Turning on a power supply without a PC

All you have to do is carefully connect the GREEN wire (normally thinner than the rest of the cables) to ANY BLACK wire. This can be done easily with a short piece of wire.

If the power supply doesn’t immediately turn on (within 2 seconds) remove the wire, you may have connected it to the wrong wires. Nearly every power supply won’t allow a short to damage the unit, but it’s not a wise plan.  

Once on, the power supply isn’t loaded so the voltages won’t be as stable as if they were used in a full PC, however it’ll be sufficient for most uses. It’s not a good idea to leave a power supply on in this fashion for a long period of time too.

AGAIN here's the simple instruction:

To be able to test your computer's power supply, you can follow the instructions below:

• 1. Turn off your power switch and plug it out from the power supply.
• 2. Remove the cable from the CPU and start unscrewing the power supply section in order to remove the whole power supply (square metal box) from the CPU.
• 3. Once done, locate for the main ATX connector and find the green wire.
• 4. Also locate for the black wire near the green wire and with the use of a paper clip, connect and safely bond the two together with a piece of isolating tape.
• 5. Put back all the power supply and switch the system on again and ensure that your system ventilation is working.
• 6. To check whether the connection is correct, the Molex in between the yellow and black wire should display at least 12 volts.