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From: "Phil Allison"
References: <email@example.com> <8npR9.16063$jM5.firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
Subject: Re: Low leakage parts
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Date: Tue, 7 Jan 2003 10:40:49 +1100
NNTP-Posting-Date: Tue, 07 Jan 2003 10:30:42 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Tom Bruhns" wrote in message
> Indeed, Phil, I think your doubts are quite reasonable!
** I am releaved you did not think I was calling you a liar !!
> 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.
** Isn't polystyrene even better ?
> 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.
** High voltage electros often hang on to their charge for days rather
than minutes as the data would suggest. I think they do it conscioulsy to
get revenge on techs.
> 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
** Improvements in films have made plastic caps much smaller than a
decade ago - I suspect this is a bigger sales point than the number of
> 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. :-)
* I *did* attempt to make pre-amps for condenser mics once ( 25 years
ago) and could not source the essential 1000 Mohm resistors in small
quantities so I resorted to drawing the component, zig zag and all, on the
PCB with a fountain pen filled with resistive ink.
I got my 1G resistors OK but the PCB leakage was in the same order
order of magnitude if the weather was even slighly damp.
Any clear coating I applied over the area to solve this ate the
resistor. Very frustrating.
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