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Re: X-ray film capacitors

Original poster: "Dr. Duncan Cadd by way of Terry Fritz <twftesla-at-uswest-dot-net>" <dunckx-at-freeuk-dot-com>

Hi Fred, All!

>With some of the recent discussion of home made capacitors, I decided
to try to
>make my own. I used a 4x10 sheet of Kodak dental x-ray film, with a
sheet of Al
>foil on each side. I tested it up to 15 kvac, and the discharge was
very hot
>and loud. My only concern was what material the film is made out of.
Could it
>be lossy at RF like Mylar? How many sheets would I need for 30nf?

The formula for a parallel plate capacitor's capacitance will give you
an idea of how many sheets you need.  This is:

C = 0,0885 K { [(N-1) a] / d }
or rearranging:

N = [ (C d) / (0,0885 K a) ] + 1; rounded to the nearest integer.

where C is in pF (30000 for 30nF), K is the dielectric constant, N is
the number of plates, a is the area of one side of one metal plate in
square cm and d is the plate spacing/film thickness in cm (1 inch =
2,54cm; 1 square inch = 6,45 square cm).  The dielectric constant of
most plastics is around 2,2 give or take a bit so if you use that you
won't be far off, but I don't know what is used as the carrier/backing
for dental x-ray film, it could be mylar as you say or it might be
cellulose acetate (or one of half a dozen other things).  Maybe it's
lossy at rf, maybe it isn't.  If it works, it doesn't matter, if it
doesn't work, it matters even less ;-)  Al's experiments with ordinary
clay gave him a working cap when quite honestly I thought it wouldn't,
so don't be afraid to try.  You haven't got much to lose!

Geek#1113 (G-1)