How to Test Your Computer’s PSU With a Multimeter

If you have a digital multimeter handy, it’s fairly easy to test your power supply and rule out power gremlins as the cause of your computer problems.

Why use a digital multimeter?

Standalone PSU testers are great and we always keep one on hand for quick results. They can even give you useful readings like the Power Good (PG) value, which shows you how quickly your power supply is going to full power – something a multimeter can’t do.

But many people already have digital multimeters handy and don’t have a power supply tester lying around. So while it’s nice to have a PSU tester for those little extra functions like PG value, you can get almost all of the same data with a multimeter using a more hands-on approach.

How to test your power supply with a digital multimeter

While using a multimeter is a little more convenient than just hooking up a PSU tester, it’s perfectly safe if you follow a few basic guidelines.

Warning: At no point will we open the power supply ourselves. If you do this without proper precautions, knowledge, and tools, you could be in for a fatal shock.

Before proceeding, we would like to highlight a few points. First, it is very safe to test the output of your power supply using the methods outlined below. Opening up the actual power supply to access the “guts” of the device is not the case and exposes you to both power from the wall and the capacitors in the power supply. Touching the wrong thing in the body of the power supply can make your heart stop.

If your power supply isn’t working properly, it’s safest to simply replace it. Attempting to replace large capacitors, transformers, or other internal power supply components is an advanced electronics repair and hardly worthwhile considering how relatively inexpensive power supplies are.

Familiarize yourself with the ATX pinouts

Before we continue, let’s take a look at the 20/24 pin connector to familiarize ourselves with the layout and expected voltages.

We used a handy pinout planner created by Reddit user /u/JohnOldman0 to create the diagram below and recommend the tool to anyone planning a custom cable project.

Holding the connector with the clip facing up, the numbering scheme starts at the bottom left, reads 1-12 on the bottom row, and then 13-24 on the top row for a 24-pin connector. When we use the term “above” in this article, we mean “clip up”.

For a 20-pin connector, it’s 1-10 and 11-20, respectively, although it’s worth noting that even if the pin number changes, the location of the actual voltages doesn’t change. The standard 24-pin ATX connector simply adds 4 extra pins to the 20-pin connector while maintaining the original layout.

Turn off the power supply

If your power supply has a switch, turn it off. If it turns on automatically when plugged into an outlet, unplug the power cord.

In either case, you must turn off the power supply, not just turn off your computer, before proceeding with the next steps.

Disconnect the component cables

You don’t have to remove your power supply from your PC when trying to troubleshoot the existing power supply, but you should unplug all power cords (not just the one you’re testing) to be safe.

While testing a particular cable is unlikely to damage neighboring components, there’s no reason to risk it when it only takes a few seconds to unplug the power cables to your GPU, drives, etc.

Bridge the Power On pin

The first pins to look out for are the power supply pin and adjacent grounds. You need to jumper the power on pin (that’s pin number 16 on the 24 pin display, fourth from the top left) with the ground pin on either side as shown in the ATX pinout diagram above.

You can jump the 16 pin to either the 15 or the 17 pin (both are ground pins). In the photo above you can see that we skipped the 15 and 16 with a short U-shaped paper clip. The lack of isolation isn’t a big deal here since the jumper only carries 24 volts and you won’t be touching it during testing.

You can also use scrap pieces of 18AWG or 16AWG wire. There are also simple ATX 24 pin PSU jumper bridge tools.

The bridge tool has small numbers stamped on it for each of the pinout positions, which is useful when you want a clear indication of which pin is which without counting. (Be forewarned, however, that some multimeters have probes that are just a tad too short to get through the jumper, making it difficult to tap the pins and check the voltage.)

Turn on the power supply

Once you have connected the power pin to a ground pin, turn the power supply back on. You should hear and see the fan on the PSU spinning. Some PSUs have a fan that only spins up briefly during power-up and then idles until the PSU’s temperature rises – so don’t be alarmed if the fan spins and then stops a few seconds later.

Test the pins with your digital multimeter

Testing your power supply with a digital multimeter is not much different than using a power supply tester. The main difference is that instead of having a little microchip doing the calculations and giving the thumbs up or down, you get the hands-on experience of being the microchip and interpreting the data yourself.

At this point you need to turn on your multimeter and set the reading to DCV. If your multimeter is auto-ranging you don’t have to do anything, if you need to range set it to 10V.

Place the black multimer probe on one of the grounded pins. On a standard 24-pin ATX connector, this would be pin 3, 5, 7, 15, 17, 18, 19, or 24. We’re using pin 15 because it’s easy to identify because of its location right next to the power jumper.

Touch any other pin with the black probe on a grounded pin and verify that the reading is as expected.

For example, if you ground pin 15 and touch pin 24, the reading should read 3.3V (or within ±5% of 3.3V). You can see in the photo above that our 15 pin to 24 pin connection is dead with a 3.3V indicator.

Repeat this process for all pins and confirm that the voltage reading is within the acceptable range. If the values ​​are not within the range, it is time to replace the power supply. Here is the pinout of the ATX power connector again for reference.

And here are the pinouts for the 8-pin (4+4) ATX/PCIe, 8-pin (6+2) ATX/PCIe, and Molex drive connector if you want to test those pins too.

As with the larger 24-pin power connector, simply ground your black multimeter probe to a known ground (each of the black pins at the top), then touch the red probe to the other pins to check its voltage. You should check them for the same ±5% range.

In the interests of protecting your hardware, we won’t even suggest gameplay parameters here. If one or more readings are outside the ±5% range, simply swap out the power supply and save yourself the headache of a failing power supply.