Hi, I have a couple of old AT power supplies that I want to use on the bench, as I often need 12V and 5V supplies. I hooked one up last night to the outlet, switched it on and the fan came on, but when I connected a load across one of the 12V supplies the fan stopped and I got no power. I know that ATX supplies need one of the motherboard header wires shorted through a resistor to enable it to work, but I thought AT supplies don't require that. Any idea what I need to do to get my AT PSU to work on the bench?
I did this to a 250W ATX supply last week, and it too is touchy about loads on the 12V output. I'm using it to power a car amp and it takes a careful hand to get it to start up. Here's what I usually have to do: -start the power supply and let it run for a minute with no load -carefully touch a wire from 12V to the remote terminal to turn it on
When I tried using a switch between the 12V and the remote, the power supply would just shut down. I think the supply doesn't handle changing currents very well, so if the switch bounces it can't keep up and shuts down.
I've used an ATX supply sans motherboard with one of those ATX power supply testers. The "tester" connects to the motherboard plug and looks to have some big power resistors, some transistors, and an LED. With the tester connected I was able to fire up the supply and tap a 12v connector.
These are pretty easy to find on the web (Google "ATX power supply tester"), though I'm not sure if they make a similar product for AT supplies.
If all else fails you could scavenge parts from the AT supply to make dedicated 12v and 5v supplies.