Possible Power Issue on Asus P5N-E SLI

Discussion in 'Asus' started by TheNobleDuke, Oct 17, 2008.

  1. TheNobleDuke

    TheNobleDuke Guest

    I recently installed a second video card on my P5N-E SLI and my
    displays stopped functioning once Windows starts (see
    I've done a bunch of testing and I'm beginning to wonder if my PSU has
    enough wattage for my hardware, however trying to calculate this is
    becoming almost as much of a pain as the original problem.

    I've been able to figure out approximate wattages for the dual 8800
    GTS cards and my SATA drives, but I still can't find any information
    on the power draw of the P5N-E SLI. Anyone have any ideas?

    Thanks for your help.
    TheNobleDuke, Oct 17, 2008
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  2. TheNobleDuke

    Paul Guest

    You should be using a 24 pin power supply, for best results. That
    provides two wires for 12V current flow, to power the motherboard
    video slots and the fan headers. The 12V1 rail is used for that.
    Typically, no more than 4.5 amps would be drawn from a video card
    slot, and even some of the high end cards, draw less. It depends
    on how the loading was split. And Xbitlabs has numbers, obtained
    by actual measurement. A 20 pin connector can safely carry 12V1
    at 6 amps, and two video cards makes a 20 pin connector a dicey
    choice. A 24 pin is better suited.


    In the detailed measurement, the slot power component is 36W, which
    amounts to 12V1 flowing at 3A per slot. 2*3A + 0.5A (fans) is
    6.5A on the main power connector, and while a 20 pin would work,
    the single pin for 12V would be getting a bit warm.


    Core2 Duo processor - (65W / 12V) * (1/0.90) = 6 amps on 12V2
    (assumes 90% Vcore efficiency)

    8800GTS = 108W from all sources of 12V1, 108W/12V = 9 amps
    Two cards is 18 amps total from 12V1.

    Four hard drives - 4*2.5A during spinup, for the first ten seconds.
    - 4*0.6A in the desktop

    CD/DVD - 12V1 loaded at 1.5A, but mainly when media is spinning.

    Fans - 12V1 loading of 0.5A, check the fan labels for better accuracy.

    Motherboard - 50W, which rail used is unknown - includes RAM

    Standby power - I allocate 5VSB @ 2A for USB and standby loads, or 10W.

    So a quick estimate would be

    12V2 = 6A (only the CPU is on this one)
    12V1 = 9 + 9 + 2.4 + 1.5 + 0.5 = 22.4A

    Next step, is a total power calculation.

    Total power = 12*(6 + 22.4) + 50 + 10 + 4*5 (HDD controller) + 1*7.5 (CD controller)
    = 428.3W

    As long as the 3.3V and 5V ratings are for 20A or more, the lower
    rails are probably OK.

    If you'd mentioned the supply make and model, I could have worked out the
    details and checked it. If the supply was a "650W" you bought for $20,
    then no, it ain't gonna work (cheap supplies don't live up to their

    In terms of sizing, I might look for something like this. One large
    rail for 12V, takes care of that nasty 22.4A load. Many supplies stop
    at 20A on 12V1. And some others involve more than two 12V output rails,
    which complicates the analysis (because of the poor documentation that
    typical accompanies power supplies). Your total load would be using
    2/3rds of the 49A rating of the single 12V rail.


    I could understand your symptoms, if you "winked out" just as
    a game started. I don't understand how the Windows desktop could
    be doing it. There shouldn't be a large transient at that point.
    If there was a problem, then when you game with one video card,
    the box would probably fall over. It sounds more like a driver
    problem or something.

    Have you tried swapping card1 and card2, in the slots available ?

    How is your "paddle card" set ? For "dual card" mode ?

    Paul, Oct 18, 2008
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