I make bench power supplies out of atx power supplies for my friends. Basically you ground the green wire and put 1 to 2 amp fixed load on the 5 volt side and you end up with a nice 12-volt supply for your work bench. I have recently run in to a couple atx supplies that wont work like this. They start for a millisecond and then turn off. They both came out of working computers. Any help on what the problem might be would be appreciated. Dan

 

All of the output rails on a PC power supply should see at least a minor load - not anywhere really more than 10 mA or so - barring of course the two negative supplies that are rarely used anymore anyways. It could be that yours is looking at the main 12V rail. Sometimes they also look at the +5V standby line.

 

Google ATX bench supply

You will find hacks for almost every brand, make, model of PS out there.

The 5v line USUALLY needs a resistor to power up the rest of the system, but there are exceptions.

 

Some newer supplies need a load on the +12V line as well, or they will go out of regulation and shut down, possibly even the 3.3V line, though I haven't seen that yet.

 

Almost everything nowadays depends on the main +12V rail(s) so that makes sense.