[Top] [All Lists]

[AMPS] isolation question

To: <>
Subject: [AMPS] isolation question
From: (Richard W. Ehrhorn)
Date: Tue, 16 Feb 1999 12:10:15 -0700
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 

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 []
Sent:   Monday, February 15, 1999 9:32 AM
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" <>
>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
>FAQ on WWW:     
>Administrative requests:

You don't need to buy Internet access to use free Internet e-mail.
Get completely free e-mail from Juno at
or call Juno at (800) 654-JUNO [654-5866]

FAQ on WWW:     
Administrative requests:

FAQ on WWW:     
Administrative requests:

<Prev in Thread] Current Thread [Next in Thread>