[sdiy] Failure modes on bypass caps...

[Tim Daugard]

>
From: "Aaron Lanterman" <lanterma at ece.gatech.edu>

I've been reading up the sage advice on bypass caps, and something
started
to worry me...

When caps fail, do they fail into "open circuits" or "short circuits?"

If the later, it could be scary if the cap runs from the power supply
to
ground...
>

They can fail shorted, It's rare they fail at all though. I was
working a new test station for the USAF when a cap failed. It was fed
by a 50 A power supply. The software recorded a failure and then
happily continued on in a diagnostic path. One of the contractors
engineers was standing by at the time. I noticed flames rising behind
a gap in the wrack panels and asked if the engineer thought we might
want to shut it down. The program kept chugging on. After about
another 30 seconds we shut the thing down. The diagnostics never found
the problem.

Found out after the analysis was returned, that a bypass cap had
shorted. The 50 A power supply had no problem feeding the power to the
cap (cable and trace impediances high enough not to overload the
supply.) Everything was fine until the power trace got so hot that the
fiberglass PCB caught on fire.

Bypass caps where rated low probability failures in all the analysis,
it was just an infant mortality failure. The only bypass cap I've seen
fail in 30+ years of electronics. (However, a bypass cap failing open
will probabilily have no noticeable effect.

Tim Daugard
AG4GZ 30.4078N 86.6227W Alt: 12 feet above MSL
http://home.sprintmail.com/~daugard/synth.htm