HOW TO BUY A PSU (or how to determine if your old one is up to it)

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Ice Czar

Jul 8, 2001

This is not as hard as all this copy makes it appear, just read it
there are a few special case instructions, that more than likely simply dont apply to you ;)

Here is your basic tool
takaman's Power Supply Calculator

or if takaman is down use ZIppy's claculator It is an exact clone

this PSU calculator will break out the amps per rail
in other words how much current each of the 3 main voltage rails needs to power a given configuration,
these change quite a bit, and Watts alone mean next to nothing these days
especially if there are amps on under employed rails something that is quite common

*The eXtreme Outervision Calculator is NOT recommended or supported due to only providing wattage estimates.

Figure the Theoretical Maximum

1. Set the Utilization
at the top set the Utilization to 90% and apply 100% utilization to drives and fans

2. Select the Basic Power Scheme
select the basic mobo, this is very important as an AMD board without a +12V mobo connector (little 4 pin 2x2 connector or an inline 6 pin) or a P3 mobo will be powering the CPU off the +5V rail, whereas a modern board will be powering the CPU off the +12V rail
(P4 mobos and AMD boards with an Auxillary +12V mobo connector)

3. Select additional components
make as close a match as possible, if your parts are not listed you can add the amp values manually from below or from the specs
this is especially useful in the case of Processors not listed or AGP Cards
Here are some realworld figures:

Power Consumption of Contemporary Graphics Accelerators: ATi
Power Consumption of Contemporary Graphics Accelerators: Nvidia

Power Consumption 7800gt
Power Consumption 6800GS

Power Consumption Radeon X1800XT, and X1900XTX

Power Consumption GeForce 7800 GTX, GTX 512

Breakdown of where Nvidia 7xxx series and ATI x1xxxx series draw power from

click on the charts for a given card for instance the ATi X800XT
here you have both a stock speed and an overclocked set of values
employ either but pull the values from the load current column, and you need to add both the power supplied from the AGP slot and the auxillary connector
in this case
+3.3V (AGP only) 2.71A
+5V Aux 3.22A + AGP +5V 0.11A = 3.33A
+12V Aux 3.11 + AGP 0.14A = 3.25A
now leave this window up as we will be using it later
enter the values in the PSU calculator you will have to round to the nearest tenth
so if your overclocking round up, probably a good idea in any event
then double check the combined watts from the calculator to the load summary power column of the chart, in this case
63.8 Watts in the PSU calculator to 63.23 Watts on the chart
(double check like this where possible)

The Processors are pretty up to date, however if you have something older, or a CPU not listed, refer to > Processor Electrical Specifications
you figure and then add the amps based on which rail would be powering the CPU
as mentioned above, that is going to be either the +5V rail on older boards or the +12V rail on newer ones
you will be working from either the Maximum Power Distribution values in Watts,
Total Thermal Power values in Watts, or the
Thermal Design Power values in Watts

as an example we take an AMD Athlon64 3400+
which has a Thermal Design Power of 89 Watts
since AMPS = Watts / Voltage
so 89 Watts divided by 12 Volts equals 7.41Amps
and double checking back to the calculator (in this case)
we have 7.4Amps and 88.8 Watts

Hard Drives as provided by DougLite:

DougLite said:
I will identify +12 figures in yellow and +5 figures in red as that is their respective color on PSU wires. You are free to look at the detailed data I have dug up, or you could just read this:

You will want 3A for spinup as a conservative estimate. Most drives will come in under 2A, but some specify 3A. I am merely supplying data, I will leave the decision on the final startup draw figure that makes it into the calculator to Johnny, Ice, and Spectre. It is unlikely that the relatively short lived spinup current draw will seriously stress quality modern PSUs with beefy +12 output, but I am no expert. For normal operation, you will want to have 1A and 1A (one amp on both rails). Also note that Maxtor drive consume significantly more power than any of their competitors that are detailed here, although they stay within the summary envelopes outlined above. For a good estimate of Maxtor 7200RPM ATA drive power consumption figures, I would raise the +12 figures for WD2500KS by 50%. I will now supply data to backup this summary.

Seagate Barracuda 7200.8 Seagate specifies a peak spinup current draw of 2.8A. Seek power is quoted at 12.4W, read write for 12.8W, and idle of 7.2W, once again not broken down by rail. I would estimate the +5 draw to be around 3.5W/.7A in all cases except spinup, with the balance pulled from +12.

Hitachi Deakstar 7K250 1.72A and .068A peak for spinup, Idle consumption totals depend on many platters the drive has - 7.0W for a 3 platter, 5.9W for 2, and 5.0 for 1. These idle figures are for SATA drives, PATA tacks on .6W to all idle figures. Hitachi does not provide read/write or seek power draw figures.

