In real life the actual rf breakdown voltage of a capacitor commonly is
determined by unintended/unrecognized/unknown regions of unexpectedly high
voltage GRADIENT - often many times higher volts per inch or mm than a
simple calculation of (Etotal)/(dielectric thickness) would indicate.
Such regions can and often do occur (a) WITHIN the dielectric, due to
inhomogeneities such as air bubbles or other contaminants, and (b) at the
dielectric surface, typically due to sharp (small radius) edges of
electrodes OR air gaps between the dielectric and the electrodes. The
latter is almost inevitable unless electrode edges are of large radius and
very smoothly polished, and the electrode-to-dielectric interface is filled
with a material soft or fluid enough to eliminate air pockets and of
dielectric constant roughly comparable to or greater than that of the main
dielectric.
This problem of dielectric inhomogenieties which create locally large
voltage gradients, which in turn lead to corona discharge, which "eats"
holes in and eventually breaks down the insulation, is at least one reason
why HV caps are often potted and HV transformers are either vacuum
impregnated and potted or filled with oil. (Of course cooling also is often
a consideration for filling transformers and even caps with oil.) I believe
that relatively soft dielectrics like teflon and nylon generally are more
susceptible to erosion by corona than are harder ones like mica, glass, and
high quality ceramic.
Despite design precautions, the end result is that rf dielectrics generally
must be many times thicker than their "dielectric strength" spec would
indicate. It's also why the insulation in critical power transformers -
e.g., utility distribution transformers and ALPHA power transformers(!) -
is applied on the basis of ~50V/mil rather than the several hundred or even
1000 volts/mil that the insulation's ratings suggest.
73, Dick W0ID
-----Original Message-----
From: Mark S Graalman [SMTP:wb8jkr@juno.com]
Sent: Monday, February 15, 1999 9:32 AM
To: sm3uzs@telia.com
Cc: amps@contesting.com
Subject: Re: [AMPS] isolation question
Probably, about 30-40 KV DC maximum. (on paper)
Mark WB8JKR
On Mon, 15 Feb 1999 12:09:18 +0100 "Johan Nordin" <sm3uzs@telia.com>
writes:
>
>Hi there
>
>Does anyone know how much DC voltage
>0.25mm (0.01 inch) teflon sheet can handle
>used as a dielectricum in a decoupling cap (made of brass)
>
>Any help appreciated!
>
>SM3UZS Johan
>
>
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