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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).

https://imgur.com/xpOqHZa

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

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  • \$\begingroup\$ Sounds broken to me. \$\endgroup\$ – Eugene Sh. Jan 16 '19 at 17:38
  • \$\begingroup\$ Try a load that isn't so heavy like 10k and see what happens \$\endgroup\$ – Voltage Spike Jan 16 '19 at 17:40
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    \$\begingroup\$ Measure the resistance of your 12V 55W car lamp when it's cold and then do the math to see how much current it will draw in that state. \$\endgroup\$ – brhans Jan 16 '19 at 17:50
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    \$\begingroup\$ Michael, there's an image upload button on the editor toolbar that allows you to post the photos in the post rather than on an external link. There's an edit link under your question ... \$\endgroup\$ – Transistor Jan 16 '19 at 18:19
  • \$\begingroup\$ Is the 3V3 sense wire connected to 3V3? Did you keep 12V1 and 12V2 separate? \$\endgroup\$ – Justme Jan 16 '19 at 19:47
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I'm getting similar results with a small 12v load in my application. I've tried more than one supply. Some shutdown, others don't. ALL are new, out of the box supplies. I'm thinking it has to do with flakey short circuit or over current protection circuitry in some power supplies.

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