PSU Rails ??

Discussion in 'Hardware' started by gnome, Jul 31, 2008.

  1. I was looking at the system requirements for a video card... and it said "12v @ 22A".

    Even for 600W PSUs I'm finding few with >20A... many have 3-4 rails of 18A though.

    So, are all of these PSUs with <22A not capable of running this "12v, 22A" requirement card?

    Is there something "additive" about the power of multiple rails for running one card?
  2. Gnome, there is nothing wrong with going with a multi-12V rail, even though it doesn't meet the single rail specs that you have mentioned.

    Again, total wattage ( ie. 600w ) means absolutely nothing. It's ALL about how many volts are supported by each rail, and primarily the +12V rail.

    I have had a PCPower & Cooling PSU running in an old Dell Dimension 8300 ( my back-up computer ) for 5 years now. No problems at all. You can feel the quality in how heavy it is. They make PSU's with strong single +12V rails.

    Here's a 500w unit from them that comes with a +12V rail rated at 35A, and
    only costs $79.99

    Quiet, and low heat.
    2 PCI-E connectors too!
    5-year warranty.

    Even their 370W ATX12V model has a +12V rail that is rated at 27A.

    Check out their company website.
    They also specialize in PSU's for Dell's.

  3. I did a little reading, and it looks like (1) if there is only one rail, it is shown with however many amps the PSU can output, and (2) if there is more than one rail, the rating takes the total output and divides it among the rails.

    I guess that means my Dell 375W, with 2, rails of 18 amps is more or less equivalent to a single rail of 36 amps... and adequate to run the video card I was checking into...

    That sound about right?
  4. Gnome, that is correct. The power supply has a single 12v regulator generating the total capacity of the power supply, but it is split into multiple current-limited outputs in order to protect the wiring from the power supply to the rest of the computer from overheating.

    Your graphics card probably gets power from multiple sources - one or more drive connectors, and the PCI-E power traces - in order to avoid exceeding the current capacity specified for each source. Violating these limits would risk melted wiring, fire, or tripping current limiters in the power supply.

  5. correct, the currents add up. The voltages remain the same in parallel.

    Like running batteries in parallel. You increase the overall current sourcing capability of the original single batteries, but retain the same voltage.

    Power sourcing increases likewise.
    Pt = vt*(i1 + i2 + i3 + ...) per rail.

    If I recall correctly, my nvidia GeForce graphics card required two separate 12v molex rails connected for that reason.

    The PSU manufactures are trying to accommodate a wide variety of loads as efficiently as possible. While the video card manufactures are aware of this they also make provisions; sometimes when
    unexpectedly hungry loads like GeForce come along, that is why they provide for two sources connected in parallel to get the wattage capability required from the supply rail.
    Separating the rails this way helps in both current limiting under unknown loads as well as maximizes efficiency.

    P.S. Just make sure your video card has more than one molex power connector to add more than one rail. Or else, the card itself will limit you. I.e. in the case where you required a current much larger than your supply outputted per each +12V molex connector, but only had one female molex connector available, you would have to either buy a supply with a much larger power available per single rail connector output (like some of those shown earlier on Landis' post), or you would have insufficient drive from the less equipped multiple rail supply.