Jumping in with both feet?

From:  SBJohnston-at-aol-dot-com [SMTP:SBJohnston-at-aol-dot-com]
Sent:  Wednesday, January 28, 1998 12:59 PM
To:  tesla-at-pupman-dot-com
Subject:  Re:  Jumping in with both feet?

Bert wrote:

>It's not clear why he constructed it in 2 parts with a 3" gap 
>in the center. Perhaps he wanted to have modular
>building blocks to test different coil systems(?). 

Perhaps, but the wire winds between the sections continuously (no splice),
and the wooden coil form is one piece.  I can't see any functional difference
with the 3" space -- it is like you grabbed a B&W coil by the ends and pulled
it slightly apart in the middle.

>You may want to make a simple small toroid from 3-4" 
>aluminum dryer duct - check the archives for details.  

Thanks for the tip -- I downloaded some material on toroidal terminals and
I'll check it out tonight.  Any suggestions on the diameter of a dryer-duct
torroid for this rig?

>You didn't mention anything about taps on the primary,
>so it sounds like he was using the entire primary winding. 
>When combined with the tank capacitance of about 0.02 uF, 
>the primary tank would have a center frequency of about 121 
>kHz.  The overall resonant frequency of the secondary and top
>terminal combined would be about 140 kHz, so it sounds 
>like the system was roughly in tune

Well, there is a spot on the 9th turn of the primary where it looks like
connections were made at times.  This would raise the resonant freq to about
134 Hz, I guess - closer to the resonant freq of the secondary.

>If the primary is only being driven off half of the neon 
>(7.5 kV at 30 mA), the system sounds like it's severely 
>underpowered. Also, at this input voltage and current, the 
>neon transformer is capable of driving a 0.011 uF tank cap 
>optimally - the 0.02 uF cap sounds like its a bit too
>large to permit rapid recharging after each gap firing. It 
>would be better driven by half of a 15 kV 60 mA trannie. 
>It also sounds like he may have been constrained by the 
>voltage handling capability of his tank caps. 

I'll have to try and see if I can find any markings on these strange-looking,
finned capacitors.  Perhaps they could not handle 15 kV, so he used half the
output winding.  Or it could be that he wanted to ground the case of the
transformer for some reason -- in this unbalanced circuit that would make it
impossible to use the other half of the winding.

Oh - how about this... If I cut one cap loose, that would drop the total C to
0.015 uF - closer to what you spec for a proper transformer load.  This in
turn would change the res freq of the primary to about 140 Hz (with the full
11 turn primary coil).  Sounds about perfect!  Hmmm..?

Thanks for all the excellent information, Bert.  

Steve Johnston


A coil this size can handle much more input power, with
commensurate increases in spark length. Doubling the tank cap voltage
(gap firing voltage) quadruples the coil's energy level and dramatically
increases output performance. You will probably need to replace the tank
caps with more robust ones.

The single spark gap is also a weak area, since it will not provide very
good quenching. This will further limit the coil's performance, since
secondary energy can transfer back to the primary tank circuit if the
gap does not extinguish at the proper time. For testing purpose, you
should be able to set the gap to 0.100". Because of the underpowered
power source, you may not be able to set the gap very much wider and
have consistent firing.