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Weird electrolytic failure mode

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throbscottle

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I haven't proved this fault by replacing the cap yet, but I don't see at the moment what else it could be since it's connected direct to the output of a bridge rect. connected to the mains.

Anyway, the cap charges slowly, over a few seconds, to 81v, then suddenly discharges, then it starts to charge again, over and over. Such a strange thing, behaving as a relaxation oscillator all by itself. (I should find out what is going on and market it...)

This is the main smoothing cap in a cheap after-market laptop psu, pretty basic unit. Two of the bridge rectifier diodes had failed s/c, mains fuse had gone. I'd already replaced this cap a few months ago with a better one than the original (still a board pull though) since the old one had developed a high esr.

Has anyone seen this behaviour before?
 
Sounds like an internal, intermittent short where the dielectric, somewhere in the cap, is breaking down at 81VDC.

Odd that's it's that specific (and repeatable) a potential. I've seen it before (though not in a long time) in older, medium wattage (SPS-40) radar systems.
 
What made the bridge rectifier diodes fail?
Maybe the cap get shot with AC.
 
After I replaced the cap, it was still doing the same thing. This was decidedly very weird. I discovered that the wire-ended fuse on the board had blown, but was still conducting enough to charge the cap slowly. Now, this is where it goes all the way through annoying and way, way out the other side. After I replaced the fuse, it all worked correctly. But, the heatsink for the main switching transistor needed to be settled down on the board a bit more, since I'd taken the tranny out to check it, so I re-worked the tranny's lead joints so I could push it in a bit more. Plugged it in.... BANG! Fuse exploded, piece of pcb track vapourised, switching transistor s/c, 3 paralleled 1R resistors blown. Replaced that lot. Still doesn't work. (though it charges the main cap ok) I think I must have made a solder bridge between the transistor's source and drain leads. Fed up now, think it's time to just get a new charger....

Don't know what caused the diodes to fail in the first place. Possibly because the original smoothing cap it came with had been replaced by me with 2 in parallel, (cos I didn't have one big enough) and that did something strange maybe. One of those appears to have gone o/c. I know caps aren't supposed to do that, but there you go. Or maybe because the whole thing's just cheap.
 
As one thing goes in one end, something else has to come out the other end? Is that it?
 
Circular filing cabinet = trash can

Hiya Buddy :)

You have been active here today :cool: Everywhere :)
What do you think about the Billy Mayo links..

Ahem...cough.. cough...never call me Sir again....

I think I am a pal....don't Sir me again. Ever.

Signed: the Terminator (aka Arnie).
tvtech
 
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Hiya Buddy :)

You have been active here today :cool: Everywhere :)
What do you think about the Billy Mayo links..

Ahem...cough.. cough...never call me Sir again....

I think I am a pal....don't Sir me again. Ever.

Signed: the Terminator (aka Arnie).
tvtech
OK pal
Thanks for those.
I liked the one in witch he was trying to find shorts on avionics boards by applying high currents to then until a chip smoked.
I can't remember the name of the tread.
 
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