Re: MMC cap preferences? (fwd)

["High Voltage list" <hvlist@xxxxxxxxxx>]

Original poster: <sroys@xxxxxxxxxxxxxxxxx>



---------- Forwarded message ----------
Date: Sat, 12 Mar 2005 19:02:44 +0100
From: Finn Hammer <f-h@xxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: MMC cap preferences? (fwd)



High Voltage list wrote:

>From: Gomez Addams <gomez@xxxxxxxxxxxx>
>  
>
>>
>>The prefered capacitors have metal foil instead of metallized plates, 
>>they
>>can hadle more current.
>>    
>>
>
>  Hmm, Ross Overstreet says metallized film is better than metal foil 
>due to its "self healing" properties.  You and Terry say that metal 
>foil is better due to increased current capability.
>
>So, who's right, or is this one of those religious arguments that may 
>never be settled?
>
>  
>
I don`t think there ever was an argument about this, because both Terry 
and Ross are right, however only when both properties are present in the 
cap at the same time.

The foil is needed, because only foil offers enough crossection area at 
the connection at the ends of the cap. This is where the foils extend 
slightly, and are metal sprayed with an end cap of alluminium so that it 
is possible to connect to the outside, IE the leads.

These foils extend almost halfway into the capacitor, and since they 
don`t overlap, there is only negligible capacitance formed between them.

There is, however, 2 layers of dielectric between the plates, and the 
one of these layers is metallized on the surface pointing towards the 
other sheet of dielectric.
In effect, there are then 3 plates in the cap, with the metalized plate 
floating in the midle.

If the dielectric breaks down and a hole is formed, then the discharge 
trough that hole vaporizes the metalization in a circular area around 
that hole, and this area is so large that the voltage cannot arc across 
it from the hole afterwards.

The cap has self healed.

Both properties, the self healing from the intermediate, floating, 
metallized dielectric, electrode and the high peak current handling, 
secured by the superiour electrical contact of the foil plates, is 
needed for a capacitor to survive in Tesla Coils.

Hope this helps.

Cheers, Finn Hammer