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Re: TC Output Voltage
Tesla List wrote:
>
> >From lod-at-pacbell-dot-netMon Nov 11 23:18:12 1996
> Date: Sat, 11 Nov 1995 21:37:05 +0000
> From: GE Leyh <lod-at-pacbell-dot-net>
> To: tesla-at-pupman-dot-com
> Subject: Re: TC Output Voltage
>
> Malcolm Watts wrote:
>
> > Point taken :) I think effective arc resistances can only be measured
> > for and be singly applicable to a well defined set of conditions for
> > a particular system. I remember doing a rough estimate on one running
> > coil of several kilohms. Channel current and channel length are both
> > rather variable quantities. A low Zo is going to give a rather low
> > unloaded Zshunt which equates with low voltage production for a given
> > Zbase IMO. (Zo^2 = Zbase x Ztop)
> >
> > Malcolm
>
> How did you estimate Zarc? Do you think that it can be determined by
> watching the decrement of the current waveform at the base of the
> secondary coil?
> If I remember correctly, a resonant tank ckt is critically damped
> when SQRT(L/C) = 2R, and max power xfer occurs at SQRT(L/C) = R.
> TC secondaries seem to ring for quite a few cycles, indicating that
> the typical Zsec is actually much _lower_ than Zarc. If this is the
> case, how does adding a toroid help with spark production?
>
> -GL
Greg and Malcolm,
It should be possible to measure the secondary Q while the coil is
producing heavy streamers, and derive the amount of energy lost, and
therefore an "average" Zarc. Admittedly, this is tough to do, as the
arc's characteristics are constantly changing. I've had some limited
success by taking a number of successive measurements on my 10" coil by
using a storage scope and trying to estimate the "average" time for the
output waveform to decay , deriving Tau, and then estimating Zarc.
My primary Q, with gap firing, is about 11. The secondary Q runs at
about
19 (single shot breakout case), and further declines with increasing
power levels to around 11-12 under full power (full streamers). I
believe the streamers to air should appear as non-linear "leakage
current" across the secondary output capactiance. As Greg suggests, just
looking at the resulting Q with full streamer production _seems_ to
imply a fairly high "average" arc impedance versus Zo (about 460K Ohms
for my coil versus secondary/toroid surge impedance Zo of 42000 Ohms at
F0=91 kHz). Direct strikes to ground are another matter entirely...
However, I also have little doubt that peak streamer currents are much
higher than the "average" implied by Zarc. Below a certain voltage value
the current drops sharply as streamers can no longer be maintained.
Above this level, currrent climbs rapidly as the channel is formed and
grown on successive secondary voltage peaks. In effect, although the
average energy loss could be grossly modelled with Zarc via Q
measurements, I'd expect that the actual arc current versus voltage
would change _very non-linearly_ as we initially break down the air,
extinguish the arc as we go through the zero crossing, and then
re-ignite the previously formed channel, and perhaps grow it, as we head
to the next voltage peak. The voltage:current curve probably also
exhibits significant hysteresis during these transitions.
As we increase power levels, the average arc "impedance" will certainly
drop, since we'll be losing energy to longer and fatter arcs at a faster
rate.
I would suspect that anything we can do to lower the coil output
impedance versus Zarc will allow us to drive more current (and energy)
into the streamer once it has formed. Once formed, the negative
resistance characteristic of the glow or arc should help us extend
streamer length as long as we can force more current into the decreasing
arc impedance. Each discharge may initially "reignite" as a glow
discharge (since thermionic emmision is probably NOT occuring at the
toroid surface) and can rapidly progress to the more efficient arc
discharge _if_ we have a low enough source impedance. Lowering Zo
(either by making a large diameter coil or using a heavy top-load)
relative to Zarc at the desired power levels should allow us to
accomplish the same results - longer, hotter sparks.
I'm going to try taking some more measurements under high-power, but I'm
a little concerned about "frying" my storage scope - I get "zapped" from
near-field effects just touching the scope as I make adjustments. Maybe
a chicken-wire screen to block the electrostatic field betwwen the coil
(and me) and the scope??
Anyway, safe coilin' to ya!
-- Bert --