Testing your power supply before it fails

A failing power supply often shows itself through random shutdowns, no power at all, or a computer that turns on briefly then dies. Before you buy a replacement, you can run a few tests to confirm the power supply is the problem. The most reliable method is a paperclip test, which forces the power supply to run without a computer attached so you can listen and feel for signs of failure. You can also use a multimeter to measure the actual voltage coming from each cable, which tells you whether the power supply is delivering the right amount of power to each component.

Both tests take 10 to 15 minutes and require no special skills. The paperclip test costs nothing if you already have a paperclip. A multimeter costs $10 to $30 and is useful for other household electrical work, so it is a reasonable investment if you do not own one yet.

Key Takeaways

  • The paperclip test turns on a power supply outside your computer so you can hear and feel whether the fan spins normally and the unit stays cool.
  • A multimeter measures the voltage on each power cable to show whether the supply is delivering the correct amount of power to your motherboard, CPU, and drives.
  • If the fan does not spin at all or spins erratically, or if voltage readings are more than 5% below the labeled value, the power supply is likely failing.
  • A power supply that passes both tests is probably not your problem; random shutdowns or no-power symptoms usually point to the motherboard or CPU instead.

The paperclip test: checking whether the power supply turns on

The paperclip test works because the power supply will not start unless it senses a load on the motherboard connector. By bridging two pins with a paperclip, you trick it into thinking a motherboard is attached. This lets you run the fan and listen for problems without risking damage to your computer.

First, unplug the power supply from the wall and from your computer. Locate the 24-pin motherboard connector—the largest cable coming out of the power supply, with a plastic housing and two rows of pins. Look at the pins from the front (the side facing you). Find the green wire and the black wire next to it. Insert a paperclip so it bridges the green pin and one of the adjacent black pins. The exact black pin does not matter as long as it is next to the green one.

Plug the power supply back into the wall outlet. The fan should start spinning within a few seconds. Listen for a smooth, steady hum. The fan should spin continuously without stuttering, stopping, or making grinding noises. Feel the metal case—it should be warm but not hot enough to hurt your hand. Leave it running for 30 seconds, then unplug it from the wall and remove the paperclip.

If the fan does not spin at all, spins only briefly then stops, or makes grinding or squealing sounds, the power supply is failing and should be replaced. If the fan spins smoothly and the case stays warm, move on to the multimeter test for a more detailed picture.

Using a multimeter to measure voltage on each cable

A multimeter measures the actual voltage coming from the power supply. This tells you whether the supply is delivering the correct amount of power to each part of your computer. You will need a multimeter set to DC voltage mode and a way to power on the supply—either by leaving the paperclip in place or by bridging the green and black pins again.

Set the multimeter to DC voltage (marked as V with a straight line, not a wavy one). Insert the black probe into the COM port and the red probe into the V port. Plug the power supply into the wall and bridge the green and black pins again so the fan runs. The power supply will stay on as long as the paperclip is in place.

Locate the 24-pin motherboard connector. The pins are labeled on the connector housing or in your power supply manual. You are looking for pins labeled +12V, +5V, +3.3V, and GND (ground). Touch the red probe to a +12V pin and the black probe to a GND pin. The multimeter should read between 11.4V and 12.6V. If it reads 12V exactly, that is ideal. If it reads 11V or 13V, the supply is drifting out of spec. Repeat this for a +5V pin (should read 4.75V to 5.25V) and a +3.3V pin (should read 3.14V to 3.46V).

Write down the readings. If all three are within the ranges above, the power supply is delivering stable voltage. If any reading is more than 5% below the labeled value—for example, 11V when it should be 12V—the supply is failing and should be replaced.

Testing the 8-pin CPU power connector

The CPU power connector is a separate 8-pin or 4-pin cable that supplies power directly to the processor. It sits near the top of the motherboard, close to the CPU socket. This connector carries +12V power, and a failing supply often shows up here first.

With the multimeter still set to DC voltage and the paperclip still bridging the green and black pins, locate the 8-pin CPU connector on the power supply. Touch the red probe to any of the yellow wires (all carry +12V) and the black probe to any of the black wires (ground). The reading should be between 11.4V and 12.6V, the same as the motherboard connector.

If the CPU connector reads significantly lower than the motherboard connector—for example, 11.2V when the motherboard reads 12V—the power supply is struggling to deliver stable power under load. This is a sign of failure.

Checking the SATA and Molex connectors

SATA connectors power modern hard drives and SSDs. Molex connectors (the older 4-pin type) power older drives and some fans. Both should deliver stable voltage if the power supply is healthy.

For a SATA connector, touch the red probe to a red wire (should read 5V) and the black probe to a black wire. The reading should be between 4.75V and 5.25V. For a Molex connector, test the red wire against black (should read 5V) and the yellow wire against black (should read 12V). If either is more than 5% low, the supply is failing.

You do not need to test every connector—testing one of each type is enough. If the motherboard and CPU connectors are stable, the SATA and Molex connectors usually are too.

What to do if the tests show a failing power supply

If the paperclip test shows the fan does not spin or spins erratically, or if the multimeter shows voltage more than 5% below spec on any connector, the power supply is failing and needs to be replaced. Note the wattage (usually printed on a label on the side of the unit) and the connector type (24-pin motherboard, 8-pin or 4-pin CPU, and how many SATA or Molex connectors you need). Buy a replacement with the same or higher wattage and the same connector layout.

If both tests pass—the fan spins smoothly and all voltages are within spec—the power supply is probably not your problem. Random shutdowns or no-power symptoms usually point to the motherboard, CPU, or RAM instead. In that case, consider having a technician test those components, or try swapping in a known-good power supply from another computer to rule it out completely.

Safety reminders when testing a power supply

A power supply stores electrical charge even when unplugged. Always unplug it from the wall before inserting or removing the paperclip. Do not touch the metal case or any internal components while the supply is plugged in. If you are uncomfortable working with electrical components, ask a technician to run these tests for you.

The paperclip test is safe because you are only bridging two pins on the connector, not touching live wires. The multimeter test is also safe because you are only touching the probe tips to the connector pins, not to bare wires. Keep the paperclip in place during the multimeter test so the supply stays powered on; remove it as soon as you are done testing.

Frequently Asked Questions

Can I test the power supply while it is still plugged into my computer?

No. Always unplug the power supply from the wall and from the computer before inserting the paperclip. Testing while connected risks damaging the motherboard or other components. The paperclip test is designed to run the supply in isolation.

What if the fan spins but the computer still will not turn on?

A spinning fan means the power supply itself is working, but it does not rule out other problems. The motherboard, CPU, or RAM could be failing, or the power supply might not be delivering enough power under load. If the multimeter shows all voltages within spec, the issue is likely elsewhere in the computer.

Do I need to buy a multimeter, or can I test without one?

The paperclip test alone tells you whether the power supply fan spins and the unit stays cool. If the fan spins smoothly, the supply is probably working. A multimeter gives you more detail and confidence, but it is not required if you are just trying to rule out a dead power supply.

How long can I leave the paperclip in place?

You can leave it in for several minutes while you run the multimeter test. The power supply will stay on as long as the paperclip bridges the green and black pins. Remove it as soon as you are done testing. Do not leave it in place for more than 10 minutes, as the supply will heat up without a load to regulate.

What if the voltage is slightly low but the computer still works?

A power supply that is slightly out of spec may still work for a while, but it is degrading. If voltage is consistently 5% or more below the labeled value, the supply is failing and will likely cause problems soon. It is safer to replace it now than to wait for a shutdown during important work.