Dual computer PSUs, switching, and common ground

[arcfour]

Hello folks,

I have a prebuilt HP PC with a proprietary (non-ATX) motherboard power connector. I want to use a GPU in it that requires more juice than the piddily 280W stock PSU can supply. While I wait on an adapter to arrive that will let me use a standard ATX PSU in it I wanted to power the GPU externally through a separate PSU.

I've successfully done this by jumping the secondary PSU's PS_ON wire to GND without much issue. However, it's annoying having to turn two power supplies on/off. There's no easy way to splice another wire to the PS_ON connection of the primary PSU, so I decided to build my own version of one of those Add2PSU modules. From what I can tell, these modules tap SATA or Molex 5V from the primary PSU to control a relay which shorts the PS_ON of the secondary PSU to turn both supplies on/off simultaneously.

My question is - why use a relay instead of a MOSFET here? To my knowledge, ATX GND is tied to mains earth, so both supply GNDs should be the same and there's no need for isolation...is the only reason for a relay to account for improperly-grounded or multi-grounded house wiring? If I know my PSUs share the same earth, then I could just use a MOSFET, correct? I have so much electronic junk piled up in my tiny apartment that I can't find a 5V relay at the moment (plenty of 24V ones though ), but I do have some 5V FETs on my desk.

Any other gotchas I should be aware of? I'm not new to electronics but I've never worked with PCs at an electrical level so I want to be cautious :-)

Thanks in advance!

[NiHaoMike]

It will work just fine. Also, surplus server PSUs are a really cheap way to get a lot of power at 12V.

[Berni]

You can actually just connect together the grounds of both PSUs (Caution high current here) and the ON signal of both together so they both get turned on at the same time.

It is a bit of a kludge solution, better idea to get a new more powerful PSU, but this works in a pinch

[PKTKS]

Yes it works fine when properly arranged...

It will cost less and prevent you from buy over priced PSUs.

These adapters will come very handy when mounting the whole
thing into a chassis..

https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20211216023346&origin=y&SearchText=extension+cable+ATX+24Pin

It will also let you select good simple PSUs to escape the
over priced landscape of these "modern" bestial units...

Put a single dedicated one to the hungry GPU...
the other to CPU and mind about the heat flow..

Very used in crypto mine setups...

Paul

[coromonadalix]

yep that should do it, BUT  some server psu  have load balancing between them, they communicate between them or use other tricks

Sometimes you can purchase 2 server psu with their hotswap backplane, the load balancing is easily done ...