Tesla Coil builder sites are good places to read about different
builder's experiences with high voltage caps.
Their advice is to limit the voltage across each capacitor to no
more than 5KV. Many of them are using Polyethylene sheet as
insulation for their high voltage caps and they recommend
immersing them in oil. They subject their capacitors to tremendous
peak voltages and currents as most Tesla coils are based on spark-gap
generators of RF. Their capacitors are all fixed ones though, not
variable.
Nowadays many Tesla Coil builders have switched to Multi-Mini-Capacitors
or MMCs. These are strings of 1500 - 2000 volt DC rated polypropylene
caps wired in series-parallel. They said it is best to use the metal
foil/film types
and not metallized/film types.
They say that if the voltage is 5KV or less Corona is usually not a
problem.
Also some companies do make grades of Teflon that are highly Corona
resistant so a lot
depends on the grade of Teflon used for a high voltage capacitor.
So far I have had good luck building trombone-style high voltage variable
capacitors for a transmitting loop antenna using a thin sheet of Standard
Teflon
wrapped tightly over copper pipes that slide inside slightly larger copper
pipes
for the loop tuning variable capacitor. This would probably work even
better with
modified-grade Teflon but don't know how expensive that stuff might be.
So far no flashovers or any problems with heating at a calculated nearly 4
KV RF
in the loop and 8 amperes of RF current using standard Teflon sheet. Very
inexpensive to build. Teflon makes very high Q low-loss capacitors needed
for a
transmitting loop.
73 Todd WD4NGG
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