Explain this one to me.

[MassiveJim]

I am just testing a system for someone.
X570 board, 5950x, Asus RTX4090
Corsair HS1000i
however, they haven't supplied me with enough PCI-E power cables, so I decided to use a spare PSU to test.

corsair HX1000i installed inside the case, connected to everything on the board, and 2 x PCI-E connectors on the supplied adapter for the GPU

secondary PSU connected to the 2 remaining PCI-E power connectors.


as soon as I turn on the secondary PSU all of the fans lit up and started spinning, and the motherboard diagnostic LED displayed some numbers.
I then turned on the system, it booted fine and ran superposition, score was as expected.

I then shutdown the machine, turned off then disconnected the spare power supply, went to turn off the PSU installed in the case and it was already off!!


just to check I wasn't going crazy I tested it again and the system will power up with the installed PSU switched off when the external PSU is connected ONLY to 2 of the GPU PCI-E connectors.

I then ran another benchmark, this time with both PSUs switched on and halfway through the system just powered off.
I then ran a 3rd benchmark with the spare power supply connected to all 4 of the PCI-E power connectors and it ran the benchmark just fine (albeit a slightly lower score)
NOTE: this time the fans did not light up when I turned on the external PSU, only when i pressed the power switch to boot the system.


I have worked on PCs for 20 odd years and never seen this happen before.

 

[MassiveJim]

PSU definitely not faulty.

with the 2nd PSU not connected and the power switch off on the internal PSU, no power is fed to the machine and it will not turn on.
Also the installed PSU is pretty much brand new.

Also, with the external PSU connected to all 4 of the GPU connectors, the above behaviour did not occur.

so somehow the external PSU was feeding power to the internal one via the GPU power cable