Re: Caps

To: tesla-at-pupman-dot-com
Subject: Re: Caps
From: "Reinhard Walter Buchner" <rw.buchner-at-verbund-dot-net> (by way of Terry Fritz <twf-at-verinet-dot-com>)
Date: Sat, 01 May 1999 19:20:26 -0600
Approved: twf-at-verinet-dot-com

Hi Adam, Scott,

>Original Poster: Adam <psycho-at-tradewind-dot-net>
>> >Nobody answered my questions, so I'll ask again.
>> >Doorknob caps:  How suitable are they for a NST powered Tesla coil?
>> >Do HV ceramic caps work poorly, or not at all?
>> >Does anybody know of good commercial sources for new and/or surplus HV
caps.
>
>If you look through the archives, you'll find a couple messages saying that
>doorknob caps are doomed in Tesla Coil service, because the electrodes wear
>faster than you would like.  I have no other knowledge about this, except
>that I have also heard that HV ceramic caps are a bad choice for the Tank Cap,
> though I'm not sure why...

Depending on the type of ceramic cap, they have two distinct disadvantages:

1.) They are very RF-lossy due to design and materials.
2.) The "endplates" are just vacuum applied silver vapor deposits. These plates
      won´t stand the high currents for long. Each time a little silver is
      removed, the capacitance drops and the contact resistance gets a little
      higher (chain reaction). Sooner or later, you will be left with a cap w/o
      end plates, so its not really a cap any more then :ö).

And if they DON´T have these disadvantages, they will have another (equally
bad) disadvantage: the pricetag. RF rated ceramic caps are very expensive
(if purchased new). Another disadvantage is the pretty small capacitance in
which they are available.

Hope this helped.

Coiler greets from germany,
Reinhard