[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

*To*: tesla-at-pupman-dot-com*Subject*: Re: Spark length, SSTC vs. spark gap*From*: "Tesla list" <tesla-at-pupman-dot-com>*Date*: Thu, 08 Feb 2001 18:13:39 -0700*Resent-Date*: Thu, 8 Feb 2001 18:13:43 -0700*Resent-From*: tesla-at-pupman-dot-com*Resent-Message-ID*: <o3kyG.A.spE.DR0g6-at-poodle>*Resent-Sender*: tesla-request-at-pupman-dot-com

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. Regards, Malcolm > 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 > > > >

- Prev by Date:
**Re: [Fwd: Spark gap not firing]** - Next by Date:
**Re: Superconducting 1/4 wave resonator** - Prev by thread:
**Spark length, SSTC vs. spark gap** - Next by thread:
**Re: Spark length, SSTC vs. spark gap** - Index(es):