make them "easier to digest". You'll find similar groupings on many manufacturers' specification sheets (but some just provide a long laundry list of specs). Although not every maufacturer will include every specification we cover here in their published specification files, they should be able to provide the information if you request it. Beware any manufacturer that can't, or won't, provide operational specifications for their power supplies. That's a good hint right there that you may not be dealing with the best of companies--or equipment.


Physical Specifications

In this section we will take a brief look at the physical specifications normally listed for a power supply. These are the routine matters of how the power supply is physically situated, and what the characteristics are of its physical pieces. Nothing fancy here.

Form Factor: The form factor of the power supply. Occasionally, you will see a power supply specified as a form factor that is actually that of the case in which it normally fits. The most common case of this would be a "microATX" power supply. In fact, there is no such power supply form factor; they mean an SFX power supply, which is what normally goes into a microATX system.

Dimensions: The physical dimensions of the case. Normally these are specified as W (width) x D (depth) x H (height). May be given in inches (in) or millimeters.(mm). One inch is 25.4 mm. The standard sizes for the various power supplies are shown in the table on the form factor comparison page.

Weight: The weight of the power supply, in pounds (lb) or kilograms (kg). One pound is 0.4536 kg.

Motherboard Connectors: The number and type of connectors used to interface the power supply to the motherboard. Normally the manufacturer will not say if the connectors are of the AT, ATX, SFX or WTX varieties; you will have to deduce this from the form factor specifications, and from looking at the number of pins listed for each connector. In the case of ATX, SFX or WTX styles, the manufacturer should indicate which, if any, of the optional or auxiliary connectors is provided.

Drive Connectors: The number of drive connectors provided as standard equipment with the power supply, as well as how many of them are the larger "D-shape" type, and how many are the smaller "mini-plug" type. Bigger supplies, as well as those of higher quality, provide more connectors.

Fan Characteristics: Characteristics of the power supply fan that you want to know before purchasing the power supply. Some specs on the fan are commonly provided on spec sheets, but many are not: For example, most manufacturers do not say explicitly whether ATX power supplies blow into the case or out of it. You will have to figure this out from the illustration, or ask them. Here are some items you may see listed:
  • Fan Size: Size of the power supply fan, usually given in mm. Fans are normally square, and this is the nominal length of one side of the fan. (Sometimes the thickness of the fan is also specified, but usually not.)
  • Fan Bearing Type: Whether the fan's motor uses sleeve or ball bearings. See the discussion of the power supply fan to understand why this matters.
  • Voltage: The voltage used to power the fan. If not specified, the default is +12 V.
  • Capacity: How much air the fan can move, usually specified in CFM (cubic feet per minute). Larger numbers are better and mean the fan has more cooling power.

Environmental Specifications

Environmental specifications refer to the conditions that the power supply requires in order to function properly (or in some case, conditions that must be met for the power supply even while it is in storage.)

Operating Temperature Range: The minimum and maximum acceptable ambient temperature range for the power supply while it is operating. (Ambient temperature means that of the room in which the power supply is functioning, not the temperature inside the power supply itself.) A typical range might be 0º C to 50º C (32º F to 122º F). Operating the power supply outside this temperature range could potentially lead to damage.

Storage Temperature Range: The minimum and maximum acceptable temperature range for the unit when in storage. If it is specified, this range will typically be wider on one end or the other than the operating temperature range. Don't assume that if this is not listed, that the manufacturer doesn't care about its storage temperature. Ask, and if you cannot get the answer, the safest thing is to use the operating temperature range for the storage temperature.

Warning: When storing components in very cold temperatures, they should be acclimated before being put into service to avoid damage due to condensation from the sudden temperature change.

Humidity Range: Acceptable humidity range for operation of the power supply. A typical specification might be "10% to 90% RH" where the abbreviation stands for "relative humidity". Excessive humidity destroys computer equipment.

Altitude Range: Some manufacturers specify a range of acceptable altitudes for use of the equipment. This is rarely an issue unless you plan to drag your tower PC onto an airplane or climb Mt. Everest with it on your back or something.


Input Voltage Requirements and Tolerances

Input specifications refer to what the power supply requires for its electrical power input--in other words, what it wants to see coming from the wall, or from your UPS. Most electrical input specifications will be provided as a range, because while the power supply may require 115 V as input it of course doesn't need exactly 115 V. The range of acceptable values on a specification are sometimes called the specification's tolerances.

Input Voltage Range: Acceptable range of input voltages. Since most power supplies can function on nominal 115 V or 230 V electricity, you will usually see two sets of numbers. For example: "85 to 135 V AC" and "170 to 270 V AC". The input range is not usually all that critical in determining the suitability of a power supply, because most utility power stays fairly close to the nominal level under normal circumstances. However, the minimum voltage level can have some impact on how well the power supply rides through brownouts.

