r/pcmasterrace • u/Front_Cookie_1198 • 1d ago

Hardware PSU fails dramatically

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u/CyonHal 21h ago

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.

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u/SoulWager 19h ago

Not this time. You can hear the mains frequency in the arc, and the only output that should be powered is 5vsb.

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u/7orque 17h ago

So you’re saying this is on the input stage prior to the rectifiers?

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u/SoulWager 16h ago

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.

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u/ReaperAteMyPrawnsuit 8h ago

This thread is too electrical smart for me to understand what’s going on

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u/4236W 8h ago

Something between the power switch and the thing that makes the wall power into PC power wasn't made right, and wall power got somewhere it shouldn't go

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u/Flying_Dutchman92 4h ago

Thank you for that translation

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u/OutlandishnessHour63 2h ago

That isn't a bad translation. I like it 😊 To make it a wee more complicated but more comprehendable when reading the raw jargon.

A rectifier takes AC, or alternating current, (ie; electricity out of an outlet) and concerts it to DC, Direct Current. (is; power from a battery)

What was being plaused upon earlier: The theory for the short is that the power moved from AC and before it hit the rectifier, it shorted to the case of the power supply due to a missing piece of insulation.

Comments back and forth were discussing whether it was pre-rectified or not based on how the arc sounded and occured and based on the fact that the circuit breaker for the outlet (AC power) was not tripped.

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u/Flying_Dutchman92 1h ago

Today I learned about circuitry, thank you for expositioning:D

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u/SoulWager 8h ago edited 8h 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 7h ago

Nice overlay

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u/stuck_in_the_desert 1h ago

There was too many pixies, which caused it to stop choochin’

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u/SoulWager 8h ago

I found a review with PCB photos, the burn holes are right under where the mains enters the board. Probably wasn't trimmed after soldering, and missing insulation sheet between pcb and case.

https://i.imgur.com/SLsQmmu.gif

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u/Inaniae 18h ago

Correct.

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u/Good_Background_243 5h ago

Oh shit yeah I hadn't noticed it but on a second watch yeah that's definitely high-frequency AC, the arc sounds fuzzy.

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u/CaptOblivious 17h ago

There's a crowbar shutdown circuit on every low voltage supply in that box, that was ac side.

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u/7orque 17h ago

Ye looks like 12v dc ground fault to me, those arcs sounded quite spicy

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u/twilighttwister 15h ago

60A at 12V is 720W, basically the full PSU load.

However it's still not thaaaat big, and certainly less than a dead short on the mains AC supply.

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u/ba123blitz 4790/ Z97/ 16gb/ GTX1060 6gb 15h ago

Can sadly confirm 12v with enough amps can burn down a car let alone a psu lol

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u/64590949354397548569 13h ago

It became a DC welding machine.

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u/Xalara 4h ago

If the breaker has arc fault detection it should trip immediately.

Sadly, building codes in the US have only recently started requiring arc fault breakers on all circuits, and that’s still not required in many places. Hell, even requiring arc fault detection on breakers for circuits in sleeping spaces is a recent requirement. Nevermind that many older houses are wired in such a way that makes arc fault breakers impossible to install without tripping almost immediately due to a false positive 🙃 

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u/Pretend_Giraffe_1288 3h ago

Nah, that's not DC voltage arcing. If mains were shorted to ground, surely either a GFCI or breaker should have tripped.

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u/ComplexSupermarket89 3m ago

The rated spec can be significantly exceeded in the event of a short. The rated spec is what it will allow to be pulled before the safeties cut off power. If it is a dead short, there isn't always time for the safety to stop things before 100+ amps gets sent through a PCB rated for 60 amps max. The caps must have exploded in this situation. At should take a lot more than the rated amperage to do that, assuming the unit was built properly.

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u/Kirianfx 21h ago

Nah, the 12 Volts on those poorly built ones do not have that kind of power.
The video clearly shows a mains power failure.
And do not ask me how I know.

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u/LeBakalite 20h ago

Please, how would you know such a thing ?

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u/Kirianfx 18h ago

More that once I have blow up something plugged to mains.
List includes, a full bridge rectifier, a extension cord, one light switch, and a angle grinder.
Last one left a hole on the floor.

I also have seen what a short on the 12V rails of a poorly build supply can do, even in car batteries too, they do not do that kind of fireworks, if they attempt to do it, they would not last as long.
A good built one could, but it will not, as it would shut down before it can do that amount of damage on itself for that long.

Look at the place of the burn, it is exactly when the mains power plugs.
now, I do not know this PCB layout, but one would have to be very crazy to put a 12 volt Rail right on top of were the EMI filter, full bridge rectifier and mains voltage capacitor would be.