Using PC power supply for powering other circuits than PC
PC power supplies are specifically designed for PC electronics.
Typically they work well in the current range a typical PC takes. If you
take too low current form the ouputs, the power supply myght not work
properly. If you load only one output, you might run into troubles of
poor regulation. Typically PC power supplies regulate the output voltage
of all outputs according the voltage in 5V output pin. If you don't put
enough load to +5V output, you don't get stable +12V output from the
power suppply.

I found the above, on the web, hope it helps...

Warmest regards,
John

wrote in message ...
For testing purposes, I'd like to make an PC power supply operate when
NOT installed in a PC. I've heard that the AT supply needs at least
SOME load on its 5-volt output before it will operate; is an ampere
or two sufficient?

For ATX supplies, I visited a friendly computer-repair store today and
examined the "cheater" into which they plugged UNconnected ATX
supplies;
it had three pairs of connections:
1) A jumper from Ground to PS-ON
2) an LED and resistor from PowerOK to Ground
3) two wires from +5V and ground going into something about the size
of a small box of matches which was shrink-wrapped to the cable

They wouldn't let me disassemble it !-)
Was there probably a 5-ohm resistor in that box?
--
--Myron A. Calhoun.
Five boxes preserve our freedoms: soap, ballot, witness, jury, and
cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTXS). W0PBV. (785)
539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle,
Pistol)