Voltage Selection: If the power supply supports both 115 V and 230 V nominal voltage, does it automatically select between them, or is there a manual switch?

Frequency: Acceptable frequency of input power (50 Hz, 60 Hz, or 50 and 60 Hz). Alternately, a range of acceptable frequencies (for example, 48-62 Hz). Most power supplies can handle both nominal 50 Hz and 60 Hz input.

Power Factor: The power factor that the power supply presents as a load to the utility power line. Normal power supplies will be in the 60% to 70% range (0.6 to 0.7). Power-factor-corrected supplies will have a number like "0.99". Sometimes, the spec will just say "power factor corrected".


Output Specifications

The most important specifications that you will find listed for most power supplies are probably those that relate to its output signals. The reason for this is pretty obvious: providing the output voltages are what the power supply exists to do. Carefully check over all the output specifications for any power supply that you are considering using.

Some manufacturers will list separately the values for each of the specifications shown below. Other manufacturers may provide a table that shows all of the relevant output statistics (and sometimes, some of the electrical characteristics of each voltage level at the same time). There's really no difference, other than how the information is presented.

Output Rating (Watts): The nominal, total, maximum output of the power supply in watts. This is actually sometimes not even supplied in the specification sheet; the name of the power supply will usually have a number in it that is supposed to represent this value, and sometimes even does. :^) See the page on output ratings for more details.

Output Current Ratings (Maximum Load By Voltage): The maximum amount of current provided by the power supply at each voltage level. See the page on output ratings for more.

Minimum Current Ratings (Minimum Load Requirement By Voltage): The minimum amount of current that must be drawn by loads within the PC, for each voltage level it provides, in order for it to function properly. See the discussion of output power loading for more.

+3.3 V / +5 V Combination Maximum: The maximum amount of total power, in watts, that the power supply can provide simultaneously for the combination of the +3.3 V and +5 V signals. This is an upper limit that constrains any maximum load levels for either of the +3.3 V or +5 V signals individually. See the page on output ratings for more details on how this parameter works. Note that this limit may be in the "fine print" at the bottom of the output current rating specifications; make sure you find it. Note that this is only of relevance for supplies that provide +3.3 V power.

Peak Output: The amount of current that the voltage specified can supply for a limited amount of time. Usually this is specified only for the +12 V signal; see here. Ideally, the manufacturer will specify not only the peak output current but the amount of time the supply is rated to sustain that peak. For example, the continuous maximum for +12 V may be 10 A, the peak level 14 A, and the peak level may be sustainable for 10 seconds.

Output Voltage Range: For each output voltage, the range that the power supply guarantees its output to be within. Power supplies can't say that they will produce, for example, exactly +5.000 V. There's a range, and that's not a problem since systems are designed with this in mind. Generally speaking, the smaller the range the better, although I've never seen small differences here make a practical difference in using a supply. Either specific numbers will be provide (e.g., +4.8 V to +5.2 V) or a "+/-" percentage will be given (which would be +/- 4% to result in a range of +4.8 to +5.2 on a +5 voltage.) Also see the description of load regulation in the section on electrical characteristics.

Efficiency: What percentage of the total energy supplied to the power supply is converted to usable form by the power supply and conveyed to the PC's components. Typical numbers for PC power supplies are 60% to 85%; the other 15% to 40% is wasted as heat. Clearly, the more efficient the power supply, the better! Not only will you save electricity, you will ensure that the power supply runs cooler at the same time, making the supply's components last longer and the system work better overall. At the same time, don't give too much credence to this parameter, especially if you are comparing two units that have similar numbers (and many do.) 71% efficiency vs. 73% efficiency doesn't really translate to much difference in the real world. Efficiency is probably more important for supplies that provide a lot of power, since the percentages equate to larger numbers.

Power Good Delay: The typical time from when the power is applied to the supply, until the Power Good signal is asserted. The manufacturer may also specify a minimum and maximum time.

Auto Restart: If the power supply supports automatically restarting the system after an AC power failure, this will be mentioned in the specifications.


Electrical Characteristics

The electrical characteristics of the power supply describe the quality of the power supply's outputs, and its ability to handle special situations such as disruptions or disturbances to its input power, or variations in the loads the power supply drives. While important to know, and definitely indicative of the power supply's quality level, I must concede that in most cases the average power supply purchaser doesn't need to sweat most of these details all that much. You should ensure that the power supply's figures for these characteristics are not wildly off from those of other, similar supplies. For the newer form factors, you should also check the power supply's specs against the requirements listed in the form factor specification document. Beyond that, however, don't use small differences in these numbers between "power supply A" and "power supply B" to draw excessively grand conclusions.

