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Bipolar Tesla Coil

 * Original msg to: Esondrmn-at-aol-dot-com
 * Carbons sent to: usa-tesla-at-usa-dot-net

Quoting Esondrmn-at-aol-dot-com:

> I have been trying to help David Lawrence design a coil that is
> best described as an Ouden (can't remember how to spell it) or
> center fed Tesla coil.  

Good attempt, these are bipolar Tesla coils. Oudin has no
priority, and therefore no claim to any of the circuits he used.
These coil configurations are called bipolar Tesla coils.

> He wants to experiment with intense electric fields created 
> between two plates connected to the ends of the coil.  

There is no better system to experiment with to produce these
effects.

> He has ordered one of the Condenser Products .025 mfd 
> capacitors so I thought we shoot for a frequency that would
allow him to use that.  He has a 4.0 ft x 12" form. 

This aspect ratio is a little low for a bipolar secondary, but it
is workable. I would shoot for something a little closer to a 6:1
aspect ratio, allowing a 3:1 aspect between the mid-point on the
coil and each end.

> I did some rough calculations today and came up with some 
> numbers. For a normal 1/4 wave Tesla coil we want about 900 
> turns of wire.  With this type of coil I think we want 1/2 wave
> - or twice the amount of wire.  Using 1800 turns of wire (he 
> has #25) we come up with a self resonate frequency of about 69
> khz.  

My chart states that #25 AWG enamel winds at about 51.7 TPI
(turns per inch). Close winding, you are going to end up with an
actual winding length of roughly 35 inches. Given your coil form
diameter of 12 inches, the actual winding ends up with an aspect
ratio of less than 3, this is too low for bipolar work in my
opinion. The work I have done with these coils show that erring
on the side of a longer, skinnier coil (higher aspect ratio) is
better for bipolar coiling.

> In order to use his capacitor, we come up with a primary that 
> is about 20 turns of 3/8" copper tubing on .75 centers wound on
> a 24" form about 15 inches long.  

Humm, I detect a problem here. The bipolar coil will actually act
as two 1/4 wave coils with a null voltage point in the middle of
the winding. In order to tune a tank circuit to fire a bipolar
coils, you must calculate the 1/4 wave frequency, then multiply
by two, to get the tank circuit frequency that properly tunes for
this configuration.

Richard Quick



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