2 Power Supplies -- Backfeed?

[Kiah07]

Can someone tell me what I need to know before using 2 PSUs on one motherboard.  I've seen warnings about backfeeding power but can't find a lot of information about the correct way to do this. 

[1714923747]

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[1714923747]

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[1714923747]

1714923747

[mac-coin]

I've just added a 3rd R9 290 to my rig and it's powered with a separate psu.  I first tried using just a normal powered riser, but as soon as cgminer started the pc would crash.  Tried all different wiring configuration, but always the same results. 

I ended up using one of the USB risers which lets you totally separate the GPU power supply as the only link to the motherboard is data.  Plug it and worked first time.

Only down side is there twice the price,  But it I had got this from day 1, I would have gained 4 days mining, sound would have paid for itself.

Hope this helps

[Test User]

I've examined all my cards (a variety of 7950s and 280x) and the auxillary power is not connected to the PCI-slot power. If you connect a second PSU to the aux inputs, then there is no risk of backfeed.

Obviously, i can't vouch for all cards and all VRM designs. However, I would expect that most VRM designs in use today could
be buillt so that backfeed cannot occur.

The most important thing to make sure if you are using multi-PSUs is that the ground connections are very securely connected together. In general, cards do directy connect the ground pins on the aux connectors to the ground pins on the PCI-e slot. This way, if you are not using risers, just connecting the 2ndry PSU to the aux connectors will result in a secure ground link.

The problem comes with powered risers. Theres a big danger with these, because they may separate the ground and power connections. If you connect the riser to 2ndry PSU, then the signal voltages will all be set with reference to the 2nd PSU, and if the ground voltage separate (e.g. when one PSU switches on) then this power can be conducted into the signal connectors and fry the motherboard. I killed several motherboards this way before I worked out what the problem was.

This problem is even bigger with the USB style powered risers, because they completely separate all the power, and only have a thin shield connecting the grounds - not like a ribbon cable which has about 10 ground connectors. If you are using USB style risers, you must ensure a very strong ground connection between the two PSUs. Easy way: Power all the risers directly from PSU 1 which is powering motherboard - Card aux power can come from other PSUs. Harder way: Make custom harnesses which connect the ground wires between 2 hard drive connectors. Use 2 or 3 of these to cross link the ground connections between the PSUs - don't trust a single wire linking the grounds on the big 24pin molex connectors.

[Kiah07]

Easy way: Power all the risers directly from PSU 1 which is powering motherboard - Card aux power can come from other PSUs.

So this should be true I think. 
https://i.imgur.com/BCnc7Uh.jpg

I ended up using one of the USB risers which lets you totally separate the GPU power supply as the only link to the motherboard is data.  Plug it and worked first time.

Only down side is there twice the price,  But it I had got this from day 1, I would have gained 4 days mining, sound would have paid for itself.

Hope this helps

That is what I would rather do I think.  Can you tell me where to get powered USB risers that have only the data lines going to the motherboard?  Any other tips are appreciated.

[phaddie]

I am about to add a 3rd r9 290 to me system and I have the USB style risers.  My plan was this.

PSU -1 - OCZ gameXstrem 750 - GPU 1, motherboard

PSU -2 - CORSAIR HX-850 - GPU 2, 3 - shorted in the 24 pin connector to turn on manually (will just leave it on).

 If I am understanding what was just said I should power the risers for all 3 cards off of PSU1.  What will the power demands be?  And what are the risks if I power GPU and risers for GPU 2 and 3 off of the second PSU.

 Also when you say connect the ground wires between 2 hard drive connectors, do you mean just taking a power plug from PSU1 and a plug from PSU2 and connecting the ground?  If I do this with the second power supply on all the time will that be a problem?

And does a piece like the add2psu take care of the ground differences mentioned in the thread?

 Phad.