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SMPSU failures

william thornton

Jun 8, 2014
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I have ammased a small collection of ex computer SMPSU's which I have blown up by dead shorting the 12v Rail. I use them as audio amp psu's and have little accidents <grn> My question is this Is it likely that the output rectifier diodes alone would have taken a hit { easy fix} or would they merely have passed on the excess current demand to the inner sanctum rendering them practically un-repairable? P.S. I know the dangers inherent in those nasty High voltage caps and I will discharge them sensibly if its worth a punt.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Have you tested them for a shorted rectifier? I would consider that to be the most likely failure mode in that case.

Sorry, I didn't see your post the first time.
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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I don't know. Normally when SMPSes die, it's because of a power surge, or dry joints, or perhaps optocoupler failure, not because the output has been shorted out.

It's not hard to measure the output rectifiers, so I'd start with that. Also check the input fuse; if it's blown, it's likely that the switching transistor(s) or MOSFET(s) have died.

And before you start, check for bulging electrolytics on the outputs - ideally, buy or borrow an ESR meter - and replace anything that's dodgy.

Those are my best guesses.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Sorry, I misread your failure as seeing a dead short across the 12V rail.

If you shorted the 12V rail, I would be looking at what Kris suggested - rectifiers, mosfets, and fuses. You can get a cascade of failures, so if the fuse has blown, it doesn't mean that the mosfet is OK, and if the mosfet has did it doesn't mean the rectifiers are OK.

And there are other things that can fail.

If the fuse has blown, measure the resistance across the mains input after replacing the fuse. If it is close to zero, the input bridge rectifier or the mosfet have failed -- then check the output rectifiers and capacitors.

If the fuse hasn't blown, check the output rectifiers and caps.
 
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