Western Digital WD2500KS
Read/write: .45A/.8A
Idle: .43A/.73A
Standby: .020A/.27A
Sleep: .020A/.25A

Western Digital WD740GD
Read/write: .359A/.706A
Idle: .338A/.770A
Standby: .015A/.314A
Sleep: .016A/.184A

I would put Samsung and Hitachi power consumption figures in the same ballpark as the WD2500KS figures. Seagate will be slightly higher and Maxtor will be signficantly higher. Of particular interest is the very low current draw of WD740GD, despite it being a 10K drive, it actually comes in lower than WD2500KS.

Seagate's Cheetah 10K.7
Spinup: 1.91A/.77A
Idle: .62A/.54A
Read/write: 1.08A/.7A

Seagate Cheetah 15K.4
Spinup: 1.89A/.73A
Idle: .77A/.54A
Read/write: 1.14A/.61A

Fujitsu quotes 3A +12 spinup current for a MAU3147NP and 11.5W idle on the same drive. I'd speculate all but 3-4W of that is on +12. As in ATA, the power consumption of Maxtor SCSI drives will be higher than the data provided for Seagate and Fujitsu but Maxtor doens't tell just how much more.

wherever possible use amp values supplied by your components manufacturer
this is generally well documented in the case of HDDs and fans, less so with many other copmponents, when in doubt look through the options on the calculator and select a worse case load
following this post will be additional tables of odd components as Im able to compile them, such as baybuses, lights, pumps ect.

4.Note Down the Theoretical Maximum
The calculator will supply you with not only the totals for us to refer to but also a url link to look at your figures, post both your full config, the link and PSU Calculator figures, noting that they are the theoretical maximum, also make sure the link works before you close that window
3.3V rail MAX 5.1A or above
+5V rail MAX 9.9A or above
+12V rail MAX 17.1A or above
+3.3V + +5V rails combined 66.7 Watts or above
+3.3V + +5V + +12V rails combined MAX 272.3 or above
Total Output 282.3 Watts or above
a few notes on what those mean
first many but not all supplies have a combined caopacity for the +3.3V & +5V rail
some are truely independent so that may or maynot apply
second the difference between the 3 main rails combined output
and the total is accounted for by the signal lines, the +5V standby power,
and the largely unused negative power rails
in the latest ATX12V v2.01 spec the -5V rail has actually been eliminated,
but the -12V rail is still there

and finally that Watts figures are deceptive,
on the one hand that theoretical maximum will never happen
all the components drawing full power at the same time simply isnt real
we will be addressing that next
and on the other hand, that total watts figure would be for a supply running at the temperature it was rated at,
something your unlikely to be able to replicate, we will also address that below

Figure a Startup Draw
Any mechanical component in your computer that spins requires more power to overcome inertia and spinup, than it does once its spinning, generally the runtime draw is roughly a quarter of the spinup draw

we will be using the figures we develop here for our realworld worse case senerio that follows as the static draw, but this spinup draw can be critical in and of itself if your building a box with alot of storage
DAS (Direct Attached Storage) NAS (Network Attached Storage) and SAN (Storage Attached Network) appliances has typically required very healthy +12V rails, often also employing redundant power supplies or N+1 suuplies (a modual system where ty[pically 2 moduals are capable of powering the computer and a third hotswap, though more moduals are also common)

many storage controllers allow a staggered spinup of an array so as not to incur the full draw at the same time, this is of course a mitigating factor, and can make the difference between having to buy a much more expensive supply if your close to the capacity of a given supply

so open a calculator and notepad
and add the amp draws for

1. CPU Fan(s) if applicable
2. Case Fans
3. HDDs
4. Pumps if applicable

in the PSU calculator amp draws displayed are for a single device
for instance a 80mm 4500rpm fan is listed as .37A on the +12V rail
youd times that by the number of fans, same for the HDDs
once we have a spinup draw, we are going to quarter it for a static draw
2 x 80mm 6800rpm Delta CPU fans @ 0.5A per on the +12V rail = 1A
3 x 120mm 2600rpm midcase rackmount hotswap fans @ 0.52A per = 1.04A
8 x 40GB 7200rpm HDDs @ 2A per = 16A
18.04A on the +12V rail total
and a static draw of 4A

so for a realworld spinup draw we add the above 16Amps
plus 100% utilization of the mobo, say a RAID controller card
25% to 50% utilization for the CPU and GPU and come up with a figure
we also add in the full spinup draw of an optical drive, but dont include it a a static draw

