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

Dan -
In the CDE 942 series capacitors, the metallized film "floating plate" has another very important function.
To reiterate, the 942 design uses actual metal foil as the "end" plates. This allows the sprayed-metal that is applied to the ends of the capacitor roll to make direct contact with the metal foil, creating a very low resistance connection that can carry high current. The capacitor's leads are then connected to the sprayed-metal end coating. The "floating plate" is a very thin vacuum-deposited layer of metal on a thin insulating film. It is this thin metallized film plate that gives the capacitor its "self-healing" capability. If there is an over-voltage condition that punctures the dielectric, the resulting internal arc vaporizes enough of the metallized film around the puncture to prevent a direct short circuit. The capacitor loses an infinitesimally small amount of capacitance, but stays functional.
I have "disassembled" several of these capacitors that had been subjected to overvoltage, and the tiny circular zone where the metallized film has been vaporized around the actual puncture can be clearly seen under a microscope. 

Regards,
Herr Zapp
----- Original Message ----- From: "Dan Elyea" <fsiyfr@xxxxxxxxxxxxxx> 
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Friday, September 26, 2008 9:49 AM
Subject: RE: [TCML] DIY Tank Capacitor - Plate configuration confusion
That 942C is a clever approach to achieving certain specs. It analyzes down to being two capacitors in series in one casing. The metalized film serves 
as one plate for each capacitor, and it also serves as the connection that
places the two capacitors in series. There is no "floating" plate as far as 
the operation of the capacitor.  The "floater" serves simultaneously as a
plate for each of two capacitors and as the "strap" that connects the two
capacitors.

Dan


-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx]On
Behalf Of Lau, Gary
Sent: Thursday, September 25, 2008 9:17 AM
To: Tesla Coil Mailing List
Subject: RE: [TCML] DIY Tank Capacitor - Plate configuration confusion




> -----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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