Re: Bi-Polar coil design

[Pyrotrons-at-xxxxxxx (by way of Terry Fritz <twftesla-at-xxxxxxxxxx>)]

Hi Terry.

I have some experience with a small (270VA) twin coil. I used the schematic 
shown on Brian Basura's website....and it worked great. It's the schem. where 
the capacitor is in between each primary coil, and the spark gap is in 
parallel with this arrangement. (and the trannie is across the spark gap of 
course)

About the secondary coil's: they are independent of each other as far as the 
resonant frequency goes. Calculate the Fr of one of your secondaries using 
the normal method, and you'll then have the freq. for both of them. They're 
both one-and-the-same if you were careful in the physical construction. (same 
wire size, same form size, same winding length and you'll have it). Certainly 
don't add the inductances because they are in series (connected at the 
bottom). That's not the way it works ; ) Do the math for ONE coil, and its 
the same for the other. Nothing special about the secondary coil's.

You are correct that there is twice as much inductance in the tank circuit 
compared to a single-coil system. Add up the two inductances and plug them in 
the resonant frequency formula [ 1 / 2 * pi * (sqrt (L * C)) ] as if it was a 
single inductance (which it is...) You'll then have your tank Fr. Adjust the 
physical parameters (# turns, spacing) to match the single secondary coil Fr.

Running two 6" secondaries with 2 to 4 kVA would be really impressive. Cool 
project.

A little side note about my small twin: I've found that it is difficult to 
tune for some reason. Seems as if you have to adjust *both* primary taps to 
get it just right. Also, the primary taps were never at the same position on 
both coil's.....they were always different (makes sense).

Gotta remember that changing *one* tap changes the resonant frequency for the 
*whole* tank circuit in relation to the secondary coil's frequency. The tap 
positions are independent of each other......you can tune the whole system 
using only one primary coil tap. Also, you can use ALL of the turns on one 
primary, and only use a few on the other and it will still work great. It's 
neat. And, I've found that adding off-axis inductance near the spark gap is a 
quick-fix for not having enough primary turns. 

Another thing that I've noticed on my twin, is that the spark gap WILL NOT 
QUENCH at all when it's out-of-tune. I think the reflected "kickbacks" are 
re-igniting the gap. When it is in tune, the gap quenches great with no 
airflow at all, even at high-temperatures (ouch-o-meter). Just for the 
record, I got 20" of maximum sparklength with the following:

9kV 30mA in 
1/8" gap spacing (!!!)
.006uF MMC capacitor
3.5" O.D. secondaries 10" tall wound with 24 gauge
5 turn primaries, .165" O.D. copper, 1" clearance from secondaries 

It's been my experience that the sparklength doubles (or comes very close to 
it) in the bipolar configuration.

Hope this helps! Take care,

Justin Hays
KC5PNP
Email: pyrotrons-at-aol-dot-com
Webpage: www.hvguy-dot-com