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Re: 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:

 >> 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. Ed S.

 > 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. Richard Quick

ES> It sounds like we should be using a 6" dia form about 36" 
ES> long.  That would get us right at 6:1. 

Yup, this has worked very well in my experience.

 >> 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.  Ed Sonderman

 >> 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. Ed Sonderman

 > 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

 ES> Understanding that we probably want to change the form, but  
 ES> for discussion, using the 12" dia form with 35" of windings, 
 ES> I calculate a resonate frequency of about 69 khz.  Are you   
 ES> saying we want to design the primary for 138 khz? 

Yes. 

What you have in a bipolar coil are two 1/4 wave voltage peaks
that are working off of a null voltage nodal point in the center
of the winding. To properly excite any coil to produce 1/4 wave
sparks you must tune the tank circuit to the same frequency as
the 1/4 standing waves you desire to produce. But what is the 1/4
wave frequency of a bipolar coil where there are two voltage
peaks?

The actual 1/4 wave "length" of the bipolar coil (null voltage
node in the middle) is 50% of the 1/4 wave length of the same
winding with a null voltage node on one end. The simplest way to
figure this, given the abundance of 1/4 wave math on this
subject, is to calculate the 1/4 wave frequency of the entire
winding, then multiply this 1/4 wave frequency by two. This gives
the correct tank circuit frequency required to excite the coil in
order to produce two 1/4 wave voltage peaks. 

Richard Quick



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