Explain this one to me.

MassiveJim

MassiveJim

MassiveJim

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22 Feb 2014
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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 said:
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.
Click to expand...
I suspect the installed PSU is feeding power even though it is switched off. Sounds like it is faulty or damaged.

You could confirm this by unplugging the kettle cord from the PSU mounted in the machine as a means to making sure it really is 'off' and running the test again; I suspect it won't boot.

If you have a lower power GPU lying around that doesn't required power directly from the PSU you could run an additional test with only the installed PSU, and no spare PSU, as an additional test.

I would consider replacing the PSU in the core system, and disposing of the broken one safely.
 
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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
 
It makes sense to me. The gpu will have a rail for its supply … connect one source of power to one part of the rail (ie a single pcie power connector), then all of the rail will be powered (all connectors).

Because those other connectors are plugged back into the internal psu, that power will flow back to the psu …. Which itself may likely have a single rail power supply as well … so anything else connected to the 12v rail in the PSU will get power flowing to it … like the ATX motherboard and the cpu power supply.

We assume that power flows only from the PSU but it’s not necessarily the case … if all the rails are joined, then anywhere in the rail can be used to put power into it.
 
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