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From: firstname.lastname@example.org (Tom Bruhns)
Subject: Re: Low leakage parts
Date: 6 Jan 2003 12:46:17 -0800
References: <email@example.com> <8npR9.16063$jM5.firstname.lastname@example.org> <email@example.com>
NNTP-Posting-Date: 6 Jan 2003 20:46:17 GMT
Indeed, Phil, I think your doubts are quite reasonable! When I
started my capacitor experiment, I biased myself with a look at a
graph in a reasonably new edition of Sams' "Reference Data for
Engineers." It shows insulation resistance (ohm-farads) for various
dielectrics commonly used in film caps. Polyprop was the best, but
the graph shows it with only an 8-day time constant at 20C. So I
expected I'd see a rather rapid decay over a couple weeks. Polyester
is shown about an order of magnitude lower resistance, so that should
have taken only a couple days to drop to a tenth the initial charge.
I was not prepared for the >10^4 factor relating what I actually
measure and the values in the graph. Also, the manufacturers' data
for the caps I'm testing matches the graph's values pretty well.
Again, I believe the attention to process details and to improved
materials has had some big benefits outside the reasons I suspect
they've been done in the first place. I doubt if very many people
were asking for caps with 100-year time constants, but I'm sure a lot
were asking for more reliable caps, and I'll bet a lot of the
improvements are the result of a constant search for better
I wouldn't _rely_ on such performance in the products I design and put
into production, but that doesn't keep me from playing with circuits
and measurements that let me know how good things actually are. With
my hobby circuits, I don't have to please anyone but myself. :-)
"Phil Allison" wrote in message news:...
> "Tom Bruhns" wrote in message
> > Perhaps Phil lives in an environment with an abnormally high number of
> > ions floating around in the atmosphere, but the roughly 0.1fA of
> > leakage I'm seeing in 0.1uF polyprop caps at 10V suggests about 10^17
> > ohms effective shunt resistance,
> ** The insulation resistance for some ( LCR brand) polypropylene caps
> ( if that is what you used) is quoted in the Farnell cat as 1 x 10^11 ohms.
> For LCR polystyrene it is given as 5 x 10^11 ohms.
> Your suggested figure is some one million times better.
> My doubts are reasonable.
> ..................... Phil
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