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From: Winfield Hill
Subject: Re: Design for long interval timer
Date: 7 Dec 2002 07:44:42 -0800
Organization: Rowland Institute
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> Spehro Pefhany wrote:
>> Fred Bloggs wrote:
>>> I am also wondering about your choice of film caps in these
>>> high-impedance timing applications where there may not be
>>> enough current to clear a dielectric fault.
>> Dielectric faults are the province of caps subject to damaging
>> transients. Film caps at low voltage just about never fail,
>> they are about as reliable as it gets!
> This is not quite true. Films are reliable because of process and
> materials improvements which include gridded metalization patterns
> designed to fuse the connection to the fault at low currents.
> There are several reasons for dielectric faults which include
> latent film defects, film faults induced by dielectric stress due
> to electric field gradients, and heat. By using the film in an
> ultra high impedance circuit you may be negating the apparent
> reliability of the part because the circuit cannot clear the fault-
> or allow the cap to self-heal in your lingo. Low voltage helps but
> it does not eliminate consideration of temperature effects on
> dielectric deterioration, or inherent defect activation, in
> storage, use, or aging of the part.
Fred, I'll go with Spef's view, rather than yours. In my personal
experience with about 250k of the mil-spec CK05-style ceramic caps
I prefer using - perhaps 1 million-cap-years, I've experienced
only four failed parts, of which two were DOA. By contrast, for a
somewhat smaller number of film caps used at 5 to 10% of the rated
voltage in leakage-sensitive applications, I have experienced zero
failures. But talk is cheap; so I'm going to ask you to kindly
provide us with references for your view.
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