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

Discussion in 'Power Supplies' started by Ice Czar, Dec 14, 2004.

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  1. Ice Czar

    Ice Czar Inscrutable

    Jul 8, 2001
    ALWAYS U N D E R - C O N S T R U C T I O N

    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

    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
    Hard Drives as provided by DougLite:

    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
    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
    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
    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
    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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