The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: firstname.lastname@example.org (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.
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup