Re: Creeping Sparks and Homemade Caps... (fwd)

---------- Forwarded message ----------
Date: Sun, 26 Apr 1998 14:24:27 -0700
From: Jim Lux <jimlux-at-earthlink-dot-net>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Creeping Sparks and Homemade Caps... (fwd)

> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Creeping Sparks and Homemade Caps... (fwd)
> Date: Saturday, April 25, 1998 8:20 PM
> ---------- Forwarded message ----------
> Date: Sat, 25 Apr 1998 12:04:24 -0700
> From: "Antonio C. M. de Queiroz" <acmq-at-compuland-dot-com.br>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Re: Creeping Sparks and Homemade Caps... (fwd)
> Thomas McGahee wrote:
> > A few of my observations:
> >...
> Excellent post. Thanks.
> I have been thinking and experimenting with a way to make dry capacitors.
> What I can comment is:
> The tabs coming out of the plates for external connections are the worst
> problem. The simple presence of the thin plate material coming out of the
> assembly produces corona in all directions. It is difficult to charge
> a capacitor built in this way to more than a few 10 kV due to corona
> at these tabs. And the corona to air easily finds a way to the other side
> of the capacitor, producing that nasty surface tracking sparks.
> Maybe some form of enclosing of shielding of the terminal areas can help.
> For example by covering the terminal areas of a flat capacitor with metal
> foil, or metal tubes with rounded ends, without any sharp edge of corner.

Polyethylene melts at a pretty low temperature. How about heat sealing
under vacuum. Make the stack of plates and insulation, pull a vacuum to
remove the air (possibly backfill with a better gas: SF6, Freon, etc.),
compress it, then heat it up. The polyethylene will bond around the tabs
coming out, essentially making them an insulated wire, with no air space
(That's why you do it under vacuum) for minimal corona. 

Or, you could put some sort of semiconductive paste or film around the
plates where they leave the "body" of the capacitor to reduce the voltage
stress, like they do with HV hookup wire.