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Re: Capacitance

In a message dated 12/10/1999 3:24:26 AM Mountain Standard Time, 
tesla-at-pupman-dot-com writes:

>  So
>  what's the deal?  What kind of capacitance do I really need?  
>  
>   Thanks,
>         Ryan

Ryan,
    As you read in  Gary's post, the impedance match sizing of a cap is just 
a general guidline that, in my opinion, is most dangerous to NST's due to the 
resonant rise of voltage that results from the transformer and primary cap 
being in resonance at 60 Hz.  
    Others have had great success with LTR(larger than resonance) caps with 
their NST power systems.  The real beauty of this is that you won't exceed 
the output voltage rating of your NST because the "extra"  capacitance loads 
down the transformer and keeps the voltage from getting way too high due to 
the mains, or 60Hz resonance.  If you think about the energy storage 
calculation for a capacitor(E = 0.5 *C*V^2), you will see that if the 
capacitance is increased by a factor of four, the voltage would be half of 
the original in order to store the same amount of energy.  Now realizing that 
your cap will "see" a higher voltage during the resonance ringing while the 
spark gap is firing, it would seem to make sense to have more capacitance in 
the circuit to help limit the voltage of the primary tank circuit.  This puts 
less stress on the filter circuit and transformer.
     Another reason to use more capacitance is that your primary to secondary 
capacitance ratio(and thus, your secondary to primary inductance ratio)  will 
be larger and perhaps lead to greater output voltage of the secondary.  I 
have heard mixed opinions on this matter, due to the larger current that your 
gap has to quench, but if you can quench the arc successfully with such a 
cap, then you should get better output.
     I have read another method for sizing the primary cap that may be of 
use.  It is simply to take the 1/2*C*V^2 * BPS =  Transformer output wattage. 
 For static gaps, you would use 120 breaks per second and for rotary gaps, 
you would use what ever the RPM/60 * the number of electrodes.  
     I am sure there are some who might differ in opinion on the above, but 
this should give you an idea of how much leeway you have in designing your 
primary tank circuit.
Regards,
Mike