Re: Good Caps for Tesla Coiling

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Steve,

At 05:06 PM 12/19/2002 -0500, you wrote:

>>Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
>
>>The GE42L's are all metal film.  They are pretty tough, but we still see 
>>problems like this:
>>
>>http://hot-streamer-dot-com/temp/Pb280016.jpg
>>
>>Cheers,
>>
>>         Terry
>
>Now this has been bugging me for some time now.  Why is it that the plates 
>actually BURN?  I had these caps before with this problem.  Now what would 
>happen if you over currented these at a lower voltage?  It was my 
>understanding that the current was destroying the 42ls.  But would that 
>actually happen at say 500v or whatever they are rated in AC?  So this 
>leads to my next point,  what if there are enough of them in series so 
>that there is only the rated AC voltage across them?  Is there than ANY 
>reason for them to fail?  Somehow i cant see lower voltages doing this to 
>a cap.  Maybe it would disconnect and that would be the end of it, becuase 
>the low voltage can arc across the break?  Just curious as i still have a 
>LOT of .68uf 1200v caps.
>
>Steve Ward.

In metalized caps, they take bare poly film and vacuum evaporate a thin 
(like millionths of an inch!!) film of aluminum on the film and call it a 
plate.  The plates themselves can actually take the current since it is 
spread out.  However, the problem is connecting the super thin film to the 
end caps.  At the edge of the film you have 500 micro inches of poly and 2 
micro inches of aluminum.  They just can't make a good connection there and 
that is the weak spot current wise.

In foil caps, you have 500 micro inch solid aluminum foil to bond to and 
the connection is far stronger.

If you could put more in parallel, then you could lower the current on each 
cap to a safe value.  But that is hard to do with big 0.68uF caps :-(

It does not have much to do with voltage, just the current it takes to blow 
the end cap to plate connections.

Cheers,

         Terry