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Re: Spark length, SSTC vs. spark gap

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Gary,
         I think your note sums up the various conundrums about power 
vs spark length well. It is theoretically possible to generate 
extremely long sparks using a very low average power input. Your 
approach performs that sort of trick. It seems to me that there is a 
need for a list of definitions so that in discussing length/power, we 
can refer to a specific set of parameters. For example, huge sparks 
could be generated by the ringing-up approach (given a suitable 
topload) but for a limited power input could occur at a rate of just 
a few per second. Another case is a gap driven coil producing sparks 
at say 200BPS so that they appear continuous to the eye. Perhaps it 
is all a matter of perception and expectation.

On 8 Feb 01, at 16:55, Tesla list wrote:

> Original poster: "Gary Johnson by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <gjohnson-at-ksu.edu>
> One very desirable feature of Tesla coils is a long spark.  The longer
> the better!  I have recently looked at spark length as a function of
> power for a solid state driven coil and the results will be
> interesting to some.
> John Freau reports that the spark length in inches for a
> well-constructed spark gap coil is 1.7 times the square root of
> wallplug power in watts. My tests show a spark length for a solid
> state coil of 0.17 times the square root of peak apparent power into
> the coil. In other words, my sparks are one tenth the length of his
> for a given power figure. Most list members will assume that I am not
> as good a coiler as John, but I think the difference is in how power
> is measured and how sparks are formed rather than differences in
> technical ability. I think we are both correct, and an explanation of
> how that can be should be instructive.
> Power comes with many descriptive terms: real, reactive, apparent,
> average, peak, and to our audiophile brethern !ugh! rms. John's
> wallplug watts would be real or average power measured with an analog
> (or digital) wattmeter. The needle will wiggle a little during Tesla
> coil operation, but there is no problem getting the true value to
> within 2 or 3%.
> In a solid state system power can be measured at three points: ac
> input to the rectifier, dc output of the rectifier, and rf input to
> the coil. The dc power is also easy to measure. Just multiply the
> average volts by the average amps. It will be slightly less than the
> wallplug watts due to losses in the rectifier. When we get to rf
> power, things get complicated. Wattmeters are not readily available
> and the price would discourage use in a Tesla coil environment. My HP
> 54645D scope calculates the rms values of two voltage waveforms and
> the phase angle between them, (one voltage proportional to current),
> so average power can be calculated from the product of rms voltage,
> rms current, and cosine of the phase angle. There are some other
> issues involved, since neither waveform is a single frequency
> sinusoid. It appears that the average power is about 0.9 times the
> apparent power VI in my system. Rather than try to make that
> correction each time and have a formula expressed in average power, I
> just use 0.17 times the square root of apparent power VI for spark
> length.
> For my base driven coil, voltage stays fairly constant and current
> rings up over many cycles, maybe 500 to 2000 microseconds, until the
> spark occurs. The apparent power builds also. I get a screen of 50
> microseconds length just before the spark occurs, read off the rms
> voltage and current, and call the product the peak apparent power.
> Another word besides peak might be better since peak power might imply
> peak voltaqe and peak current, which is not the case here. Anyhow, if
> one uses enough words, the meaning of peak apparent power should be
> understandable.
> A spark gap coil will build up power much more quickly than a solid
> state coil. The spark may occur within 2 to 5 rf cycles, or within 10
> to 25 microseconds for a 200 kHz systems. Once the spark occurs, power
> decreases rapidly. Richie Burnett has a waveform on his web site that
> shows this effect. If a spark does not occur, power sloshes back and
> forth between primary and secondary, but when it does, dissipation is
> rapid. 
> I believe that if we were able to measure the apparent power in John
> Freau's secondary, we would see a peak about 100 times his wallplug
> watts. The power in the secondary would rise to this peak in a few
> tens of microseconds, stay there for a similar length of time while
> the spark was present, then decay to a lower value for a few hundred
> microseconds, and go to zero until the next spark, 8333 microseconds
> later for a 120 bps spark gap. There are many curves of power
> variation that would satisfy the observation of the peak being 100
> times the average.
> The peak power establishes the length of the spark. The power that
> flows into the spark after the peak establishes the thickness of the
> spark. In my solid state system, at a particular voltage it might take
> 1 ms for a spark to occur. A 2 ms burst will produce a thin spark. A
> 10 ms burst will produce a white, rich spark that looks similar to a
> spark gap coil spark. A 100 ms burst will produce a spark with a very
> thick trunk for one third to two thirds of total length, say 1 inch
> thick for a 10 inch spark.

> There is a hypothesis among coilers that ions remaining in the air
> after the previous spark will help the next spark to grow to a greater
> length. I have looked carefully for that effect and have been unable
> to see it. Ions will help the next spark start, but this reduces the
> peak apparent power and reduces the spark length. In CW mode, the peak
> power is just the average power, and sparks are shorter than in the
> disruptive mode, where the peak power can grow to a larger value. 
> Neither the case of several short bursts closely spaced nor a longer
> single burst will increase the length of a spark.

I'm not too sure about the ions as such but I certainly buy that the 
hot air channel helps. Perhaps it's the same thing. It's easy to 
demonstrate - just start out your NST coil with fixed static gap by 
slowly turning the variac up. It first fires very slowly with sparks 
of some length (which depends on a bunch of parameters - of course!)
but then as you continue to crank it up, the sparks coalesce and grow 
progressively longer, the only thing changing to any great degree 
being the breakrate. In fact I was amused to see my work coil visibly 
reaching out and connecting with objects a few feet away in real 
time, quite slow enough for the eye to follow. I have also seen such 
growth on a tabletop coil running about 150W.


> I hesitate to say that the hypothesis is wrong for all cases, but I
> don't think it works for sparks up to 12 inches long. Sparks are so
> nonlinear that it is difficult to make sweeping generalizations about
> them. I have noticed a nonlinearity that might explain the difference.
> I have commented in the past that once a spark occurs, the combination
> of secondary and spark become a constant current sink. Actually the
> current will grow during a long burst. For example, I was applying
> about 750 V rms to the base of my 14 ga coil. Current rises to about
> 7.5 A at 0.44 ms. The spark occurs and current drops to 2 A at 0.69
> ms. Then the current grows to 3.4 A at 10 ms, remaining constant for
> the next 20 ms. The apparent power into the coil is 5500 VA at peak,
> 1500 VA at 0.69 ms and 2500 VA at 10 ms. This type of nonlinearity
> could conceivably cause spark growth at much higher power levels.
> I am still trying to get all this stuff written down. Hopefully it
> will happen in this lifetime.
> Gary Johnson
> Manhattan, Kansas