For an AT power supply, you need only hook a load up to it, like a fan, or
a hard drive, plug it in, turn it on and check the yellow and red leg on
any power connector. To do this put your meter on DC volts, plug the
positive (red) probe into the yellow and then red wire, and the negative
(black) probe into any black wire. Yellow should be at 12+ and Red  should
be at 5+ regardless of the load. There is no reason why you can't run this
test on a running PC , while the PS is under load. In fact that would be
the ideal test.

For an ATX powersupply you will need to override the signal wire in order
to turn it on. They sell a plug in that will simulate the motherboard
signal switch, but you can make your own with a paper clip. On the ATX
Motherboard Power  Block ...put one side of the paper clip into the green
wire (signal) slot, and the other end into any black (ground) wire. Plug in
some kind of load , like a fan. and turn the PS on by either plugging it
in, or the best way is to use a surge protector with a switch on it.

Rode
The NOSPIN Group
http://freepctech.com


>If anyone knows how to do this, I'd be interested to hear.  Several times
>I've wanted to test a power supply on the work bench, but when they're not
>connected to a computer, they don't function at all, ie you can feed power
>in, but nothing comes out the other end.
>
>I have an idea that you can short out a couple of the connector pins, to
>simulate the motherboard switch, but I don't want to try it out without
>knowing the right ones.
>
>Incidentally, if the original poster (sorry, I forgot who that was) is still
>wondering how to use a meter, set it to 10 volts DC.
>
>The power supply puts out two voltages, 5v and 12v.  On my non-digital
>meter, there are several scales on its' face, so 12v doesn't kick it off the
>clock.
>
>Ian Porter

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