RE: The 1500t secondary myth (long)

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Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>

I thought I better put in my 0.006 GBP on this one ;)

>It's not a myth --- it's physics, pure and simple.
>The potential developed across an inductor is V = -L di/dt.
>It is very apparent that the larger the inductance, the large the output
>potential

This is true. But also, the larger the inductance, the lower the operating
frequency. Hence you don't really push any more current through a given
streamer capacitance than you did before (since I=C dv/dt)

By this line of argument I wouldn't expect massive differences between
higher and lower frequency coils. And indeed in practice this seems to be
the case. There does seem to be a sweet spot around the 1000-1500 turn range
but it's nothing drastic.


The new theory that I have been working on (with a lot of input from Terry,
Malcolm Watts, and others) suggests that the sweet spot is not any
particular number of turns, or even any particular inductance. Rather, it's
when the resonator has a characteristic impedance (Zo) of around 36,000
ohms. Zo is a function of both secondary inductance and secondary/toroid
capacitance:

Zo=sqrt(L/C) or Zo=2*pi*f*L or Zo=1/(2*pi*f*C) where f is your coil's
resonant frequency

To check my theory, I have calculated Zo for various coils with good
performance documented on the net, and they all come out around 40 to 50k.
See my earlier posts for details. John Freau's TT-42, which is still one of
the most efficient coils ever, has Zo=44k. John found that adding a bigger
toroid to his TT-42 (which would lower the Zo towards 36k) increased the
spark length.
And, Richie Burnett found that a coil with Zo=22k performed poorly, but
changing to a resonator with Zo=~50k gave much bigger sparks with the same
bang energy.

Hence I think there is fairly good evidence for my theory. If anyone knows
of any good coils with Zo greatly different from 36k, please let us know.


So my recipe for an optimised coil is-

1) Decide what length of sparks you want

2) Work at around 100 bps

3) Use Freau's efficiency equation to get the required bang energy

4) Size the toroid so it will just break out at this bang energy

5) Choose the secondary height big enough to avoid flashovers and racing
arcs (ie more than 1 meter of height per megavolt- you can get a more
accurate figure from Paul Nicholson's TSSP voltage gradient plots)

6) Now choose the secondary diameter and number of turns so that Zo is 36k.
If this is inconvenient (maybe it gives too high an operating frequency for
your *SSTC) you can increase the inductance provided that the "Q-limited
streamer length" works out bigger than your target streamer length. I think
I already posted how to calculate this.

7) Finally choose the primary and tank capacitor to match your power supply
voltage/current and secondary frequency, and deliver the required bang
energy. The procedure for this is well known already (at least for spark-gap
coils and OLTCs)



PS. For what it's worth, I favour using a "fat" secondary like Dr. Resonance
does. A fat secondary is better at "catching" the magnetic field from the
primary than a thin one, so you can achieve high coupling without getting
the primary dangerously close. And it makes the coil look mean. It also has
a higher unloaded Q than a long skinny one, but in the light of the new
theory I don't think this is much of an issue (as the loaded Q of our
optimal coil is only 6)

Steve C.