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Daniel McCauley's 1st coil
Original poster: "K. C. Herrick by way of Terry Fritz <twftesla-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>
Hello Daniel (& all)-
Here's a run-down on my first (and only) coil. As I have commented on in the
past, in List postings, I've concluded that s.s. coils will never, in
practicality, achieve spark-lengths as long as those of spark-gap-type coils.
And I say this after having spent 5 years or so developing my s.s. system.
It's because s.s. drives cannot cram enough power into the 1st 3 or so cycles
of the excitation. It's during that time that the spark length can become
I'll also paste-in below my "recipe" for making a secondary.
Solid-state Tesla Coil Design in Progress:
1. Input, 117 VAC, 60 Hz, 0-1800 W.
2. Physical size, 2 ft. square x 5 ft. tall.
3. Primary design:
3.1 3 equivalent untuned electrical turns driven by power MOSFETs.
3.2 Present configuration, 24 pairs of 500 V, 85 A MOSFETs (IRFP 460A’s) in a
patented circuit (U.S. #6,069,413) switching 4, ~150 Vdc power sources in
series into the 3 equivalent primary turns at the secondary’s Fr. There are 6
pairs of MOSFETs plus associated energy-storage capacitors per printed circuit
board. With each such board is associated an additional circuit board holding
the MOSFET drivers and an off-line switching-current-source to both charge the
capacitors and keep mains current limited to a selected maximum.
4. Secondary design:
4.1 12.5" coil diameter x ~38" height.
4.2 Approx. 550 turns of 20 ga. wire space-wound on a sonotube. Q = 80-100 -at-
~140 KHz resonance.
4.3 Top electrode, 6" x 24" Landergren toroid.
5.1 Highest voltage to ground ever present anywhere in primary circuit, 160 V.
Highest MOSFET turn-off transient, n.g.t. ~400 V.
5.2 Self-tuned: Secondary is the sole resonant element in a feedback
oscillator incorporating the power MOSFETs. Always dynamically tuned.
5.3 Spark rate continuously variable from 1 per button-press to 120/sec
depending on power-line current-capability and spark duration.
5.4 Controlled via a small "wand" at the end of a 15 ft. cable. Wand
incorporates a collapsible ground rod for optionally inducing sparks.
5.5 Spark-length in free air, currently ~33".
5.6 Approx. 300 us secondary-voltage rise time to commencement of spark. Spark
duration selectable in 5 steps from 64 cycles of excitation to 1024.
Suggested secondary-construction recipe
1 "sonotube" fiber concrete-form, 12" nom. dia., "light-wall", x 36". Examine
prior to purchase for smooth exterior. My source: White Cap (distributor), San
2 pail covers, clear butyrate plastic, 12"; Cambro Co., Huntington Beach, CA
#RFSCWC12. My source: East Bay Restaurant Supply, Inc., Oakland, CA. Purpose:
end-caps for the sonotube. Note: Cambro's white cover will not fit White Cap's
sonotube; these do, very nicely.
As req'd, clear RTV rubber; Dow Corning 732 recommended over GE brand. Source:
As req'd, Jasco Water Sealant II. Mfr: Jasco Chemical Corp., Santa Ana, CA.
Source: Local distributor.
As req'd, twisted unpolished cotton twine, 10 ply, .06" nom. dia. My source:
McMaster-Carr Co., Los Angeles, CA, #1929T28 or eq. (One 2 1/2# cone is
plenty). Purpose: to provide a spiral track & cushioning for the wire and to
space the wire turns for increased Q and decreased effective resistance. Note:
I suggest .06" dia. twine to make the winding easier. In the prototype I used
.05" twine. The secondary’s Fr with .06" twine will increase somewhat and the
quantity of turns will diminish.
2 rolls, 43 ft. each, "military grade" teflon thread-sealant tape, 2" x .003";
McMaster-Carr #6802K77 or eq.
About 8#, 20 A.w.g., (~.033" dia.), "heavy build" (.0015"; 5400V) varnished
solid-copper wire. Sugested mfr: Phelps-Dodge. Suggested distributor: EIS Co.,
Los Angeles, CA. (my local distributor: Wirenetics, San Leandro, CA, 10#
spool). Be sure to specify 1 piece with no splices.
1, 2-oz. bottle, "Q-dope" (liquid polystyrene); mfr., GC Electronics; available
through local electronic parts distributors. Purpose: to immobilize the wire
Misc. hardware: 2 short insulated stranded-wire lengths; 2 solder lugs; 1
"banana" or "RCA" panel-mounting receptacle; toroid mounting and connecting
means; 4, 6-32 nylon screws; 4, non-metallic washers. Sources: Local retailers.
4 securing straps; length a/r. Suggested: McMaster-Carr #94905K52 5/8"
hook/pile nylon tape (one 30' roll is plenty). Purpose: to connect from coil
top to apparatus base-plate, to hold the coil upright on top of the primary
Purpose-built winding machine. Source: Up to the builder.
Absent a long-suffering and wholly cooperative spouse or friend, design and
build a winding machine, motor-driven and foot-switch controlled. I used a 1/70
hp gear-motor belt-driving a horizontal winding jig. Said jig I made starting
with two short pieces of "2x4" lumber to be C-clamped to the workbench—one to
support the motor assembly and the other to support a pilot shaft for the far
end of the sonotube/end-cap assembly. Upon each piece of 2x4 I mounted a
1"-dia. wood dowel standing upright. I drilled a 1/2" dia. hole transversely
thru each dowel and inserted short pieces of 1/2" metal rod to serve as
mandrels for the coil assembly. I then suspended the sonotube assembly
horizontally above the 2x4s on the mandrels.
