Testing a power supply tells you whether it can still deliver power to your computer, or whether it is failing and needs replacement

A power supply unit (PSU) converts wall power into the direct current your motherboard, graphics card, and drives need. When one starts to fail, your computer may shut down randomly, refuse to start, or show signs of instability that look like software problems. Testing it directly is faster and cheaper than replacing parts blindly.

You have three practical routes: the paperclip test (checks whether the PSU itself powers on), using a multimeter to measure voltage on the cables, or swapping in a known-good power supply to see if the problem follows it. The paperclip test takes five minutes and requires no tools. A multimeter test is more thorough but requires a basic understanding of how to read one. A swap test is the most reliable but only works if you have access to another computer or PSU.

Key Takeaways

  • The paperclip test shows whether your power supply can turn on at all by bridging two pins on the 24-pin motherboard connector with a paperclip while the PSU is plugged in.
  • A multimeter can measure the actual voltage output on the 12V, 5V, and 3.3V rails to see whether they are within the normal range of roughly 10 to 13 volts, 4.5 to 5.5 volts, and 3 to 3.6 volts.
  • If the paperclip test fails, the PSU itself is likely dead and needs replacement; if it passes but your computer still will not start, the problem is probably elsewhere.
  • Swapping the PSU with a known-good one from another computer is the fastest way to confirm whether the power supply is the cause of your shutdown or startup problems.

The Paperclip Test: Checking Whether the PSU Powers On

The paperclip test is the quickest way to find out whether your power supply can turn on by itself. It works because the PSU needs a signal from the motherboard to start, but you can fake that signal with a paperclip.

Shut down your computer completely and unplug the power cable from the wall. Open the case and locate the 24-pin motherboard connector—the large flat plug with two rows of pins coming out of the PSU. Look at the pins from the front (the side facing you when the case is open). The pin you need is in the bottom row, fourth from the left. It is usually green. Next to it, on the left, is a black pin (ground). Insert a paperclip so it bridges the green pin and the black pin next to it.

Plug the PSU back into the wall outlet. The fan inside the PSU should spin up within a second or two. If it does, your PSU can power on. If nothing happens, the PSU is likely dead. Unplug the PSU again and remove the paperclip. If the fan spins but your computer still will not start normally, the problem is somewhere else—likely the motherboard, a loose cable, or a failed component.

Using a Multimeter to Check Voltage Output

A multimeter is a handheld meter that measures voltage, current, and resistance. You can use one to check whether the PSU is actually outputting the correct voltage on each rail. This test is more detailed than the paperclip test but requires you to read the meter and understand what the numbers mean.

Set your multimeter to DC voltage mode (usually marked with a V with a straight line above it, not a wavy line). Plug the PSU into the wall and turn it on using the paperclip method, or have someone press the power button on the computer while you measure. Touch the red probe to a pin on one of the PSU cables and the black probe to a ground pin (any black wire). The 24-pin motherboard connector has voltage on the yellow wires (12V), red wires (5V), and orange wires (3.3V). A healthy PSU should read roughly 12 volts on yellow, 5 volts on red, and 3.3 volts on orange, with a tolerance of about ±10 percent. That means 12V can range from about 10.8 to 13.2 volts and still be acceptable.

If all three rails read within range, your PSU is likely fine. If one or more rails are low, drifting, or reading zero, the PSU is failing. If the readings are wildly off—say, 8 volts on the 12V rail—the PSU cannot deliver stable power and should be replaced.

Swapping the Power Supply to Isolate the Problem

If you have access to another computer with a working PSU of similar wattage, you can swap it into your computer to see whether the problem goes away. This is the most reliable test because it removes the PSU from the equation entirely.

Shut down your computer and unplug it. Open the case and disconnect all the cables from the old PSU—the 24-pin motherboard connector, the 8-pin CPU power connector (usually near the top of the motherboard), and any 6-pin or 8-pin connectors for graphics cards or drives. Note where each cable goes, or take a photo. Remove the four bolts holding the PSU to the case and slide it out.

Install the known-good PSU in the same position, reconnect all the cables in the same order, and close the case. Power on the computer. If it starts and runs normally, your original PSU was the problem. If the same issue happens again—random shutdowns, failure to start, or instability—the problem is not the power supply, and you should look at the motherboard, RAM, or storage drive instead.

What to Do If the Tests Show a Failed Power Supply

If the paperclip test fails, the multimeter shows voltage that is too low or absent, or the computer works fine with a different PSU, your power supply needs replacement. Note the wattage and connector type of your old PSU—you can find this on the label on the side of the unit. Most desktop computers use 500 to 750 watts; laptops use external adapters that are not replaceable in the same way.

When you buy a replacement, match the wattage and make sure the physical size fits your case. A PSU rated for higher wattage than your old one is fine and often costs the same. Installation is the reverse of removal: slide the new PSU into the case, bolt it down, and reconnect all the cables. If you are not comfortable opening your case, a repair shop can do this for $50 to $100 in labor.

When the Tests Pass but Your Computer Still Has Problems

If the paperclip test passes and the multimeter shows good voltage, but your computer still shuts down randomly or fails to start, the problem is not the power supply. Look at other common causes: loose RAM, a failing hard drive or SSD, an overheating CPU, or a motherboard that is beginning to fail.

Check that all RAM sticks are fully seated in their slots—push down on both ends until you hear a click. Reseat the CPU cooler to make sure it is making good contact. Listen for clicking or beeping sounds from the hard drive, which can signal a mechanical failure. If your computer is old and has been running hot, dust buildup in the heatsinks and fans can cause thermal shutdowns. Open the case and use compressed air to blow out dust from the CPU cooler, graphics card cooler, and case fans.

Safety Precautions When Testing a Power Supply

A power supply stores electrical charge even when unplugged, so always unplug it from the wall before opening the case or disconnecting cables. Wait a few seconds after unplugging to let any residual charge dissipate.

Do not touch the large cylindrical capacitors inside the PSU—they can deliver a painful shock. You do not need to open the PSU itself to test it. All the tests described here work on the cables and connectors outside the unit. If you are uncomfortable working inside a computer case, or if you are not sure whether you have unplugged it fully, take it to a repair shop instead of risking injury.

Frequently Asked Questions

Can a power supply fail partially, where some cables work but others do not?

Yes. A PSU can lose the ability to deliver stable power on one rail while others still work. This is why the multimeter test checks all three voltage rails separately. You might see the 12V rail drop to 8 volts while the 5V rail stays normal, which would cause instability on devices powered by the 12V line.

What if the paperclip test makes the fan spin but it stops after a few seconds?

That is normal. The PSU fan will run for a moment and then stop because there is no load on it—no motherboard drawing power. As long as the fan spins at all, the PSU can power on. If it does not spin at all, the PSU is dead.

Do I need a special multimeter to test a power supply?

No. Any basic digital multimeter that measures DC voltage will work. They cost $10 to $30 at hardware stores or online. You do not need an expensive one or any special features.

Is it safe to touch the cables while the PSU is powered on during testing?

Yes, the cables themselves are safe to touch. The voltage on them is low enough that it will not hurt you. Do not touch the inside of the PSU or the large capacitors, but the cables and connectors outside are fine.

How often should I test my power supply to make sure it is still working?

You do not need to test it regularly. Test it only when your computer shows signs of trouble—random shutdowns, failure to start, or crashes that seem unrelated to software. A healthy PSU can run for five to ten years without problems.