Hold (or Hold-up) Time: Probably the most important electrical characteristic, this is the amount of time the power supply will keep producing its output, if it loses its input. A typical figure is about 20 milliseconds (the energy-storing components within the power supply are what allow this number to exceed zero.) This value indicates the length of a blackout that the power supply may be able to tolerate before dropping the Power Good signal. It is also important to compare against the switch time of a UPS you are considering for use with your PC. The hold time should be considerably greater than the switch time to reduce the chances of problems.

Load Regulation: Sometimes called voltage load regulation. This specification refers to the ability of the power supply to control the output voltage level as the load on the power supply increases or decreases. The voltage of a DC power source tends to decrease as its load increases, and vice-versa. Better power supplies do a better job of smoothing out these variations. Load regulation is usually expressed as a "+/-" percentage value for each of the voltages the power supply delivers. 3% to 5% are typical; 1% is quite good. (The -5 V and -12 V signals usually are no better than +/- 5% even on very good units; there's no point bothering getting them better than that since they are low-current and mostly unused anyway.)

Line Regulation: The complement of load regulation, this parameter describes the ability of the power supply to control its output levels as the level of the AC input voltage varies from its minimum acceptable level to its maximum acceptable level. Again, a value for each output level is usually specified as a "+/-" percentage. +/- 1% to 2% is typical.

Ripple: Also sometimes called "AC Ripple" or "Periodic and Random Deviation (PARD)" or simply "Noise". The power supply of course produces DC outputs from AC input. However, the output isn't "pure" DC. There will be some AC components in each signal, some of which are conveyed through from the input signal, and some of which are picked up from the components in the power supply. Typically these values are very small, and most power supplies will keep them within the specification for the power supply form factor. Ripple values are usually given in terms of millivolts, peak-to-peak (mVp-p). "Peak-to peak" refers to measuring the AC voltage from its negative maximum to its positive maximum (see here for an illustration of what this means.) Lower numbers are better.

Transient Response: As shown in the diagram here, a switching power supply uses a closed feedback loop to allow measurements of the output of the supply to control the way the supply is operating. This is analogous to how a thermometer and thermostat work together to control the temperature of a house. As mentioned in the description of load regulation above, the output voltage of a signal varies as the load on it varies. In particular, when the load is drastically changed--either increased or decreased a great deal, suddenly--the voltage level may shift drastically. Such a sudden change is called a transient. If one of the voltages is under heavy load from several demanding components and suddenly all but one stops drawing current, the voltage to the remaining current may temporarily surge. This is called a voltage overshoot.

Transient response measures how quickly and effectively the power supply can adjust to these sudden changes. Here's an actual transient response specification that we can work together to decode: "+5V,+12V outputs return to within 5% in less than 1ms for 20% load change." What this means is the following: "for either the +5 V or +12 V outputs, if the output is at a certain level (call it V1) and the current load on that signal either increases or decreases by up to 20%, the voltage on that output will return to a value within 5% of V1 within 1 millisecond". Obviously, faster responses closer to the original voltage are best.

Peak Inrush Current / Input Surge Current: The absolute maximum amount of current that the power supply will draw in the moment after it is initially turned on. This is sometimes used to indicate how much "shock" the power supply is subjected to when it is turned on. Lower values are better.

Overvoltage Protection: In addition to specifying a normal maximum voltage level, good power supplies will include protection against the output voltage exceeding a certain critical level. If for some reason the voltage of the +3.3 V, +5 V or +12 V lines goes above a certain value, the power supply will shut down that output. The number is usually expressed as a "trip point voltage" (for example, +6.25 V for the +5 V line) or a percentage (which for a trip voltage of +6.25 V would be 125%). The specification will also say what the power supply does when an overvoltage is detected; usually, it will reset.

Overcurrent Protection: If the power supply's outputs exceed their maximum ratings, some power supplies will detect this condition and reset the unit. The supply will specify what percentage over the maximum rating for each voltage output will cause this to occur.


General Quality Specifications

There are some power supply specifications that don't pertain specifically to its operation, but are just indicative of its overall quality. Be sure to pay attention to these, as they can give you a good indication of the power supply's general quality level.

Noise Level: Normally rated in dB (decibels). The larger this number, the more noise the power supply puts out. Early PCs had only two components that were constantly moving, and therefore generating noise: the hard disk drive motor, and the power supply fan. In contrast, today's PC are a veritable cacophony: higher-speed disk drives (multiple in many systems), removable drives, power supply fans, case cooling fans, CPU fans. As a result, users have begun to notice that their PCs are loud, and many have started to take the effort to buy or build systems that will cut down on the racket. While most power supplies of the same form factor and similar output will produce similar amounts of noise, some are better than others. In particular, look for power supplies with "low noise" or "silencer" specifications. The component of the power supply that primarily influences its noise level is, of course, the power supply fan.