Hinged onto one of the 2x4 assemblies I mounted the motor assembly and also
installed a V-pulley (or other), freely-turning on the 1/2"-rod mandrel and
belt-connected to the motor assembly. On the face of the pulley, I installed a
short pin, perhaps 2" out from the center and in my case just a 1/4-20 bolt, to
project out into a corresponding drive-hole put into one of the sonotube’s
end-caps. The motor assembly drove the V-pulley which in turn rotated the
sonotube assembly via the drive pin. Choose the gear/pulley ratios so that the
twine/wire speed does not exceed ~8"/second, for ease of winding.
Inspect the periphery of the sonotube for any dents; tap them out appropriately
from the inside before proceeding.
Prepare the end-cap pail-lids by cutting off their lifting tabs; "aircraft"
snips are convenient for this purpose. File or sand the cut areas smooth. Drill
an axial 1/2" dia. hole in each lid and the offset driving-hole in one of them.
At the same time, toroid mounting holes/hardware can be installed in the other
end-cap as long as such hardware does not interfere with the winding process.
Trim each end of the sonotube as req'd, assuring that each is approximately
orthogonal with the axis. Apply a bead of RTV rubber fully around each end,
working it in a bit, and then press on the lids. Stand the assembly vertically
on the floor, hold it together with a moderate weight, & allow the RTV to cure
Apply one liberal coat of the Jasco sealant to the outside of the sonotube.
Allow it to dry overnight. Then thoroughly rub down the surface to further work
in the sealant & remove the excess.
Prepare a jig or jigs for holding first the twine and then the wire. Provide
hold-back means so that a moderate tension may be maintained while winding. For
ease in acomplishing the winding, position the jig(s) 12 ft or more away from
the mid-point of the sonotube/winder assembly. Clamp the jig(s) to a firm
At the end of the sonotube from which winding is to begin, drill a ~1/8" dia
hole on a shallow angle, starting at the juncture of the end-cap flange and the
sonotube, orienting it in the winding direction, and ending at the approximate
mid-line of the end-cap's internal shoulder. This is for feeding the wire
through from the beginning position of the winding to the inner periphery of
At the "far" end of the sonotube assembly, drill a hole similarly. On both
internal end-cap shoulders, adjacent to the holes, mount a solder lug. In the
"bottom" end-cap, drill an appropriately-sized hole radially thru at the
approximate mid-line of the end-cap's flange for the return-end’s connector. I
used a panel-mount "RCA" connector.
Drill & tap 4, 6-32 holes, radially and -at- 90 degrees, on the approximate
mid-line of the "top" end-cap's outer flange-surface, orienting them as desired
with respect to the ground-end's connector-hole. These will be for attaching
the hold-down straps.
Mount the sonotube assembly onto the winding jig & assure that it turns
smoothly & freely.
Wedge the twine-end under the lip of the starting-end end-cap to hold it in
place. Then create a spiral track on the surface of the sonotube, for the
subsequently-wound wire, by closely and firmly winding on 1 full layer of
twine. Fully fill the space between the end-caps with the twine. Use a leather
glove to spare your epidermis.
Next, wind on a slightly-overlapping spiral of the teflon tape and press it
down firmly. The tape is for the purpose of pressing down any fuzz from the
twine and also to provide a more spark-resistant surface upon which to wind the
To commence wire-winding, thread the starting wire-end through the hole
provided, scrape off its varnish, pull it snug and solder it to the lug. Then
wind on the wire, carefully keeping it in the track provided by the twine under
the teflon. Maintain a moderately firm tension. Again, use of a glove is
suggested. The twine will compress somewhat, providing a residual tension that
will prevent any subsequent bowing or loosening of the wire prior to final
securing. Keep a piece of masking tape handy for holding the wire in place
When winding is completed, feed the wire-end through the hole provided in the
far end & proceed as before to attach it to the lug there. If you are
compulsive you may now inspect the winding and tweak it here & there, with your
fingernail, to improve the spacing. I don't see it as necessary & depend upon
the varnish ultimately to keep the turns apart if they should touch; it’s good
for 5400V/turn, presumably.
Next secure the winding in place by applying 8 longitudinal beads of the Q-dope
along the full length of the coil. Let dry overnight and then remove the
assembly from the winder.
Mount and connect the bottom connector, using the short piece of stranded wire
to connect to the lug. Mount and connect likewise the means for mounting the
toroid at the other end. I use 3 flanged metal pins arrayed at 120° and sized
to fit into 3 hollow metal posts that I fastened to the underside of the
toroid. One of the 3 pins connects, using hookup wire, to the solder lug. The
toroid’s 3 posts are merely set down over the 3 pins with gravity holding the
toroid in place.
Attach appropriate lengths of hold-down strap to the top end-cap using nylon
screws and non-metallic washers. In the primary apparatus, provide appropriate
mounting eyes or slots for accepting the strap ends. The straps are merely
pulled tight with the ends folded back upon themselves to effect a secure hold
on the secondary.
A coil made to this design resonates at about 140 KHz with a 6" x 24" top
toroid in place.
Ken Herrick Oakland, CA <mailto:kchdlh-at-juno-dot-com>kchdlh-at-juno-dot-com