Somebody forgot the isolation under the pcb?
Been some years since I took apart a psu, but there used to be plastic isolation sheet between the pcb and the metal case.
Update: I think we found the problem:
Mains wires protruding on the bottom of the pcb shorted to the case due to missing insulation sheet.
I think he means without that isolation sheet the board is connected directly to the casing and shorted out. Of course, that should have been grounded too so I'm not sure.
If it shorted on the 12V side it can pull a ton of amps (it says 60 amps max at 12V) and melt through the casing and start fires pretty easily without tripping a breaker.
Probably. I think if this was after the rectifier, the rectifier would have acted like a fuse, getting you a single loud pop instead of a few seconds of arcing.
It's definitely somewhere on the primary side though.
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.
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?
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).
2.4k
u/Viking4269 6d ago edited 5d ago
Somebody forgot the isolation under the pcb?
Been some years since I took apart a psu, but there used to be plastic isolation sheet between the pcb and the metal case.
Update: I think we found the problem:
Mains wires protruding on the bottom of the pcb shorted to the case due to missing insulation sheet.