Power Supply: turning on an ATX supply that isn't connected to a motherboard
**disclaimer: power supplies are dangerous, they can shock you; do this at your own risk **
Find the ATX mobo plug, find the green wire, (should be only one, if there are multiple green wires... then your PSU sucks, i wouldn't go any further), and take a piece of a wire so that you can connect something, than find a ground wire (should be 6, these should be black), and connect the green wire to the black wire, basically by shoving the third piece of a wire you have into both of those two plugs