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*To*: tesla-at-pupman-dot-com*Subject*: Re: Vortex gap loss measurements*From*: "Bert Hickman" <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)*Date*: Sun, 03 Sep 2000 17:55:58 -0600*Delivered-To*: fixup-tesla-at-pupman-dot-com-at-fixme

Gary, Dan, John, Jim and all.. :^) Excellent experiment! It turns out that introducing virtually any spark gap into a single (uncoupled) RLC circuit will induce linearly-decrementing behavior instead of the exponential decay seen with a classic RLC circuit. It's interesting to rediscover this, since it was originally reported by Dr. J. Zenneck almost 100 years ago (1904!). Because of the linear decrement, tank circuit energy goes to zero in a sharply-defined period of time. The waveform on Gary's site clearly shows this happening in 295 uSec for the Vortex Gap. An exponential decline will asymptotically approach zero but will never quite get there (at least in theory). The spark gap's nonlinear, arc-like behavior is characterized by a relatively constant voltage drop (only tens of volts) that's pretty much independent of gap current. The actual voltage drop is mostly a function of the geometry, materials used in the gap, gas pressure, and the type of gas(es) and metal(s) that form the arc. The instantaneous energy that's lost in the gap is almost directly proportional to tank circuit current (E = V*I), and it's this characteristic that actually drives the linear decrement. John H. Morecroft devotes a fair section of his book to the behavior of spark gaps in RLC and coupled circuits in "Principles of Radio Communication" (in all three editions, but the 1921 1st edition is the most thorough). Of particular interest is an equation, originally developed by Zenneck and Stone, which describes the linearly decrementing tank circuit current as a function of the Effective Gap Resistance (R): Section A: Section B: Section C: ---------- ---------- ---------- I = -E*Sqrt(C/L) * (1-(R*t)/(2*L)) * sin(t/(Sqrt(LC)) While Sections A and C of the above equation simply describe the peak current and sinusoidal oscillations, section B represents the linear decrementing term that forms the "envelope" of the waveform. If the spark gap behaved as a pure resistor, section B would instead be an exponential function of time. The current envelope declines to zero when (R*t)/(2*L) = 1. This means that, if we know L and measure t, we can solve for the effective gap resistance R and use it to compare different types of gaps! Morecroft suggests that gap resistance was actually governed by the magnitude of the FIRST current maximum. Apparently, the higher the first current peak, the larger the initial plasma channel, and the lower the effective gap resistance for the remainder of the decrement. This also implied that there was relatively little modulation of gap resistance by the oscillating RF current inside the envelope. Let's re-look at Gary's experiment now in the light of the above relationship. Gary's primary circuit resonated at 138 kHz, and per his coil specs, he was using a 0.021 uF tank cap. This implies a tank inductance of about 63 uH. The Vortex gap decremented to zero in about 295 uSec, while the vacuum gap did so about 17.5% quicker, or in about 243 uSec. This implies that the vacuum gap lost energy more quickly, and thus had a higher effective gap resistance. Solving for the effective gap resistances of the two gaps styles: RVortex = 2*L/t = (2*63e-6)/(295e-6) = 0.43 ohms (lower = GOOD) and RVacuum = (2*63e-6)/(243e-6) = 0.52 ohms Conclusion: The Vortex gap has significantly lower gap resistance, possibly because of the greater number of charge carriers available at the higher operating pressure. However, gap resistance is only one parameter making for an efficient gap. Another is its quenching capability. Higher pressure gaps often take longer to quench. However, some of Gary's earlier measurements seemed to indicate that the vacuum gap was not quenching very well, so it remains for another set of experiments to determine if the Vortex gap has better quenching ability. Great job, Gary! Safe coilin' to you all! -- Bert -- -- Bert Hickman Stoneridge Engineering Email: bert.hickman-at-aquila-dot-com Web Site: http://www.teslamania-dot-com Tesla list wrote: > > Original poster: "Lau, Gary" <Gary.Lau-at-compaq-dot-com> > > Today I found some time and performed a comparison between the gap losses of > my single vacuum gap, and my new single vortex gap. To do so, I scoped the > primary ringdown with no secondary in place. I used a Terry Fritz fiber > optic voltage probe across the primary coil and a digital storage scope to > record the results. I have not yet accurately calibrated the voltage > readout, so for now, the results are just relative to each other. > > With no secondary in place, the ringdown is a linearly decrementing > waveform, not logarithmic. As such, the slope of the ringdown indicates the > losses in the circuit and is independent of the gap firing voltage. I > performed ringdown slope measurements at a variety of gap widths to vary the > initial voltage, but the ringdown slope is a constant, independent of Vgap. > > The power to the blower motor is varied through a lamp dimmer and I tried > varying the motor speed to see what effect that had. At very low speed, the > linearly decrementing waveform became slightly logarithmic-looking, but > still predominantly linear. The gap breakdown voltage appeared to change > slightly at low speed, but this was hard to measure as it was slight and the > bang-to-bang gap breakdown voltage is not as consistent as one might hope. > > The slope decrement figures are assuming that my probe is accurately > calibrated for voltage, though I suspect it may not be, so the figures are > useful only for relative comparison purposes. > The pressurized vortex gap decremented at 200V/usec. > The vacuum gap decremented at 235V/usec (17.5% faster). > The vortex gap breakdown voltage is about 20% higher than the vacuum gap at > the same gap distance. > > Vortex gap web page: > > http://people.ne.mediaone-dot-net/lau/tesla/vortexgap.htm > > <http://people.ne.mediaone-dot-net/lau/tesla/vortexgap.htm> > Vacuum gap web page: > > http://people.ne.mediaone-dot-net/lau/tesla/onegap.htm > > <http://people.ne.mediaone-dot-net/lau/tesla/onegap.htm> > > Regards, Gary Lau > Waltham, MA USA

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