Tesla Caps


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 knobs!

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.

Other constructions:

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.

To summarize:

It is important to remember that no service for a capcitor, yet devised, 
abuses the capacitor or stresses it more than a disruptively discharged 
Tesla coil.

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