Re: A triggered-s.g. 1-turn primary

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

Original poster: "K. C. Herrick by way of Terry Fritz <teslalist-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>

Marc & all-

It's a long way from being "in action".  But I have been noodling around
with an assembly scheme & I post the drawing at
http://hot-streamer-dot-com/temp/tspk13a.pdf.

As to coupling to the secondary, the secondary sits directly on the top
capacitors; it can't get much closer.

Referring to the drawing,

1.  The gaps are to be made from large brass cup-washers.  They're sized
for 1/4" screws but I'd use 10-32 flat-heads so that the screw-heads are
under-flush with respect to the washer rims.  Thus the sparks should run
between the (rounded) rims.

2.  Each gap is surrounded with a silicone-rubber-foam washer to exclude
oxygen while firing.  The gap spacings are maintained using dummy gaps on
either side.  The dummy gaps are spaced with non-conducting shims.

3.  The wedge-shaped thingies are cut from a pair of 0.04"-thick x
18"-diameter aluminum pizza trays.  The trays are very nicely made, not
expensive, and have a rolled, reinforced and bent-up rim.  Each
wedge-image represents two such wedges, one above the other & separated
by the gaps.

4.  The capacitors are arrayed in 6, 8-capacitor blocks, each capacitor
being 4.7 uF, 600 WV.  Alternate blocks are connected between pairs of
tray-wedges, first pair on top, second pair below, etc.  Connection is
via swaged brass eyelets (which I would use) or solder-lugs, either type
of item fastened to the wedges with internal-star phosphor-bronze
lock-washers.  I would trust the washers to provide for gas-tight
connections to the aluminum.

5.  All components are in one plane, of thickness no greater than 3 1/2"
or so--about 3 capacitor diameters or less.  The secondary sits directly
on that assembly.  There would be no problem with voltage stress since
the largest voltage with respect to ground in the primary is mains + 250
V or so.

6.  For tuning,  I would switch in & out some capacitors.  I find, from
simulation, that if I change one of the six "6C"s (of the posted
schematic) from 40 uF to 30 uF and then parallel that with 10 uF in
series with 1 ohm, I change Fr by about 7%.  A plain-vanilla toggle
switch can be used to short out, or not, that 1 ohm resistor.  Several
such arrangement would be used, to give perhaps 15% tuning range.  The 1
ohm resistor(s) will have no effect on capacitor charging & thus on
trigger timing.

All I need now is a) to hear of no insurmountable problem and b) the
inclination to do it.

