[TCML] Tuning the primary

[Jim Lux]

On 4/1/12 9:01 PM, Jason Dunphy wrote:
> I have built 2 tesla coils already and will be building a third soon,
> but would like a little advice before pushing ahead. I am building a
> simple spark gap pancake style coil. In my limited experience I have
> noticed that when "tuning" the primary on a coil the more material in
> the primary I electrically connected to, the larger the sparks get. At
> one point I ran out of coil and couldn't make it any larger. My question
> is if I built this coil with a really large primary coil, and I tune it
> by using more and more of the primary, is there a point when I stop
> getting larger sparks? And if I pass this point will my sparks start
> getting smaller or will they stay the same? Can anyone shed some light
> on this?
>
> I have noticed something similar with the amount of capacitors I have in
> the circuit. The more I add, the larger the sparks got. Until I ran out
> of capacitors. Can you install too many capacitors? What happens if you do?
>
> Any help is appreciated.
>


Yes to both.

As you add more primary turns, you're lowering the resonant frequency 
(by adding inductance) and bringing it closer to the resonant frequency 
of the secondary.  At some point, though, you'll pass that point, and 
then you're moving away from resonance and sparks get smaller.

Capacitance is the same.  There are some issues with capacitance, 
because there are two resonances to worry about. Of course, you want to 
resonate with the secondary, but if you pick the "wrong" capacitance it 
will resonate with the inductance of the neon sign transformer (at 60 
Hz) and lead to higher voltages than the insulation of the transformer 
can take.  You want  your primary capacitance to either be Larger Than 
Resonant (LTR) or smaller than resonant with your power source.

Raising the primary C also (potentially) allows for a bigger "bang size" 
  (more energy per firing of the primary gap).


I figure the "bad" capacitance this way:

X = NST voltage/ NST current
     (for a 15/30 this is 15E3/30E-3 or 500,000)

C = 1/(377 * X)
	(turning around X = 1/(2*pi*f*C)
	(use 314 if you're in 60Hz land)
     (for that 15/30, then, 0.005 uF)

If you have a static gap across the NST, then resonating with the NST 
isn't as much of a problem, because the static gap will limit the 
voltage. However if it misfires (much more of an issue with a rotary 
gap), the voltage can get quite high (double), and NSTs aren't built 
with a lot of design margin.