[Prev][Next][Index][Thread]

Re: Radical Flat Cap design

Bert:

Thanks for the excellent reply!  My rejoinder:

Yes, in theory you are correct.  Thinner dielectric does indeed have greater
dielectric strength.  Practically speaking, however, most home flat cap
builders don't happen  to have a vaccum evacuator lying about with which to
vacuum down their cap.  Building a cap of any design (roll or flat) with
*out* such a vacuum will inevitable leave tons of trapped air bubbles between
the layers.  The more layers the more  trapped air.  In this case, the
tradeoff is between many layers and their greater dielectric strength but
more trapped air, and fewer layers with less dielectric strength but also far
fewer air bubbles to cause heating and melting/breakdown.

In this scenario, I believe that the advantage is with the thicker
dielectric.  Thicker dielectric, purchased from a plastics dealer is also
much higher quality with fewer imperfections than the thin roll sheeting
purchased from the local hardware store.  The reduction of dielectric
strength that occurs with increasing thickness of dielectric can be offset
somewhat by over building the cap (greater total thickness of dielectric
between plates) than what the spec sheet says.  120 mills seems like the
magic number!

As regards tight clamping:  Some method of attaching the foil plate to the
surface of the dielectric is necessary to prevent plate shifting during tesla
operation.  This is required because at working voltages and currents, the
plates in a tesla coil cap push apart and pull together with the frequency of
the power supply much like an accordion at a polish wedding.;)  None of the
regular adhesive strategies I examined were workable.  They either dissolved
in oil and/or were too thick - which over 50 plates or so adds up to alot!  

This leaves clamping pressure alone as the "glue " of choice.  Once again
another tradeoff; if the plates are clamped too loosely, they will shift,
this is not good.  Too tight a clamping pressure will cause the bogie of
trapped air bubbles and pockets leading to cap failure.  The solution of
choice for this is the paper wick between plates.  The paper I used was an
exceptionally absorbent yet relatively stiff  sketching paper by Bienfang
called "Newsprint - rough surface."

Bottom line: for the home builder without a vaccum pump, I believe the
advantage is with fewer thicker, higher quality  plates with paper wicks that
with many thin sheets.  Time *indeed* will tell.  I would appreciate anyone
who follows one  design philosophy or the other posting their
successes/failures.  Perhaps somewhat thinner layers - 30 mills thick with
paper between each layer would be a workable compromise.  My next cap
perhaps......

Gassless capping to you!

 - DavidF -