Fried Power Supply

The original fuse (white tube) after cutting away black heats hrink tubing.
The original fuse (white tube, bottom left) after cutting away black heatshrink tubing.

As a public service message, I feel obliged to share the following nugget of wisdom: before plugging a tower computer into a 220V outlet, reach around to the back of the computer and make sure the input voltage select is 220. Sounds simple, right? This isn’t something you have to think about for most laptops, which have dual voltage power supplies. They are happy plugged into either voltage and the power supply brick just works.

I’m not [arguably, perhaps] an idiot – I was aware of the switch. I just thought that the power outlet was off, but also I didn’t expect to encounter an issue until the computer itself was turned on. Wrong — ATX switching power supplies are always on. When I plugged the computer in 220V, there was a popping sound followed by smoke from the back of the unit. Never a good sign.

I pulled the power supply out, opened it up, and looked around. Nothing was obviously charred. My nose has lousy spatial resolution — it confirmed that something wasn’t right, but couldn’t help me localize the problem. I followed the wiring from the outlet inward. For a cheap supply, I was glad to see some decent capacitors on both live and return wires to ground, and across them. Also, some inductors to quell EMI. Next in line: the fuse. It had blended in because as a safety precaution, it was wrapped in heat-shrink. I cut away the heat shrink to reveal a white tube. I couldn’t see into the fuse, but my continuity tester showed it had blown. There was no fuse holder; the fuse was just soldered in by its leads, so I dutifully unsoldered it.

I looked through the rest of the supply, and it looked fine. The next component in line was an NTC thermistor, and it seemed okay, then the primary transformer, and onward towards the voltage doubler circuits (since the switch was in the 110V configuration at the time it blew). It seemed to good to be true, but I thought I might get away with just replacing the fuse. Having ransacked some other switching power supplies in the past, I did have a 250V 6.3A fuse on hand (which is good, because it would have taken at least a month to order one).

Scorch marks at the location of the former MOV, to the left of the  bridge rectifier, also removed.
Scorch marks at the location of the former MOV, to the left of the bridge rectifier, also removed.

After encasing the fuse in heat-shrink tubing, I stuck the new fuse in, closed up the power supply, wired it back into the computer, and made sure the switch was on the 220 setting this time. I put on my safety goggles and plugged it in. Puff! Another impressive pop and some smoke.

Again, I yanked the supply. Obviously, the problem ran deeper than the fuse. I couldn’t see any failed components, but between various wires and component density, plus copious amounts of sticky glue used to pot the components, it was difficult to get a good look at the whole board. I like to find and fix problems, but I’m also not crazy. I ordered a replacement power supply from New Egg and got to work on a postmortem. At the very least, I figured that I could harvest the inductors and toroids.

When I dug down, I did find some scorch marks near MOVs that had given their lives for the cause. These charred little guys failed short, so no wonder the fuse kept blowing. They were also cloaked in heat-shrink tubing, which made the failure harder to spot.

As I dug through the board, I wondered if the bridge rectifier had also been damaged. So far as I can tell, no — it tested fine. In principle, I think I could have replaced the failed MOVs  and fuse and had a working supply. However, given the layout of the board, I’m not sure I could have gotten easily to the MOVs. Oh well, at least some more parts for the parts bin. I’ll shelf the computer until the new supply comes and see where we go from there.

Two bad fuses, two bad MOVs, and one good rectifier.
Two bad fuses, two bad MOVs, and one good rectifier.

Leave a Reply

Your email address will not be published. Required fields are marked *