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More info on mica caps (was Re: .062uf 20,000 volts cap on Ebay)

Hi Bill, Malcom, and all,

My "chimes" down below ;o))

> Original Poster: wysock-at-ttr-dot-com
>
> > Original Poster: "Malcolm Watts" <malcolm.watts-at-wnp.ac.nz>
> >
> > Hi all,
> >          I had to jump in on this one:
> <snip>
>
> Malcolm, et. al.,
>
> I had to jump in on this one as well.  Sangamo mica caps (sometimes)
> if you are lucky, do hold up to repeated abuse at (or above)
> nameplate ratings.  I must take exception that mica caps (in general)
> are lossy.  Quite to the contrary!  They are very low loss: (look
> up the dielectric loss tangent of ruby mica in any CGC Handbook.)
> Mica caps are possibly the very best, at being low loss and high
> effeciency; espececially with respect to size and weight, not
> to mention low dielectric heating cooeficients, even at many MHz
> of operating frequencies.  They are, however, *much less forgiving*
> then extended foil type capacitors; especially the poly-pro family.
>
> In my more then 25 years of work with this type of capacitor I
> submit the following (rated best to worst.)
>
> #1) Cornell-Dublier
> #2) Aerovox (now mfr. under a different name.)
> #3) Sangamo
> #4) (the absolute worst) Solar
>
> I've spent many $$$ finding out these facts over the years.  The
> Solar brand will (typically) fail where the p-to-p voltages are not
> even up to the nameplate rating.  FWIW!

Another interesting fact about mica is that it is TOTALLY
unaffected by partial discharges, which plague just about
every other (esp. plastic) dielectrica. AFAIK, however,
mica is VERY sensitive to overvoltage. The caps, which both
Bill and Malcom mention, are made from VIRGIN ruby mica,
which is the key point. Reconstituted mica is worthless in
cap design. The renowned manufacturers of mica caps will
NEVER print the maximum breakdown voltage as the normal
(max) operating voltage on the label. The breakdown voltage
will always a bit higher than the max operating voltage. Mica
has two other distinctive properties. The "k" value is very nice
(high) and it is NOT lossy at RF frequencies (as both have
pointed out), which is why the ARE used in large RF xmitter
rigs, running at several MHz (well above our TC operating
frequency). Looking at Billīs quality rating (1-4), I would
also guess that the price goes along the same terms. Cornell-
Dublier being most expensive and Solar being the cheapest.
Iīll bet the mica used in Cornell-Dublier caps is of much higher
quality than that used in Solar caps, which would explain both
performance and price difference ;o)). Also, AFAIK, virgin
ruby mica, in the necessary sizes needed to manufactur this
kind of cap, is getting increasingly difficult to find, which, of
course, also further increases the price of the caps.

Also FWIW ;o))

Coiler greets from Germany,
Reinhard