[TCML] DIY Tank Capacitor - Plate configuration confusion
quarkster at att.net
Fri Nov 6 17:46:26 MST 2009
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.
----- Original Message -----
From: "Dan Elyea" <fsiyfr at okeechobee.com>
To: "Tesla Coil Mailing List" <tesla at pupman.com>
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
> to being two capacitors in series in one casing. The metalized film
> 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
> 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
>> -----Original Message-----
>> From: tesla-bounces at pupman.com [mailto:tesla-bounces at pupman.com]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 at pupman.com [mailto:tesla-bounces at pupman.com] 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
> top illustrates two foil plates at the top. These are the two terminal
> connections. Below the underlying layer of dielectric is a metallized
> which acts as a "floating" plate, electrically between the two foil
> 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
> series-connected caps, so that neither one exceeds its corona inception
> 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
>> of the dielectric (the spacing between the plates). If all other factors
>> remain the same and you increase the spacing between the conductors (foil
>> whatever), you reduce the capacitance. Multiple plates (properly
>> 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
>> with floating plates. But, just-in-case, I decided to chip in. Sorry if
>> totally missed the point being made. In a real capacitor, all the "foil"
>> connected to one terminal or the other.
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