Short Circuit Protection

September 17, 2021—I'm happy to report that my power supply has it

My PC power supply recently bit the dust. It was during a storm when the power cut out. My PC wouldn’t turn back on afterward. Maybe a power surge? At any rate, I ordered a new power supply and it finally arrived yesterday evening.

And I’m happy to report that my new power supply has short circuit protection. Here’s how I found out.

I took the cover off my PC case and freed the old power supply from its screws. Both the old and the new power supply are modular: instead of all the cables dangling out of it you only plug in the ones you need. So I unplugged all the cables from the power supply end. “This will be an easy job” I thought to myself, “all I have to do is drop the new power supply in and replug all these modular cables.”


First I noticed the main motherboard power connector (the big 24 pin one) was different. Of course it’s the same on the motherboard end, but on the power supply the 24 pins are split up into two plugs: something random like 10 pins on one plug and the remaining 12 pins on another one; two different modular slots separated by a little distance on the power supply. Did motherboards used to only take 10 or 12 pin connectors? It was clear that my old plug was not going to work.

“No problem, I’ll just use the cable that came with the new power supply.”

Next was the auxiliary 8 pin motherboard power plug. Both ends fit.


The graphics card power plug was different on the power supply side. It had the same number of pins in the same pattern, but they were keyed slightly different. I don’t really like the new cord. It is flat instead of braided so it doesn’t sit right in the case. And it has two graphics card plugs on the one end even though I only have one graphics card. Who wants an extra plug dangling in their case?

Mumble grumble. Out with the old cord, in with the new.

Finally, the two or three strings of hard drive power plugs. Again, exactly the same on the hard drive side of things, and exactly the same number of pins in the same pattern on the power supply side. But keyed differently.

“Ugh, why are they doing this to me?”

The other side of the case came off to expose the important ends of my hard drives. I carefully dissected the kraken’s tangled mess of tentacles. After some time and effort I had the old power strings in my hand. Next I cautiously inserted the new strings’ plugs.

“I hope I didn’t accidentally bump any of these SATA connectors.”

And I was done!

On went the case panels, up went my PC onto its little shelf, in went the power cord and USB plugs and display cords, down went my finger onto the power button.


“I think I pressed the power button?”

I poked it again. Still nothing.

“Hmm… Maybe the power surge destroyed my motherboard too? Why do I buy such cheap surge protectors?”

I knew I still had the bent paperclip laying around that I had used to jumper the old power supply as I diagnosed it. I grabbed that, took the side panel off, unplugged the motherboard end of the fat 24-pin motherboard power cable, and carefully inserted the paperclip into the correct two pins.

There was a very faint click from a solenoid in the power supply, then nothing.

I did it again just to see. Same thing. Again. And, there were a couple of faint clicks, like it was groggily waking up but then something was urgently shouting at it “Quick!! Save yourself!!” in the midst of other terrified screams and in a panic it would stumble over itself as it rushed to disconnect itself from the mains.

Perplexed, I unplugged everything from the power supply.

“Good thing this power supply is modular!”

And I tried again. Just a single click this time.

“That sounds like a normal turning on click to me. Not a panicky click.”

Time for process of elimination. In went the graphics card plug. Fine. The hard drive plugs. Fine. The big fat 24 pin motherboard plug and the smaller 8 pin aux power plug.

Not fine.

Actually, there was a very brief flash of light coming from somewhere in the case.

Now, a flash of light by itself is nothing unusual. They stick LEDs on everything. I hate them, but the WiFi card has one, some aim out the back of my motherboard, they’re everywhere. I feel like I had to pay extra to buy RAM without LEDs. Why would someone want LEDs on their RAM??

“Okay, it doesn’t like my motherboard. But where is that light coming from?”

I kept turning it on. Sometimes there would be two clicks from the power supply. Sometimes there would be several. The poor power supply was going to have PTSD. But I had to find that brief flash of light.

And I discovered it wasn’t coming from the open side of the case. It wasn’t coming from anything in or on the top of the motherboard, but instead I could see the flash coming from behind the lower corner.

“Oh great, did I drop a screw in there or something?”

I imagined all the things that could have fallen into the holes in the back of the case.

Out came all the SATA plugs. Out came the graphics card and WiFi card. Out came the motherboard power plugs. Out came all the screws holding the motherboard in place. Out came the motherboard.

“Hmm. No loose screws laying around behind the motherboard. Just dust.”

I flipped it around and searched for the scorch marks. Nothing.

Well there had to be something making that light. I looked closer.

And then I saw it: two strings of surface mount LEDs.

On the back of my motherboard.


“I am never buying a single LED ever for the rest of my life. I’m not even going to talk to anyone who has anything to do with LEDs or anything that LEDs go on, in, or around. I don’t even like the letter ‘L’ anymore.”

What in the world were they doing on the back of my motherboard?? Diagnostic lights? Mood lighting? Well all this diagnosing wasn’t putting me in a light mood.

Okay, it definitely wasn’t arcing. I should have known: the hue wasn’t quite right, and it was silent. Well at least the light has a legitimate claim to existence; someone wanted light to come out the back of the motherboard, and here light was coming out the back. I couldn’t blame the light for doing what it was designed to do.

In went the motherboard to the case. In went the motherboard screws.

“Hmm. Maybe I’ll do process of elimination again, just to be extra sure.”

In went the 24 pin motherboard plug (and the case power switch plug). I poked the power button.

And everything sprang to life! I had left the hard drive power cords plugged into the power supply; they were happily whirring away. My CPU fan was spinning its little heart out.

Wow! What a wonderful chorus.

“Maybe my graphics card?”

In went the graphics card power. Fine! I could just hear all the money I was saving by not replacing all these components. What a beautiful noise.

“A short in some case accessory, maybe one of the USB plugs?”

In went all the case accessories and fans. Fine!

I was getting tired of saving money. There was a problem to solve. What in the world went wrong??

“Wait a minute, what’s this one cord still dangling here?”

I had forgotten about the 8-pin motherboard auxiliary power plug. I guess it really is auxiliary after all? But it was dangling and begging for a home, so I plugged it back in.


Aha! The culprit, caught red-handed!

I looked closer at that cord and compared it to the new one. Of course the motherboard side was identical on both. And the power supply side was physically keyed the same. But there was an empty pin in one place on the one cord, and an empty pin in a different place on the other cord.

They were wired differently! I was sending 12V straight to ground! A short circuit!


The point

Don’t mix and match modular power supply cables from different manufacturers.