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Solid-state t.c.'s
The List apparently enjoys precious few solid-state t.c.-ers so I paid
particular attention to Gary Johnson's recent posting. I'm now prompted
to report on the status of my own solid-state work, which I have
commented on before.
Along with Gary, I've been working on solid-state for years...>quite a
few years!<. Somewhat differently from him, though, I've come to the
following conclusions and results:
1. Power MOSFETs "hate" voltage but "love" current. Thus, I sought a
primary-drive circuit that would minimize the former and maximize the
latter.
2. One's t.c. secondary "knows from nothing" about primary voltage: It
responds only to the magnetic flux generated by the primary. And the
primary flux is proportional to current x the quantity of turns, or
ampere-turns.
3. The primary current is, of course, proportional to the applied
voltage. But the voltage it is necessary to apply in order to get a
desired current is proportional to the impedance, which in turn is
proportional to the square of the quantity of turns.
4. Thus, I reasoned, I need to minimize the quantity of turns in order
to minimize the voltage, so as to allow me to use power-MOSFETs of
reasonable voltage capability. At one time I looked into driving the
secondary directly, at its bottom end, but I concluded that the impedance
there was way too high to get a decent current from reasonably-rated
transistors. So, I have chosen a primary configuration incorporating
just 1 equivalent turn--the absolute minimum.
5. The impedance of 1 turn at ~125 KHz, my secondary's resonant
frequency, is something less than 1 ohm. Thus, if I want to develop,
say, 500 V/turn so that my secondary will develop perhaps that much or
more--given the bad effect of poor coupling on the one hand and the good
effect of resonance on the other hand--I need something more than 500 A
of current.
6. I've devised, and have a U.S. patent pending for, a circuit
incorporating 1 or more pairs of transistors and 1 or more dc power
sources that is amenable to driving my 1-turn primary. It can
incorporate multiple such transistor-pairs and power sources into the
1-equivalent-turn circuit in order to a) establish a desired ac primary
voltage, b) control the peak voltage seen by any transistor, c) allow for
paralleling of transistors in order to accomodate the current, d) allow
for conveniently driving such paralleled sets of transistors, e) allow
for all power sources (typically, electrolytic capacitors) to be charged
from a common source (the power line) and f) allow for monitoring of
current in each individual transistor in order to prevent overload.
7. Presently I have 24, 500V/85A MOSFETs and 12, 160V power sources
(1000 uF capacitors) handling ~250 A of pulse-burst current. The 24
transistors are overkill at this time, but I plan to ultimately treble
the voltage, which will treble the current so that each transistor pair
will have to handle ~62 A. At the same time I will have to treble the
quantity of transistors in order to handle the voltage. (In my circuit,
each transistor sees no more than 2 1/2 times its associated power-source
voltage, of 160 V.)
8. The drive for the MOSFETs is derived from a gated amplifier whose
input is taken from the secondary's bottom end. Thus the whole apparatus
comprises a self-excited oscillator with the secondary being the only
resonant element and thus the only frequency-determining element. By
this means, the t.c.'s excitation is always at the instantaneous resonant
frequency of the secondary: no tuning is required.
9. I gate the amplifier on for about 6 ms for each spark, at up to
100/sec or so before running out of ac-mains current capability. With
the present 160 V applied to the primary I get ~20" branching sparks from
the 4" x 14" aluminum-duct toroid. So, I expect 2-3 times that spark
length when the whole thing is finished, which is enough for me.
10. When I have gotten to the point of incorporating at least 2
transistor/power-source sets, to boost the primary ac up to at least 320
V, I plan to get my documentation together & offer copies via this List,
for a nominal sum. Purchasers could use the patented part of my design
for their own personal purposes.
11. It may well be that what I've described is just too much for most
t.c.-ers: lots of electronics and producing a relatively modest result.
But my system does have the advantages of a) no instantly-lethal high
voltage in the primary, b) no noisy/unreliable spark gap and c) being
always and instantaneously in tune.
12. I've also designed and built a "pie wound" secondary, giving me
~1500 turns in a 30" height with at least 0.1" turn:turn spacing. I plan
to refine that design and offer it also.
Ken Herrick
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