(D) Tales of the Unexpected

[catyak]

That was impressive, I was sitting at the computer, listening to music when all of a sudden there's a "Whoomph", the music cuts off and the monitors go dark. The UPS started beeping frantically and an interesting burning smell wafted up

Comments

[g8bur]

I wonder whether one of more of the diodes in the bridge rectifier has gone, and if so, whether it was a cause or an effect of the capacitor failures?

[catyak]

It's almost certainly just the capacitors, it's the usual failure mode of switch-mode PSUs. Especially if the fan had died and exposed the circuitry to excessive heat. Once ruptured, there's probably a dead short between the foil layers which will upset the input side. I guess they could overload one of the semiconductors and cause it to fail as well, most likely the series switching element.

D

[g8bur]

It's almost certainly just the capacitors, it's the usual failure mode of switch-mode PSUs.

Thinking about it, the current rating of most computer PSU rectifiers is usually several amps, so the fuse ought to go before the diodes if they're subjected to excess current, rather than excess voltage from a transient spike. The smoothing Cs in most computer PSUs do strike me as somewhat minimal, too.

In that case, I might be moved to take another look at the big 30A 13.8V one, which released its magic smoke some years ago, but which still takes up shack space. If it just needs a new C or two, and I have something suitable in the junk box, it may be recoverable. If the high-power switching semiconductors have gone, they're probably unobtainable by now.

Not that I'd use the PSU to power the gear, due to the hash (I have three linear ones to run various parts of the station), but it'd be useful for charging batteries now and then.