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Re: Dialectrics in Plate Capacitors

Hi Rick,

I don't think your poly sheet is suitable for making the cap you want.

Punching the figures, using 10"x10" foil plates and 12"x12" poly (1"
overhang), you can stack 33 plates (using 32 poly sheets) and get 0.025uF.
You have enough poly for two of these.

But:

1)
It is not advisable to place more than 5kV(RMS) accross a single cap due to
corona damage (even under oil) so you would need 3 caps in series to cover
this.

2)
Allowing 160V(RMS) per mil of poly for dielectric strength for a
bullet-proof cap, you get 60mil poly handling 9.6kV(RMS), but this figure is
for multiple layers of thin (i.e. 6mil) poly sheet making up the total.  For
a single sheet you would need to de-rate it.

3)
The overhang of poly needs to be reasonably large because the HV can track
accross the surface of plastics and travel further than you think.  Also, a
margin for error is a good thing; you may have a foil plate accidentally
misplaced and have it get too close to the edge.

Hope this helps, and not just dissapoints   :-)

Have a read of the 'Capacitor' section on my web site.

Steve Bell (UK)
steve-at-breakfix.demon.co.uk
http://www.breakfix.demon.co.uk

----- Original Message -----
From: Tesla List <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: 18 December 1999 00:10
Subject: Dialectrics in Plate Capacitors


> Original Poster: Apollo <ollopa-at-jps-dot-net>
>
> I have two sheets of 60mil 4'x8' HDPE.  I need to make a capacitor for
> my 15kv 60ma NST.  I'd like to make a .02uf capacitor, but I'm not sure
> if I can.  Will I need to put two caps in series to handle my NST?
>
> Since I have 60mil HDPE, there's no way I can roll a cap and I've decided
> that I'd rather build a stacked plate capacitor anyway.  My research
> suggests that I should let the poly hang over the foil by at least one
> inch on all sides, but I don't understand why this is necessary if I
submerge
> the plates in an oil with a dialectric strength equal or greater than that
of
> the poly.  If it won't puncture .06in of poly, why will it arc from end to
end?
> What if there was only a quarter-inch of poly overhang that was sealed
with
> silicone sealant?
>
> Thanks - Rick Richard
>
>