Realworld Worse Case Senerio

OK employing our static Draw above
we add 100% utilization of the CPU, GPU mobo, and an optical drive spinning up
Ive quized the gaming and video card forums, and its pretty evident that 100% usage of the CPU and GPU can occur in a gaming rig, if you have a different type of box, you might consider exactly what might be hitting full draw at the same time, like for instance a few Gigabit NICs, and the worse case CPU log youve seen on a server

on a modern board the +12V rail will be the primary focus
but on an older board all the rails need to looked at carefully
hell thats true in either case :p

Figure the Individaul Draw on each +12V Rail
If you have a modern board, or if you have a server\workstation
its very likely that you will be looking at PSUs with more than one +12V rail
so we need to look at how much draw there is on each of the rails
it breaks out like this for an ATX12V v2.0(1) supply
1. Add the +12V1 up
CPU, mobo, +12V power for PCI cards and +12V power via the AGP slot
2. Add the +12V2 up
everything else, including the Auxillary Power to the Video Card(s)
(this is where you pull up the Video Card Chart we used above)

for an EPS12V supply its even more fun
EPS12V 6.1.1 12V Power Rail Configuration

There are two types of 12V rail configurations for systems: 'Common plane' and "Split plane' processor power delivery. The 'commob plane' system has both processors powered from a single 12V rail (+12V1) from the power supply. The 'split plane' system has both processors powered by seperate 12V rails (+12V1 and +12V2) one dedicated to each processor. The system in both cases, has an additional 12V rail to power the rest of the baseboard +12V loads and dc/dc converters. +12V1, +12V2 and +12V3 should not be connected together on the baseboard to ensure that 240VA protection circuits in the power supply operate properly

Table 6: 12V Rail Summary
Common Plane System........................................................Split Plane System
+12V1........Processors.........................................................+12V1........Processor 1
+12V2........Baseboard components other than processors.......+12V2........Processor 2
+12V3........Drives and peripherals..........................................+12V3........Baseboards and components other than processors
...........................................................................................+12V4........Drives and peripherals

now that you have a more realistic power draw its time to assess PSU
employing the Theoretical Maximum, Spinup Draw, Realworld Worse Case Senerio, and the Individaul Draw on each +12V Rails[/B] baselines we will adjust and weight PSUs ratings to try to match up applicable supplies

Deciphering PSU Ratings
this is wear we pass through the gates of Moria and into the netherworld of PSU marketing :rolleyes:

Rated Amps and Watts
unless specifically stated otherwise, these figures are likely taken at a temperature of 22 to 25C, the problem is that your far more likely to have an internal PSU enclosure temperature of 35 to 50C, especially if its the primary exhaust for the CPU Heatsink and case.

What we dont get to typically see is what is called a derating curve
Clearing The Fog That Surrounds Derating Curves @ powerdesign365 (registration required)

Derating is the negative slope of the power-versus-temperature graph. Specifically, it shows that as the operating ambient temperature increases, the converter's maximum output power drops to ensure reliable system operation. Derating curves provide a quick way to estimate the maximum output power of a converter at a given temperature
or simply given as
0ºC ~25ºC for full rating of load, decrease to zero Watts of power at 70ºC
(they even spin these figures as youll find if you follow the link)

so, as a rough estimate, deduct 1\3rd the rated amps per rail and start to compare that to your baselines, the surviors proceed to the next round
(following this tutorial there are some suggestions regarding lowering the temperature of the supply, both increasing its capacity and its longevity)

AC Input Voltage
another way manufacturers make their supplies look better is to rate them with a narrow range of acceptable AC Voltage, it much harder for the supply to maintain a stable reference voltage on the mail rails if the AC source power is fluctuating
if the VAC drops from 110V to 90V the supply must draw more amps from the socket to compensate, and doing that an maintaing a stable DC voltage isnt easy, thus its a real test of the voltage regulation, by limiting the range in the spec of the power supply they are fudging the figures on you, so its a sign that you might be dealing with a less reputable manufacturer, some examples
PCP&C 510.................... 90 to 264 VAC, 47 to 63Hz
Enermax EG565P............90 to 135V or 180 to 265V, 47 to 63Hz
Neopower........................100 to 240 VAC. 47 to 63Hz
Ultra-X.............................103 to 132V or 206 to 264V, 47 to 63Hz
Raidmax 470...................100 to 120V or 200 to 240V, 50 to 60Hz
Codegen 500...................115 to 230V 50 to 60Hz

to be continued
next Load Regulation, Transient Response and Baseline reference voltages
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