What a multimeter tells you about your power supply

A multimeter is a handheld tool that measures electrical voltage, current, and resistance. When your computer won't start or shuts down unexpectedly, a multimeter can tell you whether the power supply is delivering the correct voltage to your motherboard and components. You don't need an expensive meter — a basic digital multimeter costs $15 to $30 and works for this job.

The test takes about five minutes and requires no special skills. You'll measure the voltage on the cables coming out of the power supply while the computer is running or in standby mode. If the readings match the expected values, your power supply is likely working. If they're far off, the power supply is probably failing.

This test won't tell you everything — a power supply can fail under load even if it passes a quick voltage check — but it will rule out the most common failure mode and save you from replacing a good power supply by mistake.

Key Takeaways

  • A multimeter measures voltage on the power supply cables; the main 24-pin motherboard connector should read between 11.4 and 12.6 volts on the yellow wire and between 4.75 and 5.25 volts on the red wire.
  • You must turn the power supply on (either by starting the computer or by using the paperclip trick on the 24-pin connector) before testing, because an off power supply reads zero.
  • Set your multimeter to DC voltage mode and touch the black probe to a black (ground) wire and the red probe to the wire you're testing.
  • If all readings are within range, the power supply is delivering correct voltage; if readings are far outside the range or zero, the power supply is likely failing.

Gather your tools and prepare the computer

You need a digital multimeter, a computer with a power supply you want to test, and a small paperclip or wire. Before you start, shut down the computer completely and unplug it from the wall outlet. Wait 30 seconds for any remaining charge to drain from the power supply.

Open the computer case using a screwdriver. Locate the large cable bundle coming from the power supply — this is the 24-pin motherboard connector, the main power cable. You'll also see smaller cables running to the CPU, graphics card, and storage drives. These are the cables you'll test.

Do not touch any components inside the case while the power supply is running. Metal parts inside the case can carry electrical current, and touching them while the power supply is on can damage the computer or injure you. Keep one hand in your pocket or behind your back while testing to avoid accidentally touching something.

Turn on the power supply without starting the computer

To test the power supply, you need it to be running but the computer does not need to be fully booted. The easiest way is to use the paperclip trick. Locate the 24-pin motherboard connector and find the green wire — it's usually in the middle of the connector. Find a black wire next to it.

Straighten a paperclip and touch one end to the green wire and the other end to the black wire. The power supply will turn on and stay on. You'll hear the fan inside the power supply start spinning. Leave the paperclip in place while you test.

Alternatively, if the computer is already running, you can test while it's on. The readings will be the same. If you prefer not to use the paperclip method, you can also plug the computer in, start it normally, and run your tests before shutting it down.

Set up your multimeter and take the first reading

Turn on your multimeter and look for the dial or button labeled DC voltage or DCV. Select a voltage range of 20 volts or higher — this range is safe for power supply testing and gives you accurate readings. If your multimeter has an auto-range feature, turn it on.

Hold the multimeter in one hand. The two probes — one black, one red — hang from the bottom. The black probe is ground (negative). The red probe measures voltage (positive). You'll touch the black probe to any black wire on the power supply connector and the red probe to the wire you want to test.

Start with the 24-pin motherboard connector. Touch the black probe to a black wire on the connector. Touch the red probe to a yellow wire on the same connector. The multimeter should display a number between 11.4 and 12.6 volts. Write down the reading. This is the 12-volt rail, and it powers the CPU and other components.

Test the remaining voltage rails

Now test the red wire on the 24-pin connector. Touch the black probe to a black wire and the red probe to a red wire. The reading should be between 4.75 and 5.25 volts. This is the 5-volt rail, which powers the motherboard and some storage drives. Write down this reading too.

If your computer has a separate 8-pin or 4-pin CPU power connector (usually near the top of the motherboard), test that as well. Touch the black probe to a black wire on that connector and the red probe to a yellow wire. It should also read between 11.4 and 12.6 volts.

If your graphics card has a separate power connector (usually a 6-pin or 8-pin cable), test that too. The yellow and red wires should read the same as the main connectors — 12 volts and 5 volts respectively.

Understand what your readings mean

If all your readings fall within the ranges listed above, your power supply is delivering the correct voltage. This is a good sign. The power supply is likely working, and your computer problem is probably caused by something else — a bad motherboard, a failing hard drive, or a software issue.

If a reading is significantly lower than expected — for example, the 12-volt rail reads 10 volts or the 5-volt rail reads 3 volts — the power supply is failing. It's still producing some voltage, but not enough to run the computer reliably. You should replace the power supply.

If a reading is zero or the multimeter shows no number, the power supply is not delivering voltage to that rail. This usually means the power supply is dead and needs replacement. However, check that the paperclip is still touching both wires, or that the computer is still running, before you conclude the power supply has failed.

If readings are higher than the maximum — for example, the 12-volt rail reads 13.5 volts — the power supply is overcharging. This is less common but equally dangerous. Overcharging can damage components over time. Replace the power supply.

Turn off the power supply and close the case

Once you've finished testing, remove the paperclip from the 24-pin connector. The power supply will turn off and the fan will stop spinning. If the computer was running, shut it down normally and unplug it from the wall.

Close the computer case and find it with screws. Plug the computer back in and turn it on to make sure everything still works. If the power supply passed all your tests, you can rule it out as the cause of your problem and move on to testing other components.

Frequently Asked Questions

Do I need to remove the power supply from the case to test it?

No. You can test the power supply while it's still mounted in the case. You only need to access the cables coming out of it, which are straightforward to reach from inside the open case. Removing the power supply is unnecessary and makes the job harder.

What if the multimeter shows different readings each time I test?

Small fluctuations of 0.5 volts are normal. If the readings jump wildly — for example, 12 volts one second and 8 volts the next — the power supply is unstable and failing. Consistent readings within the acceptable range mean the power supply is stable.

Can I test the power supply while the computer is completely off?

No. An off power supply reads zero volts on all rails. You must turn the power supply on using either the paperclip method or by starting the computer. The power supply must be actively running to produce voltage.

What does it mean if one rail reads correctly but another doesn't?

If the 12-volt rail reads correctly but the 5-volt rail reads low, the power supply is failing but not completely dead. Different internal circuits in the power supply handle different voltage rails, and one can fail before the others. Replace the power supply.

Is it dangerous to test a running power supply?

Testing with a multimeter is safe as long as you don't touch metal components inside the case and you keep the probes on the cables only. The voltage in a computer power supply is low enough that it won't injure you if you accidentally touch a probe, but touching internal components while the power supply is running can damage the computer.