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Thread: Running 12V PCIe on seperate power supply for GPUs

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    Default Running 12V PCIe on seperate power supply for GPUs

    Hello,

    I'm a newbie here, so I should start by saying hello to everyone, this is a great site that I just recently discovered.

    I'm looking at building a system with LOTS of GPU power, partly for gaming, but also for CUDA development on nVidia cards, and I would like to use a seperate power supply for the PCIe leads on the cards to avoid running the main power supply at or beyond its rating.

    I have two concerns in doing this however, and I wanted to know what the experts here thought about it:

    1) If I simply bridge the green (ATX ON) wire and a ground wire between the power supplies (as the adapter that came with my CoolerMaster stacker does), don't I risk creating a ground loop between the two PSUs? With only one wire tying both PSU grounds together, there could be a current going through the wire bridging the grounds together, which would be bad If so, I could use a simple 12V relay to isolate the two supplies. but then I risk creating a ground loop through the computer itself, which is even worse.

    2) As a follow-on to 1), does someone know if the 6-pin and 8-pin PCIe power feeds to modern video cards are isolated from the motherboard power fed to the card via the PCIe slot? The voltage will differ slightly between the two power supplies, so the circuits would have to be isolated for this to work, otherwise undesired stray currents will be created.

    From the research I've done to date, I get the impression that they are indeed isolated, since as I saw some 5.25" drive bay PSUs for video cards that function in pretty much the same way as what I want to do. Also, if a single PSU ran the PCIe off of a totally separate +12V rail than the motherboards +12V power, the same voltage imbalance could happen depending on loading. I figure the grounds are common however, so the problem in 1) still applies.

    Has anyone done something like this in the past? Has it worked reliably?

    Thank you,

    Luc
    Last edited by RFMan; 01-15-2009 at 12:40 PM. Reason: fixed typos

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    Default

    1) Don't worry about the ground loop, since you're dealing with two isolated power supplies and their ground don't have a sustaining voltage difference hence there's no current on this wire.

    2) I'm not pretty sure, but I think the 12V pins on the PEG side and on the 6pin/8pin molex are isolated. Use a multimeter to measure the resistance between them and maybe you'll figure out. Again they share the same ground but there's no need to worry about that. However I'm pretty sure that two 6pin/8pin connectors on a single card(like GTX260, HD4870) have their 12V pins connected together, so don't plug two 6pins from each PSUs to the same video card.
    It's been a hard day's night and I've been working like a dog.

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