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Re: Pspice model of Tesla Coil

Hi all,
        A few ideas on modelling the coil.

> I am trying to model a Tesla coil using Pspice.  To start with, I 
> thought I'd try and model the lumped circuit as you would see in any 
> schematic and see what I got.  While I seem to be getting vaguely 
> believable waveforms, the secondary voltage and current magnitudes 
> seem way off (approx 10kV and 200mA peak respectively).
> 
> Has anyone else tried this, or could anyone look at my source code 
> below and tell me what I might need to do to get more beleivable 
> results.  Is Pspice capable of modeling this accurately?????

I think yes, BUT, you have to recognize that nothing is static. 
Parameters change on the fly depending on secondary discharge. Rgap
also changes as the primary rings down for reasons given below.
    I think a non-sparking succussful model is that of a string of
series inductances with a small cap connecting each junction to earth.
Total C value is that of coil self-C. Then a lump of C representing
terminal C at the end of the string to earth. The interturn C's are 
effectively in series and add up to a tiny value and I think can be 
ignored. Greg Leyh has used a similar model. The Q at this stage is
the unloaded secondary Q.
    Once sparks break out, things change. According to measurements 
made by others, corona only causes this Q to approximately halve.
I think you can assume that a solid arc causes loaded Q to drop near 
10 as this condition is required for coupling to become critical by 
definition (for k=0.1). From a transmission line point of view, you 
would still get standing waves on the line under arc conditions 
because the line is still mismatched by having both a feed impedance 
and terminating impedance considerably lower than the characteristic 
impedance.
     The impedance of components external to the tank (chokes, bypass 
caps, transformer etc. can be ignored during gap conduction if the 
gap is situated across these components. Scoping an arc reveals that 
the arc will begin with all but the largest terminal while the 
secondary is still ringing up. This also is a requirement for 
critical coupling.
     The gap must be included in the model. It is that of a pair of 
back-to-back zener diodes with a sustaining voltage around 70V. In
short, the scope reveals it has a negative resistance characteristic.
With Vgap fixed, Rgap is inversely proportional to Igap.
     Primary-secondary coupling is graded as one looks up the winding.
The result is that the secondary volts/turn is highest where coupling 
is tightest at the bottom of the winding.
     Hope this helps. No doubt others will correct me if this runs 
counter to their experiences.

Malcolm

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