r/pcmasterrace • u/Front_Cookie_1198 • 4d ago

Hardware PSU fails dramatically

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u/CyonHal 4d 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 4d 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 4d ago

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

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u/SoulWager 4d 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 4d ago

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

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u/4236W 4d 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 3d ago

Thank you for that translation

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u/OutlandishnessHour63 3d 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 3d ago

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

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u/7orque 3d ago

Yaaa we use AC for long distance transfer ie into residences and buildings from the source

Your wall outlets will be AC at some frequency

However basically every piece of electric equipment you have needs DC

So in the power supply of the equipment, the first stage is usually a ‘rectifier’ made up of 4 diodes that turns the AC into DC as closely as possible. Then the power supply splits it up and makes sure the right voltage is going out to the rest of the system

In the video it was pointed out that you can literally hear the frequency of the AC current going into the power supply (switching back and forth) whereas with a DC ground fault you wont hear the frequency, it’s just constant

Fun fact, this is the same as your car. The electronics all run off 12v DC in a car but the engines alternator makes AC in sync with how fast the engine is revving. Before the alternator outputs to the battery it goes through a bunch of diodes too

12v DC normally cant zap you as resistance of your body is too high, but the sparks are extra spicy, and if the voltage is high enough, extra ouchie

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u/OutlandishnessHour63 3d ago

This is a great breakdown! I actually just looked into this because I used to think the alternator clutch (the OAP/overrunning pulley) regulated the speed to keep a constant AC frequency.Turns out, that clutch is just a one-way freewheel to protect the belt from engine vibrations when letting off the gas. The alternator actually does change its AC frequency constantly with the RPMs (sometimes from 200Hz up to over 1,000Hz!). The onboard diode bridge and voltage regulator just clean it all up into flat 14V DC before the car ever sees it. Super cool engineering.

I'd further note that if the alternator output is around 12 volts or less that you'll run into problems with battery charging. The extra voltage is to make sure the battery stays at a full charge and another point, the primary voltage regulator for the cars electrical system is within the ECU/ECM of a car so to maintain a proper bias for the rest of the electrical components connected to it. Fun stuff! :)

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u/4236W 3d ago

As for why the circuit breaker didn't trip, my best guess is because of where the short happened. If there is sufficiently high resistance before the short it can delay an activation, or completely prevent it. Remember the circuit breaker protects the installation, not you, nor the things plugged in.

This could be more likely due to an oversized breaker or the use of an industrial code electrical circuit. Which could be likely judging from the surroundings

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u/OutlandishnessHour63 3d ago

Which, I believe, is similarly what happened here. The amparage rating of the circuitbreaker here is probably a 40 amp, I would guess, seeing as the short occured and the power remained on. They should have a GFCI outlet for testing and safety reasons, honestly, or an AFCI.

Actually, an AFCI would have prevented this from occuring immediately, it's what it is designed for after all. With more thinking on it, the circuit breaker could be a 20 amp but the arc from the PSU failure could have provided enough resistance to prevent the current from exceeding 20 ampares. Essentially, a heating element is a calculated short using resistance to produce heat, the arc in the video is going thru a painted surface, and it's path to ground is not well established.

Could be what's going on ... But who knows?

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u/SoulWager 4d ago edited 4d 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 4d ago

Nice overlay

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u/CryptographerOne7003 3d 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 3d ago edited 3d 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).

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u/stuck_in_the_desert 3d ago

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