I used an ATX power supply on one of our
team's projects. They have a minimum load in order to work. Without this load, they quickly stop working. It has something to do with the inductors needing a minimum current in order to work properly. I'm not well versed in power supply design, so I can't give you much more detail than that.
I ended up connecting some power resistors to it. You'll have to look up the specs for your supply and figure out the correct load. Depending on your exact situation, you may be better just finding a cheap supply somewhere.