How to Test a Power Supply

how to test a power supply
Knowing how to test a computer power supply unit might be one of the number one tools in your arsenal for troubleshooting your system when it goes on the fritz.

Owing to its sheer power and importance in the hierarchy, the PSU (power supply unit) is often first up on the list of hardware most likely to fail, especially for a computer with some years on it. This can cause random reboots, lockups such as the system randomly hanging, and issues that can run the gamut throughout the whole computer due to improper sensor operation.

Investing in the best power supplies is always a key. However, the efficiency of a power supply unit gradually falls with time and knowing when to replace one is even more important. This can be achieved by knowing how to test a power supply.

Why know how to test a computer power supply?

The indications of a faulty power supply unit could range from problems in the way your computer performs or “sounds”:

  • Case fans and hard disks can’t be heard or seen spinning
  • The blue “screen of death” occurs often and intermittently
  • The system doesn’t move on from the boot-up process at all
  • The system spontaneously restarts or lockouts while being in use
  • The system doesn’t power on at all or to begin with
  • System memory insufficient for function
  • Flashing lights on the motherboard
  • Overheating computer
  • Voltage fluctuations
  • Failing power rails

How to Test a Computer Power Supply

There are generally two ways to test a power supply: with a multimeter or through a device known as an automatic power supply tester. Please note, however, that the former – the multimeter test – should only be performed very carefully and usually only when you have previous experience with electrical devices and tests.

Before attempting to execute a millimeter test – perhaps the most nuanced of all the possible ways to test a power supply unit – try either the jumper test or check to see if the system is under warranty. It’s also possible that the PC not turning on at all might be the result of other factors such as disconnected wiring rather than the actual PSU having failed.

If your PC doesn’t turn on at all (especially if it’s a desktop with a separate CPU and all sorts of peripherals and wires), check to see the following:

  • Is there an external switch, such as on the back of the CPU? Check to see if this is turned on.
  • Try a different power cable and different power supply, alternatingly and with different combinations. The power supply might have a light that can help in knowing easily when it’s powered on.
  • Check the lights on the motherboard case and listen for BIOS beeps, then refer to your motherboard product manual to see what the lights and sounds combined could mean. Flashing lights, for example, almost always means the power supply is faulty.

Alternatively, the paper-clip test can be employed.

How to test a power supply with a simple paper clip

The paper-clip test (also known as the jumpstart or jumper test as previously mentioned) is relatively one of the safest ways to test the PSU, with no contact needed with live wires or a live power source.

With the jumper test, you can pinpoint issues such as failed components, a live power connection, or short circuits inside the PSU, allowing you to know if the unit needs to be replaced. If the 3-step test is passed, you can move on to other factors such as power rail failure, overheating, and voltage fluctuations (yes, the last three on our list from two sections ago).

  1. Turn off the power supply. This can be done by flipping the power switch at the rear of the PSU to the off position (essentially meaning that the ‘O’/zero symbol should be in the ‘down’ position).
  2. Insert one end of the paperclip into the PS_ON (green) pin on the 24-pin (20+4P) motherboard connector, and the other into any of the Ground (black) pins.
  3. Turn the PSU back on (by flipping the switch in Step 1). If you can hear an internal fan, your PSU has passed the paper clip test and is turned on.

How to test a power supply with a millimeter test

Learning this will also equip you with the knowledge needed of knowing how to test a laptop power supply.

A multimeter is a tool used primarily by electronics technicians to measure electrical current (in the form of amps, volts, and ohms). It is a simple enough tool to use, but caution should be taken nonetheless whenever enlisting the help of one in a DIY adventure:

  • Turn off the computer and unplug it, and then wait a few minutes to let the capacitors drain any remaining electric charge.
  • Take off anything conductive, such as metal rings, bracelets, and other hand jewelry.
  • Tell yourself when to walk away and not get too intensively invested: the possible damage to other components (or yourself) will be worth less than simply knowing the PSU is replaceable.

These measures are particularly important in a workplace context or when using a computer that’s under warranty. It should also be noted that these tests should only be used to know when to replace a PSU: do not attempt to fix one on your own unless you are a professional (who probably does not need this article at all, then).

Once the necessary precautions have been taken and everything has been unplugged from the PSU, open the case with a screwdriver, and follow the ten steps as listed below:

  1. Make sure all the power connectors are unplugged.
  2. Make sure that there has been a considerable wait period, and that you are preferably not working in the same area as the computer.
  3. Reroute and keep the power cables organized for easier execution of the final steps.
  4. Find the pins 15 and 16 on the 24-pin (20+4P) connector to short them out, preferably using an online pinout table to locate them on your specific motherboard.
  5. Ensure that the correct voltage is set on your PSU voltage switch, again preferably using an online outlet electricity guide.
  6. Plug the PSU back in, turn on the power switch if there is one on the rear, and check to see if you can hear the internal fans. Continue testing even if you hear them functioning properly.
  7. Turn the multimeter on, set the dial to the VDC (Volts DC) setting and, unless it has an auto-ranging feature, set the range to 10.00V.
  8. Test the 24-pin (20+4P) motherboard power connector, preferably every pin with a voltage. Connect the black (negative) and red (positive) probes to any ground wired pin and the first power line you want to test, respectively. None that the main power connector has lines +3.3 VDC, +5 VDC, an optional -5 VDC, +12 VDC, and -12 VDC across multiple pins.
  9. Document and note the voltage on each number and verify that it’s not over nor under the specifically approved voltage tolerance. If any of them go over the tolerance (again, refer to guides online, easily available across Wikipedia and websites for your specific unit), your PSU needs replacing.
  10. Upon completing the test, replace the cover on the PSU and plug everything back the exact way it was (see how simple it is when we already paid attention to the first step)?

The benefits of using a multimeter over an automatic PSU tester

Knowing how to test a power supply is effective no matter which way you choose between these two, but there are still merits and demerits to each method:

A “manual” PSU test (using a multimeter) can be…

  • …performed instantaneously with no other tools except a screwdriver and multimeter. Even if you own none of these, your total cost could be well under 40 bucks or so.
  • …dangerous, and requires work, careful attention, focus, and usually some previous experience in electronics work and maintenance.

Conversely, you can perform an automatic test on your PSU using a power supply tester.

  • A power supply tester gives a more conclusive result and reduces (or removes completely) the element of human error, direct exposure to live electricity, faulty components, and the need for previous experience or supervision.
  • The cost could possibly be the exact same (or even way, way less) as buying a new screwdriver and multimeter, but it would require a bit of research beforehand and a waiting period if you’re ordering one online (which is a prime way to get a cheaper model that would still get the job done reliably).

In either case, however, the necessary precaution needed cannot be forgone, lest you end up electrocuted.

Conclusion

Even if you know how to test a power supply, it’s useful to keep yourself limited to just testing. Fixing one might be a bigger hassle than just buying a new one, both mentally and financially. If your PSU has failed even a single test, whether a simple one or a more complex and rigorously detailed set of procedures, it’s time to replace your unit or to contact a professional repairman.

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