Ken Herrick

On Thu, 01 May 2003 07:33:43 -0600 "Tesla list" <tesla-at-pupman-dot-com>
writes:
 > Original poster: "marc metlicka by way of Terry Fritz
 > <teslalist-at-qwest-dot-net>" <mystuffs-at-orwell-dot-net>
 >
 > While i see where your going with this principle, I would like to
 > see a
 > magnetic profile while in action.
 >   i just don't see a strong  coupling to the secondary with this
 > unit?
 > Marc
 >
 > Tesla list wrote:
 >  >
 >  > Original poster: "K. C. Herrick by way of Terry Fritz
 > <teslalist-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>
 >  >
 >  > I take note of jimmy hynes/Terry Fritz's posting of 4/28 but
 > submit this
 >  > posting anyway, which I had been preparing.
 >  >
 >  > In another periodic retreat from solid-state exasperations, I
 > have come up
 >  > with a notional spark-gap design,
 >  >
 >
<http://hot-streamer-dot-com/temp/tspk13s2.pdf>http://hot-streamer-dot-com/temp/t
spk13s2.pdf.
 >  > You will need Acrobat Reader for that.  (I note that gif format
 > yields a
 >  > lousy image, derived using Photoshop either from my CAD program's
 > exported
 >  > pcx or from a scanned print of the drawing.  The pdf image seems
 > just
 >  > lovely--similarly derived from a print.  Computer mysteries...)
 >  >
 >  > Referring to the drawing, six "6C"s, six "1/6 L"s, six "1/6 R"s
 > and six
 >  > spark gaps are connected in a circle on the secondary's nominal
 > diameter
 >  > (in my case that would be 12").  Six 6Cs in series yield C and
 > six 1/6 Ls
 >  > in series yield L.  L and C resonate at the secondary's Fr when
 > the 6 spark
 >  > gaps fire.  Each 1/6 L is merely the sum of a 6C's internal
 > inductance +
 >  > the spark gap's intrinsic inductance + the interconnection
 > inductance of
 >  > that segment of the primary loop, as arrayed on the nominal 12"
 >  > diameter.  Each 1/6 R is the sum of spark-gap, 1/6 C and
 > interconnection
 >  > resistances in its segment.  All of those elements constitute a
 > 1-turn
 >  > primary.  The secondary is to sit right on top of it--resting,
 > perhaps,
 >  > directly on the capacitors.
 >  >
 >  > Each 6C has applied to it switched AC-mains input, coupled via
 > windings of
 >  > a T1 and a T2 operating as chokes.  The AC is a sine wave that
 > is
 >  > interrupted using two power MOSFETs, two IGBTs or a triac, not
 > shown.  The
 >  > switching circuit is turned on during each mains 1/2 cycle at
 > the
 >  > approximate voltage peak, staying on for 1/4 cycle and thus
 > delivering a
 >  > step-voltage via the T coils to each 6C.  The coils in the Ts
 > resonate with
 >  > each 6C so that, a few milliseconds after each step, the voltage
 > on each 6C
 >  > reaches about 275 V (as simulated with MOSFET drivers) with 160 V
 > peak from
 >  > the mains.
 >  >
 >  > Phasing of the T1s and T2s is such that the resonant-charging
 > currents'
 >  > induced voltages in their series-connected second windings are of
 > opposite
 >  > polarity & thus the series connections exhibit 0 V during the
 > charging
 >  > time.  All 6 second-winding pairs are connected to a bus-pair
 > that is
 >  > driven via a break-over diode from one of the 6Cs.  Because of
 > the phasing,
 >  > the break-over diode sees only the voltage on the 6C.  The Ts are
 > to be
 >  > physically located close to the physical center, to minimize the
 > Fr flux
 >  > intercepted by them and their interconnections.
 >  >
 >  > During charging, each gap sees twice the voltage on a 6C.  Just
 > prior to
 >  > the resonant-peak (absent breakdown), the diode breaks down and
 > applies the
 >  > 6C's voltage to all six T-winding pairs in parallel.  This action
 > induces
 >  > voltages into the charging windings such as to momentarily
 > increase the gap
 >  > voltages, sufficient to cause the gaps to break down.  Again,
 > during the
 >  > spark event, the coil polarities are such that each pair of
 > trigger
 >  > windings exhibits no induced voltage.  Note that alternating
 > trigger-coil
 >  > pairs are connected to the driving bus oppositely.  Thus the
 > trigger
 >  > voltages on alternating primary-loop segments are of opposite
 > polarity,
 >  > doubling the trigger voltage seen by each gap.
 >  >
 >  > The turns ratio in each T might need be no larger than 1:2 for
 > reliable gap
 >  > firing; 1:1 and perfect transformers would cause the gap voltages
 > to
 >  > increase by a factor of 1.5.  Also, all the Ts could well be
 > consolidated
 >  > onto just two cores with one trigger winding on each core.  The
 > 20 mH (in
 >  > my simulation) is not critical; a whole lot smaller and the peak
 > mains
 >  > current becomes excessive while a whole lot larger, the charging
 > time
 >  > becomes excessive.
 >  >
 >  > As soon as the gaps break down, the damped sine wave loop-current
 > at Fr
 >  > commences flowing.  The 6C-charging T-coils then act to isolate
 > the mains
 >  > from the spark gaps during the firing event while a simple
 > clipping network
 >  > at the MOSFET/IGBT/Triac output clamps the pk-pk voltage there to
 > less than
 >  > ~+/- 250 V--again, as simulated.  I've added a small R-C damping
 > network as
 >  > well, to soak up most of the Fr signal there.
 >  >
 >  > The gaps keep conducting until the Fr current becomes too low,
 > which will
 >  > occur well before each following initiation of capacitor
 > charging, ~8 ms
 >  > later at 60 Hz mains frequency.  They are to have extremely close
 > spacing,
 >  > perhaps 0.02" or so and thus will dissipate relatively little
 > power.
 >  >
 >  > Since the capacitor charges for each half mains-cycle are of
 > opposite
 >  > polarity, any tendency for metal to transfer across the gaps
 > should be
 >  > minimized because the DC component of the gap current will
 > alternate from
 >  > one mains half-cycle to the next.
 >  >
 >  > I show 115 VAC input but 240 V could be applied just as well.
 > I've
 >  > simulated the circuit using 40 uF for 6C, 33 nH for 1/6 L, 5
 > m-ohms for
 >  > each 1/6 R and 20 mH for each charging-coil of T.  I've not
 > simulated the
 >  > trigger scheme except for using a "switch" for each gap and
 > turning those
 >  > on for 500 us after the delay time.  The scheme works just fine
 > in that
 >  > simulation: Fr is about 125 KHz and the peak first-half-cycle
 > loop current
 >  > is about 9 KA with no simulated additional switch voltage-drop.
 > At 120 V
 >  > in, the line current is about 15 A RMS, 30 A peak.  The Fr
 > current
 >  > diminishes to 1 KA at the 4th cycle (no secondary present).
 >  >
 >  > A 30 m-ohm total loop resistance is perhaps too optimistic.
 > Higher
 >  > resistance will, of course, diminish the peak current and also
 > the number
 >  > of Fr cycles that will occur prior to gap-extinguishing.  With
 > 100 m-ohms
 >  > total, I get only ~7 KA peak Fr current and that diminishes to
 > 500 A in
 >  > just 1 1/2 cycles; likely not enough excitation to produce much
 > of a
 >  > spark.  Thinking that perhaps two rather than 1 turn would be
 > better (if Q
 >  > = X/R, X would be 4x, R would be 2x, perhaps, so Q would be
 > about
 >  > doubled--right?), I temporarily added 600 nH and 60 m-ohms into
 > the
 >  > loop.  I got 4 KA peak diminishing to 500 A in 2 1/2 cycles &
 > whether
 >  > that's an improvement, I don't know.  I didn't bother to alter the
 > 6C
 >  > values to bring Fr to the same frequency.
 >  >
 >  > It does seem to me that with too-high a dv/dt in the first
 > half-cycle,
 >  > there might be the risk of secondary turn:turn voltage breakdown.
 >  Would
 >  > the lower dv/dt and higher Q be better?  Perhaps someone else has
 > already
 >  > considered that kind of thing & I've not paid attention.
 >  >
 >  > Clearly more primary segments could be added for more power--but
 > cramming
 >  > more capacitors& gaps into the nominal diameter might present a
 > problem.
 >  >
 >  > I could use some informed comment on this.  Perhaps it's all too
 > fanciful...
 >  >
 >  > I'd be happy to email my SIMetrix simulation-schematic file to
 > anyone with
 >  > SIMetrix and a real interest in the design.  Or, perhaps someone
 > would care
 >  > to simulate it otherwise, using the pdf drawing as the source.
 >  >
 >  > Anyone interested enough to consider building it??
 >  >
 >  > Ken Herrick
 >
 >
 >
 >