Hi - I'm planning on using a old computer PSU I have laying around to power something I'm working on. I remember reading that to do so , besides shorting the green wire and a ground wire, you need to put a load on a certain rail for some reason or another. Does anybody know anything about this? Thanks!
Well, looking at the Antec PSU tester, it uses a 25w, 5.4ohm resistor, which I believe is on the +5v line, which would be about a 1A draw on it. If you want to do the same, you don't need a 25w, just something well over 5w (they probably chose 25w so it wouldn't be hot to the touch).
it's actually the 5V line that I'm interested in - I'll be drawing maybe 3A or so. Should that be OK? I'm keeping all the logic on a seperate supply just in case.
The design guide for ATX 1.1 PSUs says the following loads are required: +3.3V - 0.3A +5V - 1A Your 3A draw easily covers the 5V minimum and you can usually get away without loading 3.3V at all, but if you're worried then just stick a 10R 5W resistor on the rail.
The 5/3.3 and 12V rails balance each other, so to get stable voltages on 3.3 and 5V, you need to load 12V a little. I have experienced a drop of about 0,3V on the 5V line when 12V is not loaded down with a few hundred mA on several PSUs, both AT and ATX.
I've also modded a psu for external use, and did put a 5 ohm, 25 watt resistor on the 5v line, and let me tell you, it still gets pretty hot!!! Voltages are a little off across all lines, so I'll probably throw another resistor in the 12v line to see if it bumps things up a bit. I didn't put anything on the 3.3v line.
