Testing a power supply before you buy a replacement

A failing power supply often looks like a dead computer—the machine won't turn on, shuts down randomly, or freezes without warning. Before you spend money on a new unit, you can run several tests to confirm the power supply is actually the problem. The most reliable method is the paperclip test, which takes five minutes and costs nothing. You can also use a multimeter to check voltage, or run diagnostic software if your computer still powers on.

The paperclip test works because it forces the power supply to turn on without the motherboard. If the fan spins and the supply stays stable, the unit itself is likely fine—the problem may be elsewhere. If nothing happens or the fan spins erratically, the power supply is probably dead.

Key Takeaways

  • The paperclip test is the fastest way to check if a power supply is working; it requires only a paperclip, a screwdriver, and about five minutes.
  • You must unplug the computer and wait at least 30 seconds before opening the case, because power supplies store electrical charge even when off.
  • A multimeter can measure the actual voltage output from each rail, which tells you whether the supply is delivering the correct power.
  • If the power supply passes the paperclip test but your computer still won't start, the problem is likely the motherboard, CPU, or RAM, not the power supply.
  • Never touch the inside of a power supply case itself, even after unplugging—the capacitors inside can hold a dangerous charge for hours.

How to perform the paperclip test

Unplug the computer completely from the wall outlet. Wait at least 30 seconds—this allows the power supply's internal capacitors to discharge. Open the computer case using a screwdriver; the method depends on your case design, but most have two or four screws on the back panel.

Locate the 24-pin ATX connector, which is the large rectangular plug attached to the motherboard. This is the main power cable from the supply. Gently unplug it from the motherboard by pressing the small plastic clip on the connector and pulling straight out. Do not force it.

Straighten a paperclip and insert one end into the green wire pin and the other end into any black wire pin on the 24-pin connector. The green wire is the power-on signal; shorting it to ground (black) tells the power supply to turn on. Plug the power supply back in and switch the wall outlet or power switch to on. The fan inside the power supply should spin within a few seconds. Let it run for 10 to 15 seconds, then unplug it again.

If the fan spins smoothly and steadily, the power supply is working. If the fan doesn't spin, spins very slowly, or makes grinding noises, the power supply is likely faulty. If the fan spins but when ready stops, or if you hear crackling or smell burning, stop the test when ready and do not use the power supply.

Using a multimeter to check voltage

A multimeter is a handheld tool that measures electrical voltage, current, and resistance. You can buy one at any hardware store for $15 to $40. This test is more detailed than the paperclip test because it tells you the exact voltage on each power rail, not just whether the supply turns on.

Set the multimeter to DC voltage mode (usually marked with a V with a straight line above it). Unplug the computer and wait 30 seconds. Unplug the 24-pin ATX connector from the motherboard. Insert the paperclip into the green and black pins as described above, then plug the power supply in.

Once the fan is spinning, touch the multimeter's red probe to the red wire (5V rail) and the black probe to a black wire (ground). The reading should be between 4.75 and 5.25 volts. Repeat this test on the yellow wire (12V rail)—it should read between 11.4 and 12.6 volts. Test the orange wire (3.3V rail)—it should read between 3.14 and 3.47 volts. If any reading is outside these ranges, the power supply is failing.

Testing when the computer still powers on

If your computer turns on but shuts down randomly, freezes, or restarts without warning, the power supply may be unstable under load. In this case, you can run diagnostic software while the computer is running.

read and run a stress-testing tool like Prime95 (free, from mersenne.org) or MemTest86 (free, bootable from a USB drive). These programs push your CPU and RAM to maximum load, which also draws maximum power from the supply. If the computer crashes, restarts, or shuts down during the test, the power supply is likely the culprit. If the test completes without errors, the power supply is probably fine, and the problem is elsewhere.

Another sign of a failing supply under load is if your computer shuts down suddenly when you run a demanding program like a video game or video editor, but stays on during light tasks like web browsing. This pattern strongly suggests the power supply cannot deliver enough current when demand spikes.

What to check before concluding the power supply is bad

Before you replace the power supply, confirm that the problem is not something simpler. Check that the power cable is fully plugged into both the wall outlet and the back of the computer. Try a different outlet on a different circuit—a tripped breaker or a failing outlet can look like a dead power supply.

If the computer powers on but won't boot into the operating system, the problem is almost certainly not the power supply. A failing supply causes the computer to not turn on at all, or to shut down under load. If the machine turns on but the screen stays black, or if you hear beeping sounds, the issue is likely the motherboard, RAM, or graphics card.

Check that all internal power connectors are fully seated. The 24-pin ATX connector, the 8-pin CPU power connector (near the processor), and any 6-pin or 8-pin PCIe connectors for graphics cards should all click firmly into place. A loose connector can cause the same symptoms as a failing power supply.

When to replace versus repair

Power supplies are not user-serviceable. If the paperclip test shows the supply is dead, or if the multimeter readings are far outside the correct ranges, replacement is the only option. Repairing a power supply requires specialized knowledge and tools, and the cost of repair is usually close to the cost of a new unit.

When you buy a replacement, match the wattage of your current supply or go slightly higher. Check the label on the side of your old power supply for the wattage rating (usually 450W, 550W, 650W, or higher). If you are upgrading your graphics card or adding new components, you may need a higher wattage—a graphics card's power requirements are listed on the manufacturer's website.

Frequently Asked Questions

Can I get shocked by the paperclip test?

The paperclip test itself is safe because you are only touching the connector pins, which carry low voltage. However, never touch the inside of the power supply case or any components inside it. The capacitors inside can hold a lethal charge for hours after unplugging. If you must open the power supply itself, take it to a professional.

What if the fan spins but the computer still won't turn on?

The paperclip test only tells you the power supply can turn on—it does not test whether it delivers stable power under the motherboard's control. If the fan spins but your computer won't start, the problem is likely the motherboard, CPU, RAM, or a loose internal connector. Check that all power connectors are fully seated before assuming the supply is bad.

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

No. You can perform the paperclip test and multimeter test with the power supply still mounted in the case. You only need to unplug the 24-pin connector from the motherboard and access the connector pins. Removing the supply is not necessary unless you plan to replace it.

How long should I let the power supply run during the paperclip test?

Run it for 10 to 15 seconds. This is long enough to confirm the fan spins smoothly and the supply is stable. Do not run it for more than a minute, because the supply is not designed to run without a load for extended periods. Unplug it after 15 seconds.

What does it mean if the multimeter reading is slightly off?

Power supplies are allowed to vary slightly from the nominal voltage. A 5V rail reading 4.9V is normal; a reading of 4.5V is a problem. If any reading is more than 5 percent below the target, the power supply is failing. Check your multimeter's manual for the exact tolerance range, or use the ranges listed above as a safe guideline.