Re: Capacitors

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

Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Esondrmn-at-aol-dot-com>

In a message dated 12/21/00 7:15:19 PM Pacific Standard Time, 
tesla-at-pupman-dot-com writes:

<< Original poster: "Tim by way of Terry Fritz <twftesla-at-uswest-dot-net>" 
<warpath-at-wtp-dot-net>
 
 Hi to All, My name is Tim Walrath. I am endeavoring to build my first
 Tesla Coil. I have purchased a 15000 volt neon sign transformer that
 puts out 450VA at 30MA. I am wanting to build my own capacitors. I am
 planning on using LDPE sheets at 6mil thickness and aluminun roof
 flashing for the conductor plates. I am purchasing some polypropylene to
 make the cases and plexiglass to make the tops. I am having trouble
 figuring out how to figure the capacitance needed for the caps- that is
 how much LDPE to use between each conductor plate. Math never was my
 best subject but I do want to be able to figure this out. Any help would
 be appreciated. Thanks, Tim
  >>
Tim,

The formula for capacitance (in pf) is C = (.224 x K  x A / d) x N-1
K is the dielectric constant of the material. LDPE should be 2.3
A is the area of one plate in square inches
d is the distance between plates (thickness of the dielectric in inches)
N is the number of plates used in the capacitor stack.
The thickness of poly required to stand up to Tesla coil abuse with a 15 kv 
transformer is not real straightforward.  You will probably get several 
opinions on this subject.  I have built two rolled poly caps and eventually 
destroyed both of them by overvoltage - punctured the dielectric.  My 
personal opinion is that you need a minimum thickness of .090" which would be 
15 layers of 6 mil material.

Be sure to run this cap under oil.  Either mineral oil or a good transformer 
oil.  If you have a vacuum pump to evacuate the enclosure and back fill with 
oil, that would be best.  If not, fill with oil and let it set for a couple 
of days.  Occasionally bumping or tapping on the enclosure to help the 
trapped air bubbles get out.  Finally, set it up with a your transformer and 
a spark gap and run it for a while at low voltage (using a variac) and 
gradually, over a few hours, increase the voltage.  This will also help to 
get the air bubbles out.

Good luck.

Ed Sonderman