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Author Topic: How to repair shorted a ATX power supply?  (Read 1725 times)
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Hi all,

I'm learning Arduino and electronics.  I've built ten small projects on Beginning Arduino by Michael McRoberts. This is the last project I did.

I had a lot of fun with it. I found that I would a bench power supply to do other projects or electronic devices.

I converted a ATX power supply to a bench power supply lately after spending 15 some hours. It had 3.3V, 5V, 12V, and negative 12V output. It was working fine until I connected a cheap 12V USB charger without any load in reverse polarity with two jumper wires. When I connected the USB charger in right order, smoke came out from the charger. I thought only the charger was damage.

Right after that I connected a 12V cooling fan for PC to 12V for testing. The fan was running smoothly for few minutes then suddenly the power supply went to standby mode (only green LED was turned on.)

Turning on and off the power supply few times to troubleshoot out of anxiety finally triggered the house circuit break. I knew the power supply has at least one 2A fuse. So when I checked the fuse on the circuit board in multimeter, it's 0.3 ohm.

Why the fuse was fine when the power supply was draining 50A current at 220VAC?

I put the power supply away to salvage parts later on. But, I'd like to replace damage components if it's not hard to do during spare time as learning experiences.

Below are few photos:
12V output test. ^^

Now it's officially dead.  I put one 10W power resistor, four resistors for red and green LEDs, on a perforated board, five binding posts and one on/off switch.

Debian GNU/Linux

Anaheim CA.
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Faraday Member
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.3 ohm is a fine value for a fuse... short your probes together with the DMM set on R X 1 range and note the resistance, then measure the fuse, the difference in resistance is the fuse and a good one is anything less that 1 ohm (for the Mains fuse). Likely that what you let the magic smoke out of was a series pass transistor for one of the several 12V supplies. You Might consider that you are going to spend several hours finding the bad part, getting a replacement and repairing the supply. I'm just wondering if it might not be better to look for an abandoned older computer and gut it for the PSU and do it again, won't take 15 hours... You've done it once. Have you Ever attempted to repair a PSU that is so cramped and so arcane to you?.
I started in Electronics 50 years ago and today I'm still among other things considered an A grade tech and I wouldn't go near one (They're 20 to 30 dollars on Ebay).
The ATX power supplies are repairable but in this case I really don't think you have the experience to troubleshoot a device of that nature. I'm not trying to be negative but I do know a great deal about them, I've had a great deal of experience with them and I hesitate to fix one as they can usually be found with patience for free.
If you just look for an abandoned or "Older" or other PC that perhaps a friend or acquaintance has laying about... Maybe a neighbor or find discarded somewhere. If it lights up, You're in business again you have a live one. there is one caution, I'd not require more than 50 % of the 12V side for sustained high current loads (of more than 30 minutes) I am extremely conservative, most old men are so my recommendations are made in that light. This one is based on the absolute necessity of connecting the +12V must employ all of the many 12V leads the inner wire is 18Ga and that size wire is only good for about 10A so make sure that you use all the wires.
I Do hope this is helpful.


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When you measured the fuse in the ATX supply, did you remove the fuse to measure it?
If you measured the fuse in circuit, its quite likley that you in fact measured the Drain - Source resistance
of the chopper FET or Transisitor, and they are blown.
Usually also means that something on the secondary side of the transformer is also blown, possibly one or more of the
diode bridges.
Unless you are really keen , Id just get another one, they are pretty cheap.


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Given that any number of the active components may have blown, getting a replacement unit does sound a less stressful path - although you will learn more by trying to repair it (about electronics and your own patience!).

[ I won't respond to messages, use the forum please ]

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