MTBF / MTTF: Mean time between failure / mean time to failure (essentially, these are the same thing; not exactly, but close enough). These figures are an estimate of the number of hours that this type of power supply, statistically, will run before failing. Typical values for power supplies are 30,000 to 50,000 and up. It's important to realize that these numbers are both estimates and averages--they are not a guarantee for each power supply unit. Also, these numbers reflect average failure rates within the normal service life of the power supply--a 100,000 MTBF figure does not necessarily mean the manufacturer is saying it expects the power supply to run continuously for 11.4 years, even on average. See the discussion of MTBF in the hard disk area for details.

Warranty: The term, in months or years, during which the manufacturer will agree to repair or replace the power supply if it fails. While to some extent the warranty is a marketing tool, it is still the best indication of how good the manufacturer thinks its own products are--no company will put a three-year warranty on a product it believes will fail in 18-24 months. Obviously, look for the longest warranty period you can find, but bear in mind that not all warranties are created equal. Check the terms and conditions of the warranty, and also check out the reputation of the company, especially with regard to warranty service.



Virtually all power supplies have their safety and quality certified by one or more agencies. These certifications indicate that the power supply has been tested and passed a certain standard. The more certifications a power supply possesses, the more it has been tested and the more standards it meets. Different certification bodies will focus on different types of testing. Most testing of relevance to power supplies relates to safety and general quality. Other tests deal with assessing how much electromagnetic interference (EMI) or radio frequency interference (RFI) the power supply generates.

Safety and quality certification is probably the most important thing to look for. There are many different companies that do certification in different countries. Normally, the power supply manufacturer will just list the acronym of each organization that has approved the device, or the acronym of the organization's approval "mark". Here are the most common ones you will see:
  • UL: Underwriters Laboratories, Inc. UL approval is "the" standard for safety and quality certification in the U.S.A.
  • CSA: CSA International (formerly the Canadian Standards Association). The Canadian equivalent of the UL.
  • NEMKO, TUV and VDE: NEMKO, in Norway, and TUV and VDE in Germany, are the organizations that do most of the electrical component certifications within Europe.
  • CE: Indicates that the product has been given the "CE mark", certifying that it meets the standards required to allow it to be sold within the European Community.
EMI/RFI compliance rules are established in the United States by the Federal Communications Commission (FCC). You will see many companies advertise that their power supplies are "FCC Class B certified". This is really not quite accurate, because the FCC does not certify individual power supplies, only systems. So what the manufacturer is claiming, at best, is that their power supply was certified as part of at least one type of system. This is still good to know of course. In practice, reputable power supply manufacturers will test their units with a wide variety of configurations.

Finally, some power supplies are Energy Star certified. This is an EPA program established to help promote energy-efficient PCs and components. The program is voluntary, but energy star certification is seen by many as reflective of the quality of the power supply, as well as environmental conscientiousness on the part of the manufacturer. Modern form factor specifications will detail the requirements of the form factor for the power supply to achieve energy star compliance.

The lack of certification of a device does not mean necessarily that it is a bad product. It does, however, mean that the product hasn't been thoroughly tested to meet the regular standards in the industry. There could be many different reasons why this is the case, but in my opinion, it's not really worth speculating over--I would simply avoid supplies that are not listed by at least one, and preferably several, of the better-known safety and quality certification bureaus.



[H]F Junkie
Sep 9, 2003
VIII. Confusing Performance Data On Packaging And Case

In the PC store, even reading the label tends to confuse the user, because there is usually lots of conflicting performance data.

The first information, about maximum power (420 watts), is just a reference by the manufacturer to the watt level for the purpose of advertising the power supply unit. It is not important for practical purposes.

Next to this, the manufacturers usually also include the maximum combined power of the +3.3- and +5-volt lines (220 watts) and/or the maximum combined power of +3.3-, +5- and +12-volt lines (400 watts). These numbers indicate the actual power that the +3.3- and +5-volt and the +3.3-, +5- and +12-volt power buses provide in combination. However, since PC components are supplied with power through the "+"-volt lines of the power supply unit, the maximum value for the combined power of the three "+"-volt lines warrants the most attention. That's because this value indicates the maximum power that is really usable for supply the PC components with power.

The "difference" between maximum power (420 watts) and maximum combined power (400 watts) is reserved for standby power and for the -5- and -12-volt lines no longer needed by most systems.

Tip: If the value for the maximum combined power of the +3.3-, +5- and +12-volt power bus is not given, you can calculate a good estimate for this value by subtracting 20 watts from the maximum power.

