Re: rolled caps (fwd)
---------- Forwarded message ----------
Date: Tue, 21 Apr 1998 08:10:47 EDT
From: Hollmike <Hollmike-at-aol-dot-com>
Subject: Re: rolled caps
I just read your explanation of the 2X factor in rolled poly caps. The
compression of the dielectric is hard for me to see as being the cause of
doubling the capacitance.
As I see it, the unrolled poly cap has just one dielectric layer between
the foil and thus only uses one surface of the each foil layer to create the
capacitance. This would give N=2 in the cap equation. Unless I am mistaken,
the rolled poly caps require an additional layer of poly under the lower foil
layer(still unrolled). On rolling, the "outsides" of the foil layers and the
dielectric are now part of the equation, giving N=3(or even 4?)effectively.
The(N-1) factor in the cap equation then results in two rather than one for
the unrolled cap. This would hold except for the first and last wraps as they
would still only "see" one dielectric layer. So the overall capacitance would
be just under two times the unrolled capacitance.
I am certainly no expert, but this would seem to make more sense to me.