RE: [TCML] DIY Tank Capacitor - Plate configuration confusion

["Lau, Gary" <Gary.Lau@xxxxxx>]

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of Dan Elyea
> Sent: Wednesday, September 24, 2008 9:08 PM
> To: Tesla Coil Mailing List
> Subject: RE: [TCML] DIY Tank Capacitor - Plate configuration confusion
>
> I probably missed some special case that is being considered, but a
> statement was made that the layers of foil in the middle are not doing
> anything.  Not so.  Each layer of foil in a capacitor is connected to one
> terminal or the other, and takes a very active part in the operation of the
> capacitor.

I agree that plate/foil layers in the middle of a cap exist for a reason, but it is not true that all plates connect to terminals.  Reference the Cornell Dubilier spec for our MMC caps at http://www.cde.com/catalogs/942C.pdf.  The construction diagram at the very top illustrates two foil plates at the top.  These are the two terminal connections.  Below the underlying layer of dielectric is a metallized film, which acts as a "floating" plate, electrically between the two foil plates.  There is no terminal connection to this film layer.  High current terminal connections can be made to foil, but not to metallized film.  The floating plate exists so that the total terminal voltage rating is split between two series-connected caps, so that neither one exceeds its corona inception voltage.

Regards, Gary Lau
MA, USA


> The capacitance is determined by the area of the conductor
> plates facing each other, the material of the dielectric, and the thickness
> of the dielectric (the spacing between the plates).  If all other factors
> remain the same and you increase the spacing between the conductors (foil or
> whatever), you reduce the capacitance.  Multiple plates (properly connected
> to the capacitor terminals) have the effect of increasing the total plate
> area, and thus the resulting capacitance.  Look at the formula for
> capacitance and you'll see how the several factors affect capacitance.
> Increased plate area or higher dielectric  constant increase capacitance;
> increased spacing between the plates decreases the capacitance.
>
> You probably know that already, and are likely posing some theoretical setup
> with floating plates.  But, just-in-case, I decided to chip in.  Sorry if I
> totally missed the point being made.  In a real capacitor, all the "foil" is
> connected to one terminal or the other.
>
> Dan

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