Re: CD capacitor

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

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

Justin,

You are apparently one of the few remaining hardy souls still constructing
caps from scratch instead of making MMC's! 

The list of common dielectrics that combine availability, low cost, low
loss, good solvent resistance, and high dielectric strength required for
Tesla Coil use is quite short: 

  1. Polyethylene/Polythene - low density (LDPE) or high density (HDPE)
  2. Polypropylene (PP)

Other low-loss dielectrics include polystyrene (PS), Teflon (PTFE), and
Polyimide (PI or Kapton). However, since polystyrene is fairly brittle,
tending to craze/crack under stress in the presence of many dielectric
fluids, it's better suited for flat plate caps than for rolled caps. Teflon
is quite expensive, it cold flows under stress, and may be porous. Kapton
is an excellent dielectric, capable of handling high temperature and
voltage stress, but is about 20X lossier than PP or PE and is MUCH more
expensive. A couple of other relatively low loss polymers include ABS and
polyacetal (POM or Delrin), but these are also 20-30X lossier. Some other
low loss materials include muscovite (especially Ruby Mica), Quartz, and
Paraffin, but these either are difficult to obtain, hard to work, or are
otherwise unsuitable. 

Most other common polymers tend to be significantly lossier than PE or PP
at RF frequency, ranging from 30 to 1000 times lossier than LDPE, HDPE, or
PP!  <:^o   Especially lossy dielectrics include certain types of glass,
PVC, Epoxy or epoxy glass, Nylon, polycarbonate, Mylar, PET, Polyurethane,
Plexiglas (PMMA), PVF, etc., etc...

LDPE is readily available as clear drop cloth material, HDPE is used in 5
gallon plastic buckets, and clear PP is sometimes used as plastic sheet
protectors (available at office supply stores). Both dielectrics are
readily available, inexpensive, and TOP performers for RF and pulse
applications - there's really little reason to use anything else.

Good luck!

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<Cydesho-at-aol-dot-com>
> 
> I'm beginning to find from my various investigations into capacitors built
> from readily available materials that many of them will not work because the
> are too lossy (e.g., mica, glass, polycarbonate, PET, etc.)  Has anyone out
> there compiled a list of materials that will serve as good dieletrics for
> capacitors being used with Tesla coils? It seems that the number of materials
> that are unsuitable for this use outnumber those that are suitable. Rather
> than banging my head against the wall finding out what is not suitable I
> would like to just know what will work. It would definitely be more
> productive, and much better on the brain cells. If anyone out there could
> tell me where to find a list of materials, it would be much appreciated.
>                                     Justin