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Thread: Multi-rail PSUs not good for SLI?

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    Multi-rail PSUs not good for SLI?

    The Inquirer is reporting that some mobo/dual-graphics-card setups do not work with multi-rail powersupplies. (They switched to a single-rail powersupply and got their SLI system to work.) They were quite shocked by it (as am I).

    I don't get it. Weren't dual rail power supplies supposed to be better than single rail power supplies? Any explanations?

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    Maybe because the rails are split 50/50 between the cpu and GFX and using sli the GFX want more than 50%. In a single rail they can take what they want with very little chance of maxing out the rail

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    Resident abit mourner BUFF's Avatar
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    Quote Originally Posted by Artic_Kid
    The Inquirer is reporting that some mobo/dual-graphics-card setups do not work with multi-rail powersupplies. (They switched to a single-rail powersupply and got their SLI system to work.) They were quite shocked by it (as am I).
    & yet users on places like DFI-Street or Abit-USA forums could have told you months ago that you were better with 1 strong rail as opposed to 2 medium ones.
    What surprises me is that it took so long for the Inquirer to find this out.

    I don't get it. Weren't dual rail power supplies supposed to be better than single rail power supplies? Any explanations?
    Unfortunately I think that it was seen by Intel (iirc they drove it) as a way to make the more powerful PSUs required by modern CPU/mobos cheaper but in doing so they seem to have ignored the power requirements of high-end gfx.
    Probably works fine for 90%+ of the PC market though with just the very top end sufferring.

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    Quote Originally Posted by BUFF
    & yet users on places like DFI-Street or Abit-USA forums could have told you months ago that you were better with 1 strong rail as opposed to 2 medium ones.
    IIUIR (if I understand it right?), the trade-off goes like this. A single-rail PSU is more versatile (can be used under wider circumstances, such as when THREE power-hungry chips are involved -- a CPU, and 2 GPUs). But a dual-rail PSU is lower cost for the same power rating.

    I presume the problem arises when you hook one rail to the CPU, and the other rail to 2 GPUs -- because that second rail may not have enough power to drive 2 ATIX1900 graphics boards each at 250+ Watts peak.

    Unfortunately I think that it was seen by Intel (iirc they drove it) as a way to make the more powerful PSUs required by modern CPU/mobos cheaper but in doing so they seem to have ignored the power requirements of high-end gfx.
    Probably works fine for 90%+ of the PC market though with just the very top end sufferring.
    Yes, I've only heard of this problem recently, and only for the ATI X1900XTx cards -- which individually are serious power hogs, but put two of them together and one rail of a dual-rail PSU cannot likely deliver the power. I guess that's making sense now. Thanks sgoaty and BUFF.

    Is the Seasonic S12 600W a single-rail powersupply? If not, then it sounds like it would have problems with running a dual-graphics X1900XTx setup.

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    Resident abit mourner BUFF's Avatar
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    Quote Originally Posted by Artic_Kid

    Is the Seasonic S12 600W a single-rail powersupply? If not, then it sounds like it would have problems with running a dual-graphics X1900XTx setup.
    Pretty certain that it's multi-rail & there have been problems reported with Seasonics recently - I understand that they were having particular problems with DFI nF4 boards (I believe that Seasonic replaced the PSUs so perhaps they tweaked the design or it was a bad batch).
    Maybe have a look at DFI-Street for more info on that.

    As always, I'm sure that there are multi-rail designs from quality manufacturers that work perfectly well whereas cheaper iterations may not ...
    Last edited by BUFF; 01-03-2006 at 05:21 AM.

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    Yes the Seasonic is dual rail but it is Sli certified and the 600w S12 has 18amps on both 12 v rails which shouldbe more than enough.
    Yes there were problems between the DFI Expert board & the S12 but this was sorted a while ago with new revision of the S12.

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    The OCZ 520Watt has a single rail, and it used by extreme benchers still, pretty durable PSU. PC Power and cooling also have 1 rail i think.

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    Quote Originally Posted by Smokey21
    The OCZ 520Watt has a single rail, and it used by extreme benchers still, pretty durable PSU. PC Power and cooling also have 1 rail i think.
    These new ATI X1900 cards require a whopping lot of power. Check this out. These cards require 150 Watts at idle, (and allegedly 315+ Watts at full load!!!). If that's the case, then a 600 Watt dual-rail powersupply would be risky. (No matter how you hook-up the main power-hogs, the CPU-mobo and 2 GPU cards, one rail of a two-rail supply will get overloaded. Perhaps that is the reason a single-rail PSU works better in this instance.)

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    Quote Originally Posted by jimborae
    Yes there were problems between the DFI Expert board & the S12 but this was sorted a while ago with new revision of the S12.
    Are you sure?
    Last time I checked (not long ago), Revision 2 of the PSU still had issues with DFI board. Not always, but not uncommon either. Revision 3 was posted on some blog, but I never saw any official announcements.

    They should make dual railed PSU with ratio switch. Ye know, like 50/50, 40/60 or 60/40 depending on whether it is more important to have more power for the CPU or GFX card

    Ummm, *goes to patent that idea*.

    Edit: Would be interesting to see the test with other PSUs. Looks like the Seasonic is doing good though (as is the Enermax & PC Cooling & Power... which I am starting to think is not a very nice name... It's what I'd name a generic product ).
    Last edited by TooNice; 02-03-2006 at 11:12 PM.

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    So basically buy a tagan and you can chose dual or single with a switch!!
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