Not only the power figures, but also the currents of the individual strands are limited: 26 A, 42 A and 18 A are the maximum current of the +3.3-, +5- and +12-volt lines in our example. Good to know: With power supplies in the 300/320-watt class, the maximum permissible currents of the +5V line are about 25%, those of the +12V line average about 15 % less, than a power supply unit in the 400- or 420-watt class.


IX. Increased Efficiency Saves Energy

The efficiency is defined as the ratio of output power to input power, i.e. how much energy the power supply unit takes from the outlet to supply the PC system with the necessary power.

A power supply's efficiency is often underestimated. In the power supplies that we tested, the relationship of input to output varied, from 64% to 78%.


X. Stable Voltages

For steady, error-free operation of the PC it is important that the voltages of the power supply unit are as stable as possible regardless of load, i.e. within the specified minimum or maximum range. Because over- or under-voltage can lead to unstable operation or even damage to the components. Processors in particular react quite sensitively to instabilities in voltage.

Therefore, for the voltage variation range, narrow limits of +/-5% of the nominal values (3.3 V, 5 V, 12 V) were set in the ATX Standard.

When the minimum or maximum voltages of the power supply unit were exceeded or not reached, for example because individual power buses were overloaded, the power supply unit should shut down in accordance with the ATX Standard.


XI. Quiet Operation Thanks To Regulated Fan

The high power of the power supplies goes hand in hand with enormously high heat levels. Up to 200 watts and more must be expelled in turn as heat. To ensure adequate cooling of the components of the power supply units, they have one or two fans. Many manufacturers even integrate three fans.

As with CPU coolers, the fans are an irritating source of noise that make concentrated work at the PC difficult. To avoid operating noise, all manufacturers equip their devices with automatic fan control. This automatically regulates the rpm of the fan according to the temperature in the power supply unit. When the demand on the power supply unit is low, the fan accordingly revolves more slowly and is considerably quieter.


XII. Overloading Cables

The main thing to look for when buying a power supply unit is high power and low operating noise. But a look at the equipment is important, too. Crucial features are not only the number of connectors for 5.25- and 3.5-inch devices, but also the length and number of strands in the cable.

The number of HDD and FDD connectors and their distribution on the cable strands must also be considered. At high power levels it is important to distribute the load among several strands, because, if there are too few strands, when the load is high, the cables could overload individual cable strands. Six HDD connectors should be considered the minimum. In a system with two hard disks, a CD-RW drive and a DVD drive, this would almost exhaust resources.


XIII. Common PSU Questions

How many "Watts" do i need?


"Please Note: The Wattages listed below are maximum potential wattages for each item. The total amount this calculator figures is for all devices running at peak utilization. It is important to bear in mind that this amount will never be reached under typical operation. However we feel that this tool will give you a better idea of how much power your system will need. After all when it comes to power, it is better to be safe than sorry."

A wattage calculator will give you a pretty good idea of the maximum potential draw a individual device. The only problem with that is these draws are nearly impossible to reach under normal PC usage.

The wattage scale that i go by and recommend is:

250-300w = Low-End System
300-350w = Mid-Range System
400-480w = High-End System

Anything over 400-480w is really TOO MUCH overkill. Larger is not always better. A larger PSU wont be as efficient if its not under enough load and it normally has fans with a higher CFM rating but also higher db rating which makes for a louder PSU. If nothing else your wasting your money when it could be put to use in other areas of the system.

What are “Rails?”

People talk about that their “rails” are low and that they need a new power supply. First off, the “rails” that they are referring to are the 3.3V, 5V, and 12V lines that come off the power supply. The 3.3V and 5V generally power the digital devices like the motherboard, PCI slots, AGP slot, etc. The 12V is usually used for things that are motorized like the hard drives, CD-ROM, fans, etc. These lines are rated to run at a certain specification. It is usually somewhere around plus or minus 5%. Technically, if these lines stay within this margin, the system should run fine with no instabilities but if they do run out of specification a number of problems can arise. It is rare that somebody posts a problem with their power supply going over 5% above what the line is supposed to be. Usually power supplies go under the minimum voltage when used over a long period of time. When the rails are low this usually leads to blue screens and various other instabilities like random reboots. When these lines are overvolted problems arise again like drive failure, blue screens, damaged drives and damaged motherboards. Therefore, it is easy to see why it is important to have a good power supply.

How do i test my "Rails?"

