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)