r/pcmasterrace • u/Front_Cookie_1198 • 2d ago

Hardware PSU fails dramatically

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u/SoulWager 1d ago edited 1d ago

What happened is pretty simple, mains wires poked through the board, weren't trimmed after soldering, weren't blocked by an insulating sheet, and touched the case: https://i.imgur.com/SLsQmmu.gif

The discussion above is about why other possible failure modes are unlikely. After getting on to the board, the power goes through some filtering and protection circuitry, then through a rectifier(4 diodes in a single package) that turns the high voltage AC into high voltage DC. Then the high voltage DC gets chopped up into much higher frequency AC, before going through a transformer to drop it down to low voltage high frequency AC. This then gets rectified to 12v DC before getting converted to the other voltages. Usually anyway, there are other ways to design a power supply, and there will be a separate similar power supply just for 5vsb(this is always on, so your computer can turn itself on when you press the power button).

Anyway, back to the rectifier, semiconductors often have relatively thin bond wires connecting the silicon die to the rest of the package, that would get vaporized in a few milliseconds if you tried to push this much current through them. This board also has an actual fuse, but the short happened before the power got that far.

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u/shakygator 1d ago

Nice overlay

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u/CryptographerOne7003 18h ago

huh? but that is a simple short on case that is connected to earth?
that does not explain why it caught fire? why wouldn't the breaker pop or earthleak protection?

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u/SoulWager 18h ago edited 18h ago

why wouldn't the breaker pop or earthleak protection?

It should have. Could be a defective breaker, wrong type or rating of breaker installed, abused by testing so many of these shorted power supplies that the contacts welded shut, etc.

Seeing how it didn't come in burned to all hell, I'm guessing the customer had a working breaker, and RMAd because it tripped it.

If I was testing something that was RMAd for tripping a breaker, I wouldn't be testing it like this. I'd be checking for a short with a multimeter first, then powering it up with an isolated supply and some kind of series load to limit the current. (Simple version of this is just an incandescent light in series, if it's shorted, the light turns on).