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Re: capacitor leakage



At 11:14 AM 5/19/98 +0800, Jaya wrote:


Dear List,

Recently I had a problem with a pcb (printed circuit board) hardware;
it was supposed to draw nanoamps of current when in standby mode; but
was drawing current of the order of milliamps. There were about
20 chip capacitors, all connected to a common node; one of them
was leaking and caused this excess current; what i did was to take
a surface temperature measurement probe, connect it to fluke
multimeter and measure the temperature reading at each capacitor's
surface; i was lucky that one capacitor showed 29+ degree C while
all others read about 28 degrees. I replaced that cap with
high temperature and my problem was solved.

My question is: if the capacitor leakage were of the order of microamps,
the temperature effect will not be significant; is there a different
way to detect such small leakages ?

In my experience as a former electronic technician, degraded dielectrics in
capacitors are usually sensitive to temperature changes. The easiest way to
detect small leakages such as you describe would be to monitor the circuit
current while spraying each capacitor with Freeze Mist. Freeze Mist is a
chlorofluorocarbon (or whatever they're substituting these days) under
pressure in a can. When sprayed out the product expands and cools, and will
cool down anything it comes in contact with. The product can go by
different names - I've got a can of Freez-It in my lab. It is available in
most electronic supply stores such as Radio Shack.

You'll probably see small current changes when you spray good capacitors,
but the bad one will make a big current jump.


Ron Ebert
ron.ebert@ucr.edu
*******************
The brightest flashes in the world of thought are incomplete until they
have been proved to have their counterparts in the world of fact. -John
Tyndall