I have, in my arsenal, a power-supply tester: All I have to do is connect the 20 or 24-pin connector to the tester; and if all the green lights light up then the PSU is supposedly working. Is this right? Yes and no:

The power to the motherboard, not including the dedicated processor supply, is working if all the green lights light up. What it’s not testing is the power supplied to the other connectors; such as the dedicated CPU supply, (The ATX12V or the EPS12V connector.) the 4-pin Peripheral connectors, (AKA Molex plugs) the 4-pin Berg connector(s), the Serial ATA power connector(s), or any others. That renders my tester fairly useless; as if I’m not getting power to the PCIe graphics card; my tester could still read that there’s no problem.

- So I need a way of testing each individual output, individually, to see if the unit is working as it should.

A voltmeter seems a good place to start. – And where’s a good place to find one of those? On a multitester or multimeter no doubt.

OK so I have the (ATX) power-supply that I want to test on my test-bench in front of me, all wired up to the mains power and switched on. – But there’s absolutely no response – or possibly only the 5V rail is powered on. Why?

On an ATX power supply there’s a wire which goes to the motherboard which allows certain pieces of hardware, including the “Power on” button on the front of the case to bring the power-supply out of standby. Some PSUs go to +5V standby, where only the 5V rail remains powered up, others switch everything off. (Some of the much older PSUs would go to +12V standby; but not any recent models built within the last 5 years.)

That wire is the green wire on the P1 connector. (The 20 / 24-pin connector that plugs in to the motherboard.) To fire up the PSU it needs to be grounded. Fortunately the wire either side of it is a ground wire; so all you need to do is short out either way on the connector, (Short green and black.) using a short piece of wire or a paper-clip or something, and voila: Power-up has been achieved.

If it doesn’t happen; connect a DC voltmeter across green and black: Positive to green and negative to black. You should read 5V. If you don’t get a reading, or you read anything less than 2V, the PSU is faulty. Bin it.

*Power supplies can be fixed. I fixed one myself by cannibalising spare parts from 2 identical dead units. The drawbacks are:

1) It’s dangerous: there are extremely high voltages present in the circuitry when the power’s on.

2) You need to know what you’re doing; which can take up to 4 years training to achieve.

3) It’s very time-consuming: Even if you know what you’re doing; it can take hours to trace and diagnose multiple faults and repair them.

It’s easier and usually more cost-effective to bin the faulty unit and buy a new one.

Now, having powered up; check the voltages of all the connectors.

The pinouts are as follows:

(Image above from Wikipedia.org)

4-pin Peripheral

4-pin Berg

6-PIN AUX

ATX 4-pin Molex 12V P4:

Serial ATA

- And that’s how to check the voltages.

The above proceedure doesn’t test the PSU under load. In the followup article I’ll show you how to test the PSU’s voltages under full load; including a circuit that’ll fully load any PSU up to 350W