Re: El Supremo

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Ken,
        A comment:

On 12 Sep 2002, at 11:40, Tesla list wrote:

> Original poster: "K. C. Herrick by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>
> 
> Terry Fritz's groundbreaking work with the OLTC has given me pause to
> think about my own design and how it might be improved.  Also constantly
> in my mind is an appreciation of the ratio of my MOSFET failures to his
> IGBT failures: it's infinity, not to put too fine a point on it, since
> dozens upon dozens have failed for me and none have failed for him.
> 
> Terry's 1-turn-primary design is what I had had in mind in the first
> place, but my MOSFETs would not drive 1 turn and IGBTs back-then seemed
> too slow.  But now, I see a way to combine Terry's notion and mine.  I
> call it, perhaps with some want of humility, El Supremo.  With Terry's
> indulgence you can find its conceptual schematic at 
> Http://hot-streamer-dot-com/temp/tch2.gif.  See also the accompanying
> http://hot-streamer-dot-com/temp/tch2drv.  
> 
> Referring to TCH2, all IGBT "1"s conduct during one half-cycle and then
> all "2"s conduct during the alternate half-cycle.  If you follow the
> current paths you will find that the four quadrants of capacitors become
> connected in a "daisy chain" around the primary loop in alternating
> polarities, thus establishing the requisite alternating magnetic field
> perpendicular to the image.  
> Each set of IGBTs is driven by a crossover-controlling circuit like that
> of TCH2DRV.  
> 
> Note that, during each brief interval between half cycles when no IGBTs
> are conducting, the "inductive kick" of the primary loop will become
> clamped, regardless of its polarity, by the "fast diodes", as coupled via
> the loop capacitors.  No transistor will ever see any voltage greater
> than twice a capacitor voltage.
> 
> Now here is where a major difference with my present design comes in:
> 
> The fundamental difference between sstc's and spark-gap tc's is that the
> former generates its spark by application of a constant-amplitude burst
> of excitation whereas the latter generates an exponentially-diminishing
> amplitude of excitation.  This means that by far the greatest amount of
> instantaneous energy going into the spark is delivered, in a spark-gap
> system, during the first few cycles of excitation.  It does not escape
> notice that, given the same mains input-power, a spark-gap coil will
> produce significantly longer sparks than a sstc--even given the
> relatively large loss of power in the spark gap.  So, it is very likely
> to be concluded that what's wanted is to cram as much power as possible
> into those first few cycles.
> 
> So what I propose with TCH2 is that the capacitances be markedly smaller
> that what I have in my present design.  In fact, they are to be small
> enough to give the desired rate of exponential decline in voltage during
> each spark event.  Note that the current in all the capacitors is
> unifirectional, not ac and that the primary is still untuned.  The
> capacitor voltages just decline during the spark event, they do not
> change polarity.  And since the IGBTs are to be driven from an external
> signal source, that source can either be tuned at will to match the
> secondary's Fr or it may readily be derived directly as a function of the
> secondary's return-current, as I do presently, to make the system
> instantaneously self-tuned to the secondary's Fr.

The decline in primary amplitude exhibited by a cap discharge system 
is (as you must know) a side effect of the energy transfer. Probably 
a desirable one since the low duty cycle of the machine prevents the 
air from becoming continuously ionized as it does with a CW system. 
This means that a substantial buildup of energy in the secondary 
occurs before it lets go for each bang. A primary circuit that 
decrements radically all by itself can't be much of a recipe for 
success. I'm sure you know all this. Perhaps I've misinterpreted what 
you are saying.

Regards,
Malcolm
  
> Most importantly, this scheme acts to completely separate the function of
> frequency-determination from the function of spark-energy storage. 
> Change the frequency?  Just do it: the energy available to create a spark
> won't change.  Increase or decrease the primary's time-constant?  Just do
> it: the frequency won't change. 
> 
> And notice one more interesting thing:  I show the capacitors as being
> connected to the primary buses all along their lengths.  I think that it
> doesn't matter where they are connected since a) their current is
> unidirectional and b) the current in each of the buses is unidirectional.
>  There exists the same length and locus of conductor regardless of where
> a capacitor is connected.  The advantage is that a large number of small
> capacitors may be closely attached to the conductors, both maximizing
> their overall current-carrying capability and minimizing their lead
> inductances and the overall ESR.  Also, it's a handy place to put
> them--all around the periphery of the primary.  I envision two
> 1/2"-diameter copper pipes for each segment, spaced vertically perhaps
> 1/4" apart and with the capacitors' leads soldered to their outer
> peripheries.  Easy to assemble, easy to change the capacitors.
> 
> I show 4 primary segments in TCH2 but could be any even quantity.  Also,
> the capacitors could be charged in various ways, for example from
> current-regulating supplies as in my present design or resonantly as in
> Terry's OLTC design.
> 
> Current-sharing amongst the paralleled IGBTs should not be a problem:  A
> very small emitter resistor developing perhaps 0.2V at peak emitter
> current should suffice to steer current away from the stronger
> transistors into the paralleled weaker ones.  Such a resistor might well
> consist merely of the lead connecting each IGBT's emitter to the common
> point.  Further, avalanche-breakdown should never be a problem since no
> transistor can see any voltage greater than twice a capacitor voltage
> under any condition.
> 
> And finally, this is a low-voltage design.  I remember, when I was a kid,
> that the president of the ARRL was killed by his ham apparatus' high
> voltage.  That's always stuck in my mind.
> 
> It seems to me that this idea has great potential.  I hope I will
> personally have the energy to look further into it & build it and I urge
> others to contemplate doing so.  Or else...to contemplate telling me that
> it won't work.
> 
> Ken Herrick
> 
> 
> 
>