I have noticed a number of postings on capacitors, there testing etc.
Here is my two cents worth.
The best, high end, professionally constructed tesla caps should be made
out of one substance only...pure polypropylene film and foil. They
should be of end foil construction and be vacuum impregnated with oil.
End of statement!
All other cap materials and constructions are of a lesser quality and
Many, many other caps and constructions will work fine in some Tesla
Materials which seem nice:
Mica is often touted as the best RF capacitor material. This is true
with CW AM radio work where continous RF currents are of a moderate
level. For Tesla Coil work mica is just "good" no better. Its
disappation factor is at least an order of magnitude (10 times) worse
than polypropylene or polyethylene. Therefore mica caps get warm/hot in
RF service. They can take the heat, however, up to almost 800 degrees
F.!!! wow! You could fry eggs on a mica cap while in service and it
would still be far below any danger level in continous service. The
energy to fry those eggs would come from what would have been coil spark,
length though. again, mica is good, but no better. A better term might
be that they are vitually indestructable as long as you don't exceed
their rated voltage.
Teflon is a substance which has all the best properties we want! It has
a couple of big flaws though. It is full of holes like swiss cheese. In
a teflon cap multiple layer of thin dielectric are needed to guarantee
voltage standoff. The stuff "works around a bit". That is, it moves
while in service. This is due to coulombic forces within the cap and its
slippery nature. It is probably one of the most expensive of all
plastics, too. It is not generally used in capacitors.
Door knob capacitors are of high dielectric constant materials such as
barium and strontium titanate. They are all two plate, flat plate
capacitors! The two plates are few molecule thick silver films, vapor
deposited on the ends of the titanate dielectric cylinder. In general,
these are ok for Tesla use, but they have high dielectric losses (heat
up) and as the coil is used, the silver film end plates are blown to
vapor and the capacitors value drops over a period of time. How fast
this occurs depends on power input and rep rate of the gap firing. Use
only in low power systems.
Other good choices for substitute caps in coil service:
Old oil immersed kraft paper and foil capacitors used to be the pulse cap
of choice right after mica. These are excellent for Tesla use if a bit
of disappation can be tolerated. They are cheap and available surplus
for very low prices.
Probably the second best cap for all Tesla service is polypropylene,
paper and foil caps. These are used as snubber caps in large SCR
switching circuits. The disappation is still extremely low and they are
far more rugged than the #1,optimal pure polypropylene and foil caps.
These can often be found surplus at hamfests. They look like motor start
caps, but have a tell tale "10khz" frequency marking on the can.
To my way of thinking disappation factor is the number one criteria to be
sought out in a Tesla coil capacitor's dielectric.
Flat plate capactiors are bulky and have low "volumetric efficiency".
They are, however, very easy to make and are good for very small systems
where the amateur is just starting out and must build it all from
Rolled capactiors double the capacitance per unit area. These alow the
builder to advance to a higher level (<2KW) while still allowing total
home construction. Amateur construction of capacitors usually must stop
at this level and more professional grade components must be secured for
levels much above 4KW.
End foil construction is really a rolled capacitor with virtually zero
inductance. (Foils extend beyond the dielectric at both ends and are
smashed or welded together) This is a professional wind only! The
amateur cannot keep air from within a casually wound end foil type cap.
Amateur oil filling:
In amateur service, the oil is an agent to supress corona and, hopefully,
exclude air and moisture from the capacitor. This can be done casually
if air is expelled by pressure or mild tesla service at first during a
break in period. Vacuum impregnation is desirable, but few amateurs have
the necessary tools, skills and know how to pull this off to perfection.
Still, even a slipshod attempt at vacuum impregnation would be better
than just dunking the finished capacitor in oil.
It is important to remember that no service for a capcitor, yet devised,
abuses the capacitor or stresses it more than a disruptively discharged
If you make your own caps, they must be made well, or they won't survive.
Know the kind of service you will be placing them in. Don't think you
can triple your power from 1 to 3KW and have your old simple flat plate
capacitor survive. If you buy your capacitors, be prepared to pay big
bucks for premium units.
Richard Hull, TCBOR