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Here is a way to turn on an atx power supply without having it hooked up to a computer.

ATX power supplies are designed only to turn on when hooked up to a computer. This is unlike the older AT power supplies which could be used without a computer easily. Luckily it isn't too hard to modify the ATX power-supply so that it will turn on without being hooked up to a computer.

There are three pins on the ATX power connector that are needed to get the power supply running by itself. These pins are pin 9 (%VSB), 14 (PS-ON) and any pin with a black wire connected (ground pin). Connect a 1/4 watt 10 ohm resistor between pins 9 and 14 then connected pin 14 to ground. This will turn the power supply fan on and also give power to all the power supply wires (5V,12V,-5V,-12V 3.3V etc

The ATX power supply sends a 5V 720MA current to the motherboard through pin 9 on the power connector. The math behind using a 10 ohm resistor is that Voltage = current * resistance so 5V = 0.720A * resistance. Resistance = 6.94 Ohms. I was only able to get the power supply fan turning reliably with a resistor of 10 ohms. The resistance required may be dependant on the amount of current actually being drawn from the supply. I do not know if higher resistances allow more power to be drawn, but for now I am using a simple 10 ohm resistor from radioshack, and I am getting reliable operation while drawing more current that the ATX supply is rated for! (20 amps at 5V for this ATX supply)