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*To*: tesla-at-pupman-dot-com*Subject*: RE: I'm wondering about this gap and tuning issues. Am I finally *beginning* to understand?*From*: "Tesla list" <tesla-at-pupman-dot-com>*Date*: Thu, 12 Apr 2001 11:28:51 -0600*Resent-Date*: Thu, 12 Apr 2001 11:44:27 -0600*Resent-From*: tesla-at-pupman-dot-com*Resent-Message-ID*: <54yoSD.A.MgD.1le16-at-poodle>*Resent-Sender*: tesla-request-at-pupman-dot-com

Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Gary.Lau-at-compaq-dot-com> Hi Garry: Comments interspersed... >Original poster: "Garry Freemyer by way of Terry Fritz <twftesla-at-uswest-dot-net>" <garry-at-ndfc-dot-com> > >I've been reading here quite a bit, and a lot of what is written here is WAY >over my head. The hardest part I think is getting the terms straight. >Usually, by the time my mind finally coughs up the definition of a term I am >reading, I've forgotten what was said about it. Then by the time I am done >reading it again, I've forgotten the definition and by the time I recall the >definition, I've forgotten what was said again. ;-) > >So, I try to visualize it in my head and I was thinking about NST's. I seem >to remember that (Reason I say this is because I don't trust my memory any >farther than I can throw a fully loaded and operating washing machine) that >when you have alternating current that the voltage is like a sign wave, >going positive, then neutral at the point of reversal, and then negative. >There are peaks and valleys in the sine wave. I will rever to them both as >peaks. I'll call them positive and negative peaks. >So, I was thinking about how could a cap in a tesla circuit be charged when >the current is going negative and positive. I asked myself why it wasn't >canceling it all out and here is what I came up with. Someone tell me if I >am close? If you were averaging things over several 60Hz cycles, you could think of things canceling out. But the stuff we're interested in occurs in the interval between spark gap firings, often called "bangs". The charge on the cap just prior to a bang is either positive or negative. >Are these statments true or close? > >The primary and the gap spacing in a tuned tesla coil is such that the gap >fires as near the PEAK voltage (Or is this current?) so as to discharge the >cap. The close to the peak the gap fires the better. When the charge is >reversed, the gap fires on the reverse peak. If the gap fires early, output >is reduced because the charge is drained before it can maximize, and if it >fires too late, output is reduced because some of the charge has been >canceled out by the opposite charge that is now flowing into the capacitor. >True? The primary coil doesn't affect when or how often the gap fires. The things that affect it are the size of the NST, the size of the cap, and the gap width. It is largely true that "the closer to peak voltage the gap fires, the better". But there are some important and non-intuitive things that are widely misunderstood. The following assumes the use of static gaps. This may get long so please bear with me... Let's say you have an NST that has a peak voltage of 10KV. Ideally you would choose a cap such that at the end of each half-cycle, the cap would have a charge of 10kV on it, and you would choose a gap spacing such that it would fire just at or under 10kV. So at the end of every 60Hz half-cycle, the gap would fire and it would fire 120 times per second. This would be nice if it were that simple, but it's not likely to happen in the real world. What's not obvious is that if there is a charge on the cap and the gap doesn't fire at the end of that half-cycle, that energy will not be lost, but will be carried over into the next opposite-polarity half-cycle. Let's say that you turned on your NST halfway through a 60Hz half-cycle, so there wouldn't be enough energy in the first half-cycle to charge the cap all the way to +10kV. Instead it charges up to say 5kV, then starts to decline without the gap firing. The energy in the half-charged cap won't be wasted, but will be carried into the next negative half-cycle, and roughly halfway through the next negative half-cycle, the cap will achieve a charge of -10kV, and now the gap will fire. But the negative half-cycle isn't over yet. After the relatively short primary ringdown, the cap will again start to charge from zero in the negative direction. But since there's only about half of a half-cycle left, it only makes it to -5kV, then starts going back towards zero as the positive half-cycle begins. And so on. The other thing that's important to understand is the concept of resonant rise as it relates to the NST/cap. Caps are generally sized such that they are _roughly_ resonant with the NST secondary at 60Hz. It needn't be exact. Let's say that again your NST peak voltage is 10kV, but let's say that you accidentally set your gap to twice the right width and that it now fires at 20kV. One might think that it simply won't ever reach 20kV and won't fire, but it will! Remember that energy is not lost at the end of a half-cycle if the gap doesn't fire, but is instead carried into the next, opposite polarity half cycle. So if you turned on your NST at the beginning of a half-cycle, at the end it will peak at 10kV. Then it will head back towards zero, and shoot past -10kV, possibly reaching -20kV. If it doesn't reach -20kV, it will swing even higher on the next positive half-cycle, and so on, until the gap firing voltage is reached (OR UNTIL YOUR CAP OR NST IS OVERVOLTED AND SHORTS OUT). An interesting thing to do is to note that if you turn your variac down to 50% or 30%, your gap will still probably be firing, though at a much lower BPS rate. This is because while the open-circuit NST voltage is a fraction of its normal peak, the voltage on the cap builds over subsequent cycles until the gap breakdown voltage is reached. The simulated waveforms on my web site may be easier to understand than all of these words, see: http://people.ne.mediaone-dot-net/lau/tesla/gapsim.htm >The primary, acts as a kind of resistance to emf flow, halting it at the >right time (Quenching) and delaying the discharge to the right moment. True? The primary doesn't affect how fast the cap charges or when the gap fires. Quenching refers to when the gap *stops* conducting, following the gap's firing, and is affected by the gap geometry and airflow, pri/sec coupling, whether streamers have formed from the top load, and probably other things as well. >So, that's where we get the 120 breaks per minute because the gap fires on >the peaks that are double the alternating cycle of 60 cycles per second. >True? In a synchronous rotary gap one can achieve exactly 120 breaks per second, but with a static gap, the gap firing interval will in practice be chaotic. >Than a BPS of 60 would mean the gap discharges on the peaks of only the >upper or lower half of the sine wave or every other peak? Exactly. In fact I was once able to operate my static gap at 60BPS. I had to set my variac to a lower, very critical setting and the effect was not at all stable, wanting to slip back into chaotic mode, but it was surprisingly easy to make happen. >So, rotary gaps have the advantage of better control. AC would do best with >a sync gap and DC would work with either sync or assync? Probably a fair assessment. >I recall folks speaking about voltage ringup, would that be kind of like >where the relationship between the frequency and the inductance in the NST >is such that the current gives a shove to the next cycle kind of like a >person rocking back and forth on a swingset to get more swing (Amplitude). >Wow, this last one sounds a little far fetched to me. Take this one with >grain of salt if desired. That's a great analogy for what I was trying to explain earlier. Energy on one side of the middle (zero) of the swing is carried into the motion on the other side. >So, after reading all your posts out there, am I finally getting some idea >of what ye all are saying or am I still in clueless canyon? >PS: Warning! If you reply in really technical terms. I probably won't >understand you. ;-) Regards, Gary Lau Waltham, MA USA

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