Most people complain about their power supplies not giving enough voltage to the lines. Many use hardware monitors like the ones available in the BIOS, Motherboard Monitor, Winbond monitor, etc. These are all software based and can be inaccurate. They are approximate reading from the motherboards chips. So just because these software reading say lines are low does not mean it is the power supplies fault. There are many other factors included. Most of the main factors all occur within the motherboard and not the power supply. The voltages given from the power supply have to go through the motherboard before the software reading is taken. Therefore, there are different resistances on the motherboard that the power has to pass through. For instance, the power connectors on the motherboard could be damaged or not making good contact. Other factors are resistances along the way to the motherboards voltage regulators. Also over time, especially when overclocking, the circuitry will degrade on the motherboard. All of these things can play a roll on how much voltage is lost on the way to the software reading. Therefore, the only true way to test if the power supply is doing its job is to take the voltage readings direct off the power supply and not the motherboard or the software readings. We find this way to be much more precise and accurate.

To properly test the PSU the voltage readings need to be taken directly off the motherboards 20-pin ATX connector using a good digital multimeter. To do so, first the power supply needs to be plugged into the rest of the system, as it normally would be. This will help to make the reading more precise in a real world environment. After the system is hooked up and powered on, get the digital multimeter out. Place the red probe on the corresponding wire on the back of the 20-pin ATX connector. Red is the 5v rail. Yellow is the 12V line. Orange is the 3.3V line. For a ground, use any of the black wires located on the 20-pin connector. Technically speaking you could take these reading off the auxiliary 6-pin connector or a 4 pin Molex but since the 20 pin is the one that goes directly to the motherboard it will be the one that needs to be tested the most. We have tested using different connectors and the readings did come out correctly but there could be different resistances in each line so it is generally better to go ahead and just test off 20 pin.

Test the idle and the load voltages. Idle is supposed to be while the computer is not running any programs. Do not just check to see if the rails are where they are supposed to be, also check for the fluctuation in the rails. Repeat the same process while the computer is under full load. Full load can be simulated by running a benchmark or something very intensive on the PC. Again, make sure to check the voltage and the fluctuations in the line. If the voltage reading go up and down a lot at either idle or load, then the power is not supplying “clean” power. It is “spiky” if you were to look at a graph. Fluctuations like this are no good and can cause problems. We find that most of the time, low rails are often cause by the motherboard and not the power supply after testing the PSU directly. Most people do not check direct from the power supply so they would go out and buy a new PSU thinking it will solve the problem and it does not. So what now? Well this will have to be a whole other article but the user could buy a new motherboard, or find out where the resistance is coming from. It could be dirty connectors, or just degraded circuitry. Some power supplies come with potentiometers however. This gives us the ability to change the voltages that come from the power supply to compensate for the low rails. The only problem with this is that other devices besides the motherboard get over volted. Over time this could damage these other devices and if too much voltage is used, then the devices could damage in a very short amount of time.


XIV. Recommended PSU Manufacturers



Fortron Source

PC Power and Cooling





Oct 16, 2003
I take it this sticky came about because of all the PSU queries posted on the forums? (Not to mention the person who zapped himself touching a capacitor in the PSU.....) Very good post for noobs to read about PSU's before doing anything themselves. Not to mention what could almost be called a flame war because of my views on generic PSU's.... no hard feelings :)

Vertigo Acid

May 31, 2003
I've got some BTX/ATX2.3 stuff imma PM over to ya, later, lol I knew you'd give up and write one of these some time... lol I got lazy and busy w/ school and never got a chance!


Mar 5, 2004
a mod should delete all of the posts and lock this

or just delete all of the top posts to make it just continue


Oct 16, 2003
Originally posted by thebro
a mod should delete all of the posts and lock this

or just delete all of the top posts to make it just continue

Thanks for the positive feedback on our posts...


[H]F Junkie
Sep 9, 2003
There are new PSU articles coming out all the time and i usually save them so this will sort of be an on-going FAQ when i have time to update it. If any of you have something to contribute just fix it up yourself in your post and dont worry where its at in the thread. I'm going to make a larger index right now so you can just use your "find in this page" dealie to go right to the information you want to read more about.

I could use some more info for the Commonly Asked Questions area so if any of you know of some good ones that have articles to back up the info just go ahead and post it in here somewhere.

It would be a waste of time to lock it because then i'd have to have someone unlock it so i could add more stuff lol.

Something else it thought i might do is make a PSU review section. It will just be a database list of all the reviews i've got for all different PSU's. If you've got any you want added to the list just PM or e-mail them to me and be sure and have them in some form of order stating which brand and model or whatever it is so i dont have to go to each link to verify.


Oct 16, 2003
Here is a good question (i don't know if you answered it before) that a noob may ask: What is a good PSU brand/model?

That one could light up the forums.


More Human than Human
May 26, 2000
Originally posted by thebro
may be you should be a mod then.;)
...settle down, guys.

Perhaps when burningrave101 gets the feedback he wants from his early edits, he might choose to cut / paste the FAQ into a new thread, in sequence, which some mod might choose to sticky, cutting this one loose. Maybe. If he wants to.

