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From: email@example.com (Andrew Kohlsmith)
Subject: Re: Philips MKT cap failures - LV pulse application
Date: 3 Oct 2002 11:32:48 -0700
NNTP-Posting-Date: 3 Oct 2002 18:32:49 GMT
> First off, if your circuit is sensitive enough to fail when that cap changes
> from 0.012uF (12nF) to 10nF then you've got a big problem to start out with.
> A 20% variance in value in caps should be expected -- maybe not for a given
> unit but certainly from unit-to-unit.
Sorry, not 20% variance, but 200% variance. I wrote the damned number
out wrong. :-)
> Is this cap a through-hole film device? If it's a surface-mount ceramic then
> your problem is probably simple to understand, as smt ceramics are notorious
> for developing cracks due to differences in thermal expansion;/contraction
> coefficients between the cap and the pcb. Most cracks happen during the
> soldering/cooling-off phase.
It's through-hole film. I had thought of that too but can't find any
proof of that happenning with TH devices.
> If the pulse current is too high (high dv/dt), and that's what's causing the
> cap to change its value, then perhaps you could insert a small resistor in
> series with the shorting transistor to limit the cap current when the
> transistor is turned on.
My first instinct was that it was dv/dt but these devices have a
110V/us dv/dt rating, and the curve it follows when not switching its
maximum voltage showed that with a 4V swing, its dv/dt was somewhere
near 2200V/us. A simple LM3904 can't switch *that* fast (I wish!).
> You see, capacitors are like women -- you can't live without them and you
> can't live with them.
hahaha I suppose. I'm trying to track down WHAT changed -- The
markings are clearly different but I can't find a page to describe
what they mean.
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