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

["Tesla list" <tesla@xxxxxxxxxx>]

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