ATX Power Supply Testing

Sometimes you run into problems where you think the power supply of a computer is failing. Or you simply want to test that old power supply that you have lying around to see if it still works or not. In both cases you want to test the power supply without the other components of a computer either because you don’t want any influence from e.g. the motherboard or you don’t want to rip out a power supply out of a working PC just to test that one power supply.

Problem is, if you attach a ATX power supply to the mains it will simply do nothing. It’s waiting for a signal from the motherboard to turn on (usually controlled by a button on the front panel) but how do you supply that signal without hocking up a motherboard and case? Turns out that is very simple.

An ATX power supply either has depending on the ATX version it support a 20-pins or 24-pins (ATX 2.0) connector. Additionally it can have a 4 or 8 pin connector for additional CPU power. The layout of the 24-pin connector is shown below.

A 20-pin connector misses the bottom 4 pins (+12V, +3.3V, +5V, COM). Some power supplies come with a 20-pin connector and these 4 pins as a separate connector so that it can be used both with 20-pin and 24-pin motherboards.

The motherboard signals the ATX power supply to turn on by pulling the PS_ON# pin to ground. The PS_ON# pin is the only wire on the connector with a green wire so it’s very easy to find the correct pin. On the 24-pin connector it’s pin number 16. So to manually turn on an ATX power supply we only need to pull the PS_ON# pin to ground. Conveniently, the PS_ON# pin has two COM (= ground) pins next to it on the same row so the only thing we need to do is connect it to either pin 15 or pin 17. This can be done with either a wire or, as I did, with a part of a paperclip.

Simply stick it in the connector to short circuit the pins 15 and 16 and connect the mains to power supply and it should immediately power on. It should stay on as long as the two pins are connected. It’s not dangerous to touch the paperclip while the power supply is on as it’s on ground level while the power supply is on. And even when you would touch pin 16 alone it’s not dangerous as it’s only pulled up to +5V with an internal resistor, so it’s harmless to humans.

Some power supplies need a load on one or more connectors before they will turn on. This can be accomplished by attaching one or more fans or hard disks or a CD/DVD. Also note that some power supplies will not spin up the cooling fan(s) unless the power supplies reaches a certain temperature.

To test if the power supply is working correctly you can measure the output voltage with a multimeter. The above drawing shows the output voltage of each pin. The only special pins are PWR_ON, +5VSB and pin 13 (+3.3V). The PWR_ON pin is controlled by the power supply and turned to +5V when the power supply is ready and that all power has stabilized and is ready for use. The +5VSB (+5 Volt Standby) always stays on even when the power supply is in standby. Pin 13 can either be a +3.3V power supply or can be used as a sensor by the power supply to measure losses in the cabling.