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Tesla Primary

Quoting bboettje-at-su102s.ess.harris-dot-com (Bruce Boettjer):

> I have a question regarding the topology of Tesla primaries.  

> The pictures I have seen of Tesla's coils have the primaries 
> stacked in a vertical cylinder, a uniform distance from the 
> secondary...

> The pictures I have seen of modern Tesla coils (ie... R. 
> Quick's outstanding video and stills) have the primary lying in
> a horizontal plane, perpindicular to  the secondary, spiraling
> away in an ever-increasing arc...  Others even have a 
> combination of both...

> What is the difference?  Is it (I suspect) an issue of Q?  or 
> is there a tuning issue? Bruce Boettjer

Good question. I think it really boils down to the peak powers
developed when using modern pulse capacitors.

Fifty years ago the most common capacitor used in a Tesla coil
was the classic stacked plate glass capacitor. Glass has a very
high RF dissipation factor and is very lossy. Tesla patented
liquid oil dielectric capacitors that are much higher Q, but
there was not a high demand for these, so they did not see
"assembly line" production in commercial quantities. Tesla did
not have enough of these on hand to operate his larger coils.

The peak powers developed by glass caps are hideous when compared
to a plastic film unit, and even most homemade plastic caps pale 
when compared to the peak powers developed by a commercial
polypropylene cap (such as those purchased by the group from
Condenser Products).

Using a glass capacitor you need a vertical helix primary in
order to couple enough energy into the secondary for decent spark
production. Swapping a homemade plastic capacitor for the glass
plate capacitor results in "splitting" of the secondary; too much
energy coupled into the coil. The coupling must be loosened in
order to restore synergy or "critical coupling" between the two
coils and maintain efficient operation. If you are already
working with a vertical helix primary you have to lift the
secondary up to loosen the coupling. This becomes a problem when
firing indoors with restricted ceiling heights, as I have had to
lift the secondary completely above vertical helix primaries in
some cases.

The design solution is to reduce the aspect of the primary. This
first results in a primary winding that is shaped into an
inverted conical section, or "saucer" type primaries. These are
quite versatile for many coilers, especially for beginners with 
homemade plastic film capacitors.

But, then the inevitable occurs. The coiler advances; peak powers
grow with design and construction skills. Commercial capacitors
begin to make an appearance and the primaries flatten out into
"pancakes" that allow the coiler to critically couple more
efficient, high peak powered, 1/4 wave systems.

I need to inject another note into this discussion, and that is
the spark gap performance. The peak power developed in a Tesla
tank circuit is only as good as the spark gap. I guess this is
another way of saying that the best commercial capacitors will
not perform well when switched with a second rate spark gap
system. I always assume that spark gap performance increases with
the skill of the coiler. But, the very best gap performance will
still not produce astounding peak powers from a lossy capacitor.

I will also make a note concerning my 8" coil presently sitting
on the large flat primary (used on my 10" diameter coil in the
video). This 8" coil has plenty of room in this primary, yet it
begins to overcouple and split as I increase the voltage input
into the tank circuit. I am sure the reason is due to the
exceptional performance of the .025 uF 20KVAC pulse cap purchased
through the group.

To close, I want to point out that Tesla saw the problem (limited
gain) of having to reduce the coupling between primary/secondary
coils in 1/4 wave systems and went on to develop Magnifiers as
means to restore tight coupling while increasing peak powers. The
result is most certainly more spark per watt when crossing the 5
kVA input power line into more powerful systems.

Richard Quick

... If all else fails... Throw another megavolt across it!