Re: Overhead Transperency Cap

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

Original poster: Jan Wagner <jwagner-at-cc.hut.fi> 

Hi,

On Wed, 19 Nov 2003, Tesla list wrote:
 > I am considering building an A4 sized plate stack cap using overhead
 > transperency shhets as a dielectric.
 > Has anyone had any luck/ exeperience with these...or even what theyre made
 > of.

Before starting with SSTCs I once built a couple of caps like that i.e.
for traditional spark gap coils. One A5 cap and some A4 caps. What the
transparency sheets are actually made of, no idea. It varies a lot from
manufacturer to manufacturer and can be a mixture of different polymers -
polyethylene, acrylics, acetates, and others.

But the transparency sheets certainly do work! The performance is quite
acceptable. You can see some capacitor info at

  http://www.hut.fi/~jwagner/tesla/tc-small.htm

For designing your cap, a voltage rating of 500..1.5kVDC peak accross each
sheet is decent. <500V per sheet would be even better.

The old and used overhead sheets I got, worked with 2kVpeak/sheet but not
for very long, maybe a couple of hours in total over a few months.
Although, the large charred hole blasted later into the cap (same cap as
on the above web page) was probably more due to the vegetable oil drying
out or getting rancid over time, so corona finally got the hand.
Transformer oil might've been better...

These certainly lasted longer than the rolled polypropylene sheet caps I
made. It's much easier to build these A4 overhead transparency sheet caps
(properly!), than making a proper rolled sheet cap with no air bubbles
inside and good contact between the aluminium sheet and dielectric
(rolling up the cap twists the sheets and the surfaces become uneven,
not good for HV). On the other hand the A4 ones take slightly longer time
to build...

 > I also heard somewhere that you can test possible dielectrics for RF losses
 > by puting them in the microwave and seeing if they get hot? i deas anyone?

Glass doesn't heat in the microwave, yet glass is supposed to have quite
high loss at the Tesla coil i.e. low frequency range. Not a good test,
then.

cheers,
  - jfw

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  Jan OH2GHR