Re: [Fwd: Re: Capacitor search]

Tesla List wrote:
> >From DavidF4797-at-aol-dot-com Fri Dec  6 23:47:58 1996
> Date: Fri, 6 Dec 1996 11:07:22 -0500
> From: DavidF4797-at-aol-dot-com
> To: tesla-at-pupman-dot-com
> Subject: Re: [Fwd: Re: Capacitor search]
> In a message dated 96-12-06 01:37:36 EST, D. Gowin writes:
> << You can find your caps at R&D Electronics 1-800-642-1123.
>  2500 pF                6KVDC           Z5U             $0.75
>  2000 pF                10KVDC          Real Big Disc   $1.95
>  5000 pF                16.5KVDC        D7OU502M        $2.95
>  Using a parallel/series network you can build these into
>  any voltage/capacitor rating you choose.
>   >>
> It is my recollection that caps rated for DC which the above seem to be,
> usually don't make very good TC caps because of the frequency at which they
> will be asked to perform (usually somewhere between 100khz and 1mhz for
> meduim and small Tesla
> Coils).  The dielectric material in DC caps usually becomes quite unhappy and
> has a major "loss" and heating problems which  increase with the operating
> frequency, (since the dielectrics of choice for DC are not at all the same as
> the dielectrics of choice for AC.)  While these caps might work after a
> fassion, and are certainly cheap, the energy loss that they would probably
> incur would detract markedly from the output of your coil.  If you have the
> funds, I would suggest buying  comercial AC cap or building your own from
> Polypropylene or Polyethylene and aluminum foil.
> -DavidF-


You advise is quite correct!


Once again, the use of disk capacitors will work, (for a little while.)
RF energy tears the hell out of the two plates used in these capacitors 
and doorknobs as well.  These plates are always vapor depositied pure 
silver (few molecules thick).  The common failure mode is a reduction of 
capacitance over time as the cap plates are literally burned back away 
from the input leads or screw tabs (doorknobs).  Also, some of the poor 
temp coefficient types will actually drift rapidly off tune as the unit 
is used.  For best operation, you will have to off tune so that the caps 
drift into tune once warmed up!  These work great for micro coils where 
10 watts is a big coil, but larger systems will need replacement of the 
caps on a regular basis. (buy lots of 'em!)

Richard Hull, TCBOR