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Re: Variable Inductor

 * Original msg to: Rwall-at-ix-dot-netcom-dot-com
 * Carbons sent to: usa-tesla-at-usa-dot-net

Quoting: Richard Wayne Wall <rwall-at-IX.NETCOM.COM>:

 > You are getting off on the wrong foot here. The key words are
 > "Tank Circuit". This is specifically referring to the          
 > oscillating circuit between the primary coil "L1", the tank    
 > circuit capacitor "C1", inclusive of the main system spark gap 
 > G1. This forms the classic LC oscillator loop with the gap G1  
 > as the oscillator switch. Richard Quick


    TESLA COIL SCHEMATIC                             ---------
                                                     |       | T1
                                                     ---------
                                                         O
         X1                                              O
                            RF1           G1             O  L2    
         ||O---------------nnnnn--------*   *----        O
         ||O    |BC1   |            |           |   L1   O
Line in> ||O  -----    |            |           |    O   O
--------O||O  -----    * SG      -------        |--->O   O
        O||O----|------*--|grnd       C1             O   O
--------O||O  -----    *         -------             O   O
         ||O  -----    |            |                O   O
         ||O    |BC2   |            |                O   O
         ||O---------------uuuuu---------------------O   O
                            RF2                          |
                                                         |
                                                        Gnd

       X1 = Neon Sign or other xfmr with grounded center tap
       SG = Safety Gap. 
      BC1 = Bypass Capacitor
      BC2 = Bypass Capacitor
      RF1 = Radio Frequency Choke
      RF2 = Radio Frequency Choke
       G1 = Main system (tank circuit) Spark Gap.
       C1 = Tank Circuit Capacitor
       L1 = Primary Coil.
       L2 = Secondary Coil.
       T1 = Toroid discharge terminal                             
Grnd, Gnd = RF Ground.

Note: the tank circuit is the loop around C1-L1-G1.

 RW> Seems to me that the G1-C1-L1 series tank is in parallel     
 RW> across C1 which forms its own series tank circuit with the   
 RW> secondary of the power transformer.  How can a line be drawn 
 RW> that arbitrarily separates the two circuits? 

Because the oscillator must have a switch in order to function,
which the tank circuit has, but the power supply/feed lines do
not have. Without a switch the only oscillation that can occur in
the power supply/feed lines would be at 60 cycles, and the switch
in this case would be the polarity reversal on the line in. The
odds of hitting a 60 cycle tuned circuit here by accident are
rather small, and very easily corrected.

 RW> Where does the kickback come from that destroys the power 
 RW> xfmr secondary?  If they are separate and apart then there   
 RW> is no need for any anti kickback chokes, capacitors or       
 RW> safety gaps.

Just because there are no oscillations in the power supply/feed
lines does not mean that RF does not bleed over from the tank
circuit. It does. So do kickbacks. The causes of kickbacks are
many: operating while poorly tuned, poorly coupled, breakdowns
between primary turns, misfires in the main system spark gap,
failure of any component such as an open ground connection or
punctured capacitor dielectric, secondary striking the primary or
tank circuit, capacitor imbalance (some circuits), capacitor
drift (some circuits/capacitors). I have seen "textbook" examples
of all of these, but there are other causes that I am sure I
neglected to mention.

The circuits are separate in that they are functionally
different. One operates at 60 cycles, the other operates at RF.
The difference is like night and day. The purpose of protection
and filter components is to ensure that the difference remains 
distinct: you don't want RF in your power supply and feed lines
any more than you want 60 cycles in your tank circuit when it is
supposed to be oscillating clean RF. There is some RF bleedover,
and kickbacks will leave the tank circuit to seek lower
potentials, this does not mean that the circuits operate in the
same fashion, or have the same problems or tendencies: they
don't.

> Also, after tuning a primary by tapping does the distal coil 
> (portion after the tap) have self inductance and if so how does
> it affect L1 and L2 of the Tesla coil?  

Yes the open portion of the primary has some self inductance, but
it is a small value in a well designed and tuned coil. I have
found no indications that a few turns of untapped primary are
sacrificing efficiency. 

> If there is an adverse effect why not remove it? Or, why not 
> make the primary just a little bit short and tune with
> a small variable inductor off axis.

As I said, I see no real reason to remove it. Unfortunately both
solutions you propose would be detrimental to the system
efficiency. Shortening the primary is a no-no, this decreases the
tank circuit Q. Off-axis inductance for tuning also decreases the
tank circuit Q in that every percentage of off-axis inductance
introduced is a percentage point of efficiency lost.

Richard Quick


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