No offense, but you can of course do the same thing; copy and paste the text into one long text document. I think I will; it makes it easy to search the whole document for key words, etc.

/end < threadcrap >

Thanks - B.B.S.


Mar 25, 2004
Very informative, There may be a bit too much technical info for some readers, but I like it.

On a side note, If anyone is looking for a powersupply, one word:


The best PSU money can buy. Hands down.

Vertigo Acid

May 31, 2003
Manufacturers links:
Sparkle (SPI) http://www.sparklepower.com/
Fortron Source http://www.fsusa.com/
Enermax http://www.enermax.com.tw (http://www.maxpoint.com)
Channel Well http://www.cwt.com.tw/
Astec http://www.astecpower.com
Enlight http://www.enlightcorp.com/
Antec http://www.antec-inc.com/
TTGI http://www.ttgitech.com/
Superflower http://www.super-flower.com.tw/
PC Power and Cooling (PCP&C) http://www.pcpowerandcooling.com
Vantec http://www.vantecusa.com
Allied http://www.alliedtelesyn.com/
A+ GPB http://www.gpb-jge.com/
Ahanix http://www.ahanix.com/
Aspire http://www.aspireusa.net/
Codegen http://www.codegengroup.com/
Dynapower http://www.dynapower.com/
Linkworld http://www.linkworld8.com
Kingwin http://www.kingwin.com/
I-Star http://www.istarusa.com/
Foxconn http://www.foxconn.com/
Shuttle http://us.shuttle.com/
Silverstone http://www.silverstonetek.com/
Soyo http://www.soyousa.com/
Thermaltake http://www.thermaltake.com/
Works http://www.workspower.com/
Zalman http://www.zalmanusa.com/
Delta http://www.deltaww.com/
Deer http://www.deer-group.com/
Hercules http://www.hercules.com/
Inwin http://www.in-win.com/
ETT http://www.ettshop.com/
LCT http://www.lct-tech.com/
Skyhawk http://www.skyhawkusa.com/
Powmax http://www.powmax.com/
Power Magic http://www.powermagic.com/
NMB http://www.nmbtech.com/
Meridian http://www.meridiancase.com/
Super Micro http://www.supermicro.com/

Part Numbers for various connectors, Molex if not otherwise noted
First number is Connector, second is the other half

New 24-pin ATX Connector 44206-0007 and 39-01-2240
New 2x3 Auxish type connector 9-30-1060 and 39-01-2060
20-pin ATX Connector 39-01-2200 and 39-29-9202
4-pin P4 Connector 39-01-2040 and 39-29-9042
6-pin AUX 90331-0010 (no info on the mobo connector)
"Regular Molex" Drive Connectors AMP 1-480424-0, Molex 8981-04P and AMP 61314-1
SATA Connectors 675820000 and 675810000
Floppy Connector AMP 171822-4
SATA Crimp-on Connectors

Vertigo Acid

May 31, 2003
Common Connectors









Vertigo Acid

May 31, 2003
What is ATX GES?

ATX GES is a power supply form factor. This form factor was originally developed for AMD’s Athlon MP dual processor motherboards. This form factor was required due to the increased power demands of the Athlon MP processor.

Who uses it?

The majority of the power supplies today utilizes the EPS12V form factor, which is similar to the original ATX12V form factor that everybody utilized. The ATX GES form factor is used primarily by AMD Athlon MP dual processor motherboards. Please check with your motherboard manual to see if you require an ATX GES form factor power supply.

Tyan Thunder K7X (S2468) and Tyan Thunder K7 (S2462) motherboards utilize ATX GES.

What will happen if I use an EPS12V power supply in a motherboard that requires ATX GES or vice versa?

1) Motherboard will not power up

2) The connection may cause a short circuit and burn out your motherboard and/or power supply

How do we tell if our power supply is ATX GES compliant?

First check the power supply label to see if it specifies whether the power supply is ATX, EPS12V, or ATX GES. If the power supply label does not specify which form factor it follows, look for the baseboard connector. The baseboard connector is usually the biggest connector leading from the power supply and is usually labeled P1.

The regular ATX form factor is easy to identify because the baseboard connector has only twenty pins. However, both the ATX GES and EPS12V have twenty-four pin connectors.


Dell Proprietary PSUs

After September of 1998 Dell defected from the cause of industry standardization and began using specially modified Intel supplied ATX motherboards with custom wired power connectors. They also had custom power supplies made that duplicated the non-standard pinout of the motherboard power connectors. Recently they have moved back in line, and are using true ATX PSUs and mobos on their new offerings.

