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A couple of days ago my PC's 24-pin ATX power supply began to make a bad, electrical burning smell. I took it off and replaced it with another (working) 24-pin supply I had lying around. That worked okay at the time but today it refused to switch on. So I took that one off and shorted power on and ground, hoping to test the voltages. The fan turned for a split second and then stopped. The pins and connectors did the same kind of thing - a low voltage appeared but quickly went back to zero. Another computer with a 24-pin motherboard also failed to make it do any more.

Just out of interest, I have plugged it into a computer with a 20-pin board, and it works fine! That draws me to the 4 extra pins, almost as if the original motherboard has done something to that part of the supply. But the original power supply with that board does still work other than the smell - it hasn't been shutting itself down or acting odd in any other way. And surely if one computer can power it on then the other ones should be able to as well?

Has anybody ever come across something like this?

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PC power supplies are switch mode power supplies, which typically require an electrical load to operate. If the PSU is not plugged into anything, then there is no load, and the PSU will shut down to protect itself. –  sawdust Jan 6 '12 at 6:06
    
That original PSU that smells should be repaired or properly disposed. Stop using it, since it has had somekind of failure and the smell could be from toxic fumes. –  sawdust Jan 6 '12 at 6:19
    
Also, magic smoke generally is bad. –  Journeyman Geek Jan 6 '12 at 9:02
    
Thanks guys. As Keith suggested I've bought a tester so I'll see what that says. I understand about needing a load on it. Don't worry, I have stopped using it. –  Andrew Jan 7 '12 at 3:23
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I would also recommend that you get a PSU tester. It'll better test PSUs by putting a load on them as well as being automatic and safer. For example: COOLMAX PS-228 Network/ PC Service Tools

I've never seen that but when I get the inkling that a power supply is bad I promptly recycle it.

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I've bought one of those - I didn't really know something like that existed and ceratinly not at that price. I'll see what happens! –  Andrew Jan 7 '12 at 3:20
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