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A renewing s.s.t.c. direction + tuned vs. untuned

Original poster: Ken or Doris Herrick <kchdlh@xxxxxxxxx>

My prior & only (untuned-primary) s.s.t.c. having finally failed for good, I've spent the last few years f---ing around (read "fumbling" if you care to) with sundry exotic primary configurations--tuned, untuned, mechanically variable--you name it; paper-only and also hardware--all ultimately unsatisfactory and ultimately tossed out. But now I've finally found the wit to try a simulation of my prior (also exotic) design, that actually worked (See <http://www.pupman.com/current/kcherrick/>http://www.pupman.com/current/kcherrick/ image no. 2 for my photo of that.). When I built & ran that I did not have a simulation app. Now I do, I've run it, and...what do you know!...it does work! And with interesting modifications, it works amazingly well--only in simulation so far, of course, but compared to numerous other schemes I've simulated, it's the best of the lot. So what I'd like to do is describe it here & solicit comments from you solid-state types--while at the same time proceeding with the building of a new primary apparatus to this design.

I'll ask Chip to post patent-ckt3.jpeg on the above-mentioned page; it should be up, I suppose, tomorrow or the next day. You can also refer to my U. S. patent on the scheme, # 6,069,413.

In the jpeg, the circuits to the left of Q1/Q2 plus the D7/D4/D2/D3 network simulate the low-voltage stuff I currently have (not left over but completely redesigned and which works like a champ as best I can tell). The Tesla-coil proper is TX2 plus its simulated load. The stuff in the middle represents one section of the Fig. 2 drawing of the patent. My crucial changes are a) to get rid of the parallel primaries and instead use only one and b) to reduce the capacitance of the C6, C7, C4, C3 capacitors (each pair replacing the single ones of the patent). Those changes cause the simulation to work a whole lot better than without them. Would that I had known (*sob*)!...

In operation, the rectified-mains sources V1 & V2 charge up capacitors C6, C7, C4 & C3. The four damped inductors isolate those capacitors from the mains supplies for the secondary's Fr. Q1 and Q2 are alternately turned on & off at the secondary's Fr, from the feedback circuits. That action alternately puts a short, so to speak, first between C6-right and C4-left and then between C7-right and C3-left. For each transistor turn-on, a pair of capacitors is in a loop connected in series with the primary, with alternate half-cycles connecting pairs of capacitors in opposing polarity. The improvement of my changes lies, apparently, in the charging up of the 4 capacitors to double their initial charges (from the mains supplies), during the oscillatory process. My thinking is a bit fuzzy here but loosely speaking, it appears that C6/C4 and C7/C3 are alternately clamped by the back-diodes within their associated MOSFETs, causing them to gain the added charges. For each pair, during the next half-cycle, the added charges are transferred to the coil. The result is that about +/- 500 V of quasi-sine-wave appears across the primary coil. As measured in the simulation, the peak transistor voltage is 970 and the peak current, 120 A.

Capacitor C8 very handily acts to snub the transistor currents until they can turn off. I think of its action as slightly causing the phase of the drain current waves to lead the voltages, so that current goes to zero before very much source:drain voltage appears. Resistor R19 is there merely to tie the primary down to more-or-less earth-potential; otherwise it would float since it is otherwise dc-isolated..

The various low-resistance resistors scattered around are there merely to simulate the real-world a bit.

As will be noted from perusal of the patent, the circuit is amenable to expansion; I had 6 sections in the prior implementation, each containing a bunch of TO-247 MOSFETs. Way too many parts, there! I plan to add another section in this manifestation (can't simulate it in my freebie sim program) so that 4 power MOSFETs are used; they'll now be those expensive ST ones @ ~$50 a pop, no pun intended. The added MOSFETs will still see the 970 V peak and with them I will be able to send more or less the same current thru 1, maybe 2, additional coil turns. Flux being proportional to amps x turns, more spark will result. And I would expect stronger sparks than before since the mains-source will now be ~300 V instead of the prior ~150--and with that, pumped-up additionally as I describe.

This leads me to a puzzlement I've expressed before. In a s.s.t.c., why bother with a tuned primary? A "disruptive" apparatus has to be tuned by its very nature. And it may have an advantage over s.s. for the possible reason I've also commented on before. That is, that the relatively much-higher rate-of-rise of the initial half or whole cycle (due to the abrupt shot of energy thru the spark-gap) will allow more charge to be crammed onto the top electrode before the spark has a chance to proceed very far. A research paper I have a copy of has found spark propagation in air, over 1 inch, to require about 50 ns of time. That extrapolates to about 20 inches per microsecond. I conjecture that in the first several inches, the spark-loading on the electrode will not be severe. So with a high rate of voltage-rise, more charge can be applied to the electrode than can bleed off through the spark during that time. No solid-state system (or none I've heard of) can match that kind of rate-of-rise and so little or no additional charge can be accumulated, to give the spark its extra "punch".

At least, that's my take on why, according to conventional wisdom, disruptive sparks tend to be longer than solid-state ones.

So what other reason would there be for retaining the tuned primary? The reason given, as I understand, is that by tuning it, its reactance can be greatly diminished so more current can be passed through it, resulting in more magnetic flux, etc.. Well and good, but still...where's the gain? More current means more mains-power, and that is limited by what your circuit-breakers can stand. If you're not going to be able rapidly to pump in so much primary current as to "get ahead" of the spark, as I describe above, then why try to do it by tuning the primary?

So...do I protest too much? Is my exotic scheme worth another shot at it? Is it exotic at all, or merely a rescrambling of some familiar circuit? I'm getting too old for this to know, anymore...

...and here's something entertaining about the patent--my vanity-patent, so to speak. I applied for it somewhat for the fun of it; my then-patent lawyer was an accommodating type where, for a few hundred dollars, he helped with the.wording of the claims and did the filing; I did all the writing and also the drawings. And after that I was on my own with the Patent Office.

I had elected to number my paragraphs and in the first response from the Examiner he rejected all the claims. (They almost always do that, I'm told, as a matter of professional pride, I suppose.). But he really got his knickers in a twist about the paragraph-numbering (not to be confused with the patent-office's line-numbering, which is added later). He would have none of that. So after setting him straight on all of his claim-objections--ever-so-politely, of course--I right-off agreed to drop the paragraph numbers...but first, would he please be good enough to remind me of the specific Patent Office regulation that proscribed paragraph-numbering? Well of course, he couldn't cite any such paragraph because there was none. So I got my paragraph-numbers; and mine just might be the only U. S. patent ever granted with numbered paragraphs. You could check it out...

As to anyone wanting to use the patented scheme, no sweat. It's expired since I haven't kept up its maintenance. So feel free!

Ken Herrick