I want to convert my old ATX power supply to a simple bench supply for 12V devices. On the label it says it's a 300W PSU where I can use 264W on the 12V rail (see image below).
Therefore I soldered all cables with the same color together and wired the green (PWR_ON) one to any ground cable.
If I plug the cable in, the PSU starts and you can hear an annoying whirring. I need to mention that the PSU does NOT turn off automatically when there is no load attached.
Then I connect (just to test) a 12V 55W car lamp to the 12V (yellow) cables and the PSU turns immediatly off (the lamp is flashing for half a second). To keep it running again, i need to remove the cable, turn the switch on/off or reconnecting the green cable with the ground.
I measured at which amps the PSU shuts down – that's roughly 50mA @ 12V.
Interestingly, it doesn't make a difference whether the 12V car lamp is attached before i connect the cable or not. Furthermore, if I attach a 5V lamp at the red wires everything works and the PSU stops whirring.
The brown 3.3V sense cable is wired to the orange 3.3V cables.
I also tried putting a 25 Ohm resistor between the red 5V and the black (GND) cables – nothing changes.
For me it seems that the 12V rail is faulty … do you have any ideas?
Here is an overview of how it looks like: https://imgur.com/DnTi61G