External PSU for GPU

[no_connection]

This might not be 100% related to FN atm but who knows.

I need to power a GPU with external power since internal won't cut it. Making a ATX PSU or power brick to PCIe power connector isn't that complicated, what I do wonder is if I need to turn on and off power to it or if it won't mind much to have 12V going to it when PC is off?
Google found mostly unhelpful replies but I found one post saying it would be ok.
I could make it auto turn on/off but I would rather do it manually for the times it's going to be off for a longer time.

And if anyone have pinout info for PCIe power on a DL360e G8 I would be happy, don't seem to exist on the internet and some posts claim some of the cables are wired incorrectly. What I could see the middle 6 pins are ground and 12V but all the cables I saw have the rows offset.

 

[Constantin]

To me, this is a potentially really bad idea. Dual PSU rigs typically entail PSUs designed for same with a controller or some other means of ensuring that the PSUs stay happy. In your application, the GPU is not being kept electrically separate via Thunderbolt 3 external box or PCI port extender rig.

It’s not to say it won’t work but two PSUs fighting to control shared bus voltages seems like a recipe for something to go wrong - unless they are designed for it. So at the very least, I’d get the PSUs OEM to bless your proposed solution.

Whenever I have designed circuits around this problem, I used Schottky diodes (in low power applications) on all inputs (with expected voltage losses).

 

[Samuel Tai]

This is pretty common in mining rigs. What exactly are you trying to do?

 

[no_connection]

I don't think PCIe 6pin is in any way "shared" with the 16x power. Else GPUs would work without them plugged in and fry the 16x bus.
I have seen ppl use it but since I turn PC off I kinda don't want to make a box that turns on and off PSU but I guess I could if I need to.

I pretty much need to run a Quadro M4000 in a TS140 and the 280W PSU would be too close for comfort.
I also want to be able to run said card in a DL360e G8 is it could be shoehorned it.

 

[Samuel Tai]

Since neither FreeNAS nor TrueNAS need to use the GPUs, this is outside the scope for this forum. I'll move this thread to the off-topic forum. @no_connection, please look at mining forums for pointers, as this is routine over there.

 

[Constantin]

I don't think PCIe 6pin is in any way "shared" with the 16x power. Else GPUs would work without them plugged in and fry the 16x bus.
I have seen ppl use it but since I turn PC off I kinda don't want to make a box that turns on and off PSU but I guess I could if I need to.

GND is always shared and transients during startup and shutdown may hence create a potential where there shouldn’t be one. Similarly, how GND is GND on the motherboard vs. Chassis again depends on the PSU and motherboard implementation. Apple's external laptop PSUs are famous for leaking just enough to give users a tingle unless they used a 3-prong power cable.

I have seen PSU specs that allowed multiple ganged PSUs to be used, so it can be done. I bet that for those PSUs the OEM specified the same types be used in parallel vs. a mix and match approach with other OEMs.

it’s not like graphics cards use opto-isolators for every I/O pin on the PCIe bus. But I’ll happily concede that the bulk of the power may be able to be supplied separately and externally as I am not deeply familiar with how GPU power systems work. That said, I’d still prefer a high-quality single PSU to supply the whole system.

 

[no_connection]

Apple's external laptop PSUs are famous for leaking just enough to give users a tingle unless they used a 3-prong power cable.

That is due to the EMI filter where both phase and neutral is connected to ground through a small capacitor at the power input side. This causes a voltage divider and you get half the mains voltage if the ground is left floating.

GND is always shared and transients during startup and shutdown may hence create a potential where there shouldn’t be one.

While it is theoretically possible to loose ground on external PSU and force the current to go through internal 16x the same danger is present for regular connection to internal PSU as well. And the other "worst case" is that the external loop would carry some of the current instead of internal traces on the mobo which would lessen the load.
And while GND might be shared as a potential it might not be shared as an impedance. To form a potential or "ground loop" you need to share impedance or "common trace". I don't think the 12V rail would be shared in any way since it would not be able to draw different current from bus vs 6 or 8pin, at least that is my guess. But I don't know what happens when the GPU is turned off.

My concern would be if the GPU would back power the 16x slot with 12V or if the GPU itself would be unhappy having PCIe power before bus power.
I would kinda want to *know* rather than reason or assume my way to a likely way it works, maybe I could fins something definitive if I find the PCIe standard.
*edit* Well I found this:

"The +12V delivered from the standard x16 connector and the additional +12V delivered from the dedicated 6-pin power-supply connector must be treated as coming from two separate system power-supply rails.

The two +12V input potentials must not be electrically shorted at any point on a PCI Express x16 Graphics 150W-ATX add-in card.

No specific power sequencing between the slot and 6-pin connector can be assumed. A PCI Express x16 Graphics 150W-ATX add-in card must handle all possible combinations."

https://pdfserv.maximintegrated.com/en/an/AN3605.pdf

That means that as long as it's a PCI Express compliant card (which any GPU would have to be with a PCIe marking) it shuold handle it just fine.

Regarding the G8 I must have had some bad connection, I measured it again and pins are indeed offset one step and one side goes to ground the other side to 12V at the PSU. So that would be an easy cable to make for later.