Re: Coupling Experiments

Hello Coilers,
        Here is part of a dialog between Mark Rzeszotarski and Malcolm Watts 
which may stimulate some further discussion on "the list".  Apologies for 
the length.
>Mark Rzeszotarski said:
>> I have been doing some simulations of a conventional tesla coil
>> using a lumped element model, with the primary and secondary each
>> consisting of a series RLC circuit, coupled by the mutual
>> inductance M.  I tried the values you posted earlier where you
>> varied Cp and Lp from (50nF,25uH) to (25nF,50uH) to (18nF,66uH)
>> respectively, with various coupling coefficients. I made some
>> assumptions regarding typical primary and secondary R.F.
>> resistances, and assumed you used your 44.5 mH secondary with
>> about 28 pF total secondary capacitance (distributed plus toroid).
>> The clear winner in the list of combinations you posted was the
>> (50nF,25uH)  primary with K=0.2 in my simulations.
>Malcolm Watts replied:
>Presumably a larger cap would have been even better. I was limited by 
>what the transformer could do (I should have posted its spec. along 
>with the data. It's clear there is more primary loading with the 
>larger cap. I got some bright blue bolts with that combination when
>I was shot testing. But the gaps were showing real stress, and never
>eased up as the load was varied whereas the 25nF combination showed
>that transfer was much more efficient into long coronas.
>Mark said:
>> A distant second was the k=0.185 (25nF, 50uH) combination.  I am
>> curious how this stacks up with spark lengths you achieved.
>Malcolm replied:
>Not enough power to test properly - I badly need a second transformer 
>to parallel up with the first. I did get longer sparks in shot 
>testing with the 50nF combination but only marginally. What was 
>different was the quality of the discharge. The figures clearly 
>showed a better match to a low impedance secondary load.
        I, too, got much different results when I switched from a high kV, 
low mA transformer to a lower kV, higher mA unit.

>Mark said:
>> I realize this is not consistant with the general philosophy of
>> using a high inductance primary, but makes sense if the R.F.
>> resistance of the primary and secondary circuits are considered.
>Malcolm replied:
>Curiously enough, my larger coil at work that has almost identical
>inductance, capacitance etc. but is physically much different also
>works well with small Lp, large Cp, but the power supply is a low
>voltage, much higher current job. Much more controllable than the
>neon I have to say. I don't like neons at all. The internal inductance
>makes them difficult to control. I can't wait to get the rotary 
        I think there is a significant tradeoff between capacitor size, 
spark gap design and transformer "stiffness".  If one could send a high 
charging current to the capacitor, it appears that a high C, low L primary 
would work well.  Obviously, a number of individuals have been very 
successful with this combination.  However, high current capability means 
that a power arc may be difficult to quench in the spark gap.  Hence, there 
is a tradeoff here.
        Another factor is the resistive losses in the capacitor, especially 
in home-built models.  At peak discharge, a well designed primary tank 
circuit may circulate currents in the range of 50-100 amperes.  Most 
home-made capacitors have connections with fairly lossy thin wires, and are 
not designed to handle these peak currents.  As a result, the tank circuit 
operates in a very lossy fashion.  This significantly reduces the energy 
transfer to the secondary during the spark gap conduction phase, and 
shortens the dwell time, which is the time at which maximum energy has been 
transferred to the secondary (at which time you hope your spark gap 
quenches, so the secondary can ring down on its own).  Short dwell times 
require forced air gaps, rotaries, or other quenched gap architectures to 
turn off the spark quickly for efficient operation.
        BTW, my C.P. commercial capacitor has very low internal resistance, 
high discharge current capability, and has markedly improved performance 
over any of the flat plate polyethylene capacitors I have built.  Too bad my 
transformers can't supply it with what it needs (still using neons here, for 
Mark S. Rzeszotarski, Ph.D.