Rather than simply using non-standard power connectors, only the pinout is non-standard, the connectors look like and are keyed the same as is dictated by true ATX. There is nothing to prevent you from plugging the Dell non-standard power supply into a new industry standard ATX motherboard you installed in your Dell case as an upgrade, or even plugging a new upgraded industry standard ATX power supply into your existing Dell motherboard. But mixing either a new ATX board with the Dell supply or a new ATX supply with the existing Dell board is a recipe for silicon toast.

Not only are the voltage and signal positions changed, but the number of terminals carrying specific voltages and grounds has changed as well. It would be possible to modify a Dell supply to work with a standard ATX board, or to modify a standard ATX supply to work with a Dell board, but you'd have to do some cutting and splicing in addition to swapping some terminals around.

If you need a new PSU for a dell that uses a proprietary supply, you must contact dell.
Update: It appears some of my pics have died... and I don't have copies so if anyone does from perhaps saving this thread please e-mail them to me, malcolm@vibrantlogic.com Otherwise I gotta go find em again


Apr 23, 2004
Originally posted by burningrave101
Something else it thought i might do is make a PSU review section. .

I like that idea..I'd certainly put my vote in for the Vantec 420A (which Tom didn't test,he tested the lesser ION) We've been using the Vantec at boeing's data center for several years without a single failure although we do replace them when they get close to the 150,000 hour mark..even then they have alot of life left according to the employees who line up to get'm. :)


[H]F Junkie
Sep 9, 2003
Originally posted by justin_credible
I like that idea..I'd certainly put my vote in for the Vantec 420A (which Tom didn't test,he tested the lesser ION) We've been using the Vantec at boeing's data center for several years without a single failure although we do replace them when they get close to the 150,000 hour mark..even then they have alot of life left according to the employees who line up to get'm. :)

No, i didn't mean i was going to actually review PSU's myself lol. I meant i thought about making a small database of the popular PSU's that people usually buy and post several reviews for each one so that people can find a review for the PSU their thinking to buy more quickly.

Vertigo Acid

May 31, 2003
CPU Power Usage
This is the most complete guide to power draw ratings of processors in existance!

Its got everything from i386SX-16 to A64 3700+
Even Cyrix and IDT!

Vertigo Acid

May 31, 2003
How to make an ATX Spliter to run two mobos off one connector
http://takaman.jp/D/index.html?englishTHE* definitve PSU calculator
This one is basically is a spreadsheet that does the amp * volts math for you, and has some pre-programed values for common devices but is totally customizable

Data Loss Figures:

"How important is the quality of power you supply to your PC? This graph, which
shows the leading causes of data loss by category, gives you a pretty good idea."

Another good treasure trove of PSU failure info *snipped from "the imporance of a good PSU", which snipped it from "Corruption 101"*

One, if not the best PSU reviews on the planet Well, in English that is. Mike over there is equiped with all the tools necessary to do a good PSU review, including the DBS-2100 PSU load tester that basically no one else has or can find.


Limp Gawd
Feb 16, 2005
burningrave101 said:
Something else it thought i might do is make a PSU review section. It will just be a database list of all the reviews i've got for all different PSU's.


Did you ever get around to compiling this information? I'm shopping for a PSU and such a resource would be of immense help.

Mar 1, 2005
WoW, I'm impressed, which is not easy to do.

This was very well researched & written,
however the info is so out dated that anyone following the advice given would end up with real problems.



[H] Admin
Staff member
Aug 29, 2004
davidhammock200 said:
WoW, I'm impressed, which is not easy to do.

This was very well researched & written,
however the info is so out dated that anyone following the advice given would end up with real problems.


This is in the power FAQ's and tutorials that we have been saying need some updating.....but you might want ot be more specific so it doesn't just like a backahnded compliment.

And there is some good nostalga there. I remember blowing up my first AT PSU so fondly :eek: :eek: ;)

Vertigo Acid

May 31, 2003
Yeah, the section on "what uses what voltage" is definitly out of date.
I remember talking to Burninggrave101 about doing way back before there was even a PSU forum.... I need to get copies of those pictures again and rehost them and add some updated connector info :p
me said:
New aux-ish style 2x3 pin connector
Do I mean a PCI-Express video connector?
I rather think those barely existed when this was written
Mar 1, 2005
Spectre said:
This is in the power FAQ's and tutorials that we have been saying need some updating.....but you might want ot be more specific so it doesn't just like a backahnded compliment.

And there is some good nostalga there. I remember blowing up my first AT PSU so fondly :eek: :eek: ;)
No "backhanded compliment" intended. If this was early 2003, then this would be fine.

It is very well written & very well researched. A excellent job!

Now to update it would require an explination of current mobos that have a p4 connector & use the +12V to power the CPU.
Also a section on today's high +12V demand video cards & their amperage requirements.
Finaly recommending that PSU's for today's computers
have the nVidia recommended +12V@26A for non-SLI & +12V@34A for SLI.