How do I test a computer power supply without breaking anything?

You’re sitting there, staring at a black screen. You’ve pressed the power button twelve times, and nothing happened. No fans spinning. No beep. Just the cold, mechanical silence of a dead PC. It’s frustrating. Your first instinct is probably to blame the motherboard or maybe some expensive GPU you just bought, but honestly, it’s usually the Power Supply Unit (PSU). The big metal box. The heart of the machine. But how do I test a computer power supply without just buying a new one and hoping for the best?

It's actually easier than you think. You don't need a degree in electrical engineering, though a little healthy respect for electricity is mandatory. If you mess this up, you aren't just looking at a dead computer; you're looking at potential sparks. We're going to walk through the "Paperclip Test," the multimeter route, and the dedicated tester method. Each has its own vibe. One is quick and dirty, one is precise, and one is just for people who want a "yes or no" answer without the math.

The "Paperclip Test" is the classic first step

If you've spent any time on Reddit’s r/buildapc, you’ve heard of this. It sounds sketchy. Using a piece of stationary to jumpstart a high-voltage component? Yeah, it's a bit wild. But it works because of how ATX power supplies are designed. Basically, the PSU waits for a specific signal from the motherboard to turn on. When you bridge two specific pins on the 24-pin connector, you're faking that signal.

First, unplug everything. I mean everything. Disconnect the PSU from the wall, then pull every single cable off your components. Don't leave the GPU plugged in. Don't leave the hard drives attached. If the PSU surges while you're doing this, you don't want it taking your $500 graphics card down with it.

Get a metal paperclip. Bend it into a "U" shape. Now, look at the big 24-pin connector—the one that usually plugs into the right side of your motherboard. You’re looking for the Green wire. There is only one. This is the "PS_ON" pin. You need to connect that green wire's socket to any Black wire (Ground) socket right next to it.

Once that's secure, plug the PSU back into the wall and flip the switch on the back. If the fan spins, the PSU isn't totally dead. It’s alive! Sorta. The catch is that a spinning fan doesn't mean the PSU is "healthy." It just means it can turn on. It might still be failing under load, or the voltage might be fluctuating wildly enough to crash your BIOS.

Why "it turns on" isn't good enough

Electricity is finicky. A power supply takes 120V (or 240V) from your wall and converts it into three main "rails": 3.3V, 5V, and 12V. Your CPU and GPU live and die by that 12V rail. If your PSU is putting out 10.5V instead of 12V, the fan will still spin, but your computer will crash the second you try to load a game.

This is where the paperclip test fails you. It's a binary test. Yes or No. To really know if you're safe, you need a multimeter or a dedicated PSU tester. Honestly, if you're doing this more than once every three years, just buy a digital PSU tester. They cost like $15 on Amazon or at a local Micro Center. You plug the 24-pin, the CPU 8-pin, and maybe a SATA cable into the tester, and it shows you the exact voltage on a little LCD screen.

If you see "LL" or "HH" on the screen, or if it starts beeping like a bomb, your PSU is toasted. Toss it. Don't try to fix it. Opening a PSU is a great way to get hit by a capacitor that’s holding enough charge to ruin your week, even if it’s been unplugged for an hour.

Busting out the multimeter for the real nerds

If you already own a multimeter and you're wondering how do I test a computer power supply with actual precision, this is the gold standard. It’s more reliable than the cheap plastic testers because those testers sometimes have their own internal variance.

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Set your multimeter to DC voltage. Usually, the 20V range is perfect. You still have to do the paperclip trick to get the PSU to turn on, otherwise, there won’t be any power flowing through the pins to measure.

1. Stick the black probe of your multimeter into any black wire socket on the 24-pin connector.
2. Stick the red probe into the yellow wire socket. This is your 12V rail.
3. Check the screen. You want to see something between 11.4V and 12.6V. That's the 5% tolerance window defined by the ATX specification.
4. Move the red probe to a red wire. This is the 5V rail. You’re looking for 4.75V to 5.25V.
5. Move the red probe to an orange wire. This is the 3.3V rail. You want 3.14V to 3.47V.

If any of these are outside those ranges, your PSU is a ticking time bomb. High voltage can fry components; low voltage causes "spontaneous reboots" that make you want to throw your PC out a window. It’s the subtle failures that are the most annoying.

Real talk about "coil whine" and smells

Sometimes the PSU passes all the electrical tests, but it sounds like a swarm of angry bees. That’s coil whine. It’s usually vibration in the electromagnetic coils. It’s annoying, but it’s not technically "broken." However, if your PSU smells like burnt ozone or "hot electronics," stop immediately.

I once had a client who insisted their PC was fine because the "smoke only happened once." No. If the magic smoke leaves the box, the box is done. Capacitors can leak, or worse, "pop" like a firecracker. If you see a bulging capacitor through the grill of the PSU, don't even bother with the paperclip. It’s trash.

What about testing under load?

This is the hardest part. A PSU might look perfect while it's just sitting on your desk. But the moment your RTX 4090 starts pulling 450 watts, the voltage might sag. You can't really test this with a paperclip or a cheap tester.

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The best way to "load test" without specialized equipment is to use software like OCCT or HWiNFO64 while the PSU is actually in the computer. These programs read the sensors on your motherboard. Look at the 12V rail while running a stress test. If you see that number dipping below 11.4V during a heavy benchmark, your PSU is likely too weak for your build or is aging poorly. PSU components, specifically capacitors, degrade over time. A 750W power supply from 2015 might only be capable of a stable 600W in 2026.

The "Wall Power" factor

Before you go buying a new Corsair or Seasonic unit, check your wall. I’ve seen people "diagnose" a bad PSU when the real problem was a $5 power strip from 1998 that couldn't handle the draw. Or a loose wall outlet.

Try plugging the PSU directly into a wall socket in a different room. If the PC magically starts working, the problem isn't the computer; it's the circuit you're on. This happens way more often in older houses with sketchy wiring.

Actionable steps for your hardware

Stop guessing. If your PC won't post, follow this sequence:

• Check the physical switch: Seriously. Check the I/O switch on the back. Check the wall plug. Replace the power cord itself—they do go bad sometimes.
• The Paperclip Test: Do this to see if the unit is totally "braindead." If the fan doesn't spin with the pins jumped, it's dead.
• Voltage Verification: Use a multimeter or a digital PSU tester ($15-20) to check the 12V, 5V, and 3.3V rails. If they are off by more than 5%, replace the unit.
• Inspect for physical damage: Look for dust buildup (overheating) or bulging capacitors through the vents.
• Software Monitoring: Use HWiNFO64 to watch your voltages while you play a demanding game. If the 12V rail drops significantly under load, your PSU is failing or underpowered.

If the PSU is more than 7-10 years old and acting up, don't waste time trying to "save" it. A bad PSU is the only component in a computer that can literally kill every other part in the system. When in doubt, swap it out. It's cheaper than a new motherboard and CPU.

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Invest in a unit with at least an 80 Plus Gold rating from a reputable brand like Seasonic, EVGA, or Corsair. Check the "PSU Tier List" (a community-maintained database often found on Cultists Network) to make sure you aren't buying a "Tier F" fire hazard.

That’s the reality of it. Testing a power supply isn't just about finding out if it's dead; it's about finding out if it's "clean." Clean power keeps your hardware alive for a decade. Dirty power kills it in two years. Choose the multimeter or a dedicated tester if you want peace of mind, and never, ever open the metal casing of the PSU itself. It’s just not worth the risk.