We will start with the ballast coil. This is made with two
500 foot lengths of insulated #6AWG stranded wire laid side
by side. The ballast is wired in an unconventional manner.
If we call the ends of the wires on the left side of the
ballast A initial and B initial and the wires on right side
of the ballast A final and B final, we would say that A
Initial and B final go to the wall outlet and A final and B
initial go to the low voltage side of the utility
transformer. All of the current travels in the same
direction around this ballast.
Reactance for 60 Hz is rather low, so for 60 Hz the ballast acts
something like a long extension cord, merely getting power to the
utility transformer. But when the spark gap fires across the high
voltage side of the utility transformer a high current spike is
seen on the low voltage side of the utility transformer. This
high frequency di/dt has a large reactance in the ballast and
induces voltage across both wires A and B in the bifilar winds.
The induced voltage matches the voltage at the wall outlet, so for
the brief instant the gap fires no current is drawn.
Our ballast is very efficient. During extended runs on a
hot summer day it climbed only fifteen degrees Fahrenheit
before stabilizing. We gave the ballast sufficient surface
area so that no cooling fan was required.
A peek inside the ballast shows nothing but air.
Our utility transformer is a 10 KVA unit operating at 14,400
We put this unit on industrial casters so we could
move it around without breaking our backs.
For our tank capacitor we used a total of (90) CDE-940
series 3000 volt 0.1uf capacitors wired in series/parallel
for a total capacitance of 0.09uf at 30,000 volts
We originally used Maxwell capacitors but found no
noticeable difference in performance when we changed to the
940 series capacitors. We are fully aware that many coilers
use the 942 series.
The tank capacitor sets the power level of the system. The
equation I = V ω C is meant for sine waves and certainly the
spark box causes a slight deviation from sine wave behavior.
Nonetheless, the equation gives a fair approximation of the
reactive power available to the system.
Our voltage is 14,400 and our tank capacitor is 0.09uf. The line
frequency is 60Hz so the current is limited to no more than 488ma.
Thus the reactive power is restricted by the tank capacitor to
very close to 7.2KVA (7200 watts).
We use a vacuum (forced air) gap with five spark gaps in series.
The blower is from an industrial dust collector. It is a
two horse power model rated at 1550 cubic feet per minute.
A rather large flow of air is required to remove ionized
gas, heat and conductive deposits.
The spark box is made of 1/4 inch grade CE Garolite and is
held together with epoxy.
Each electrode has a total of (25) 1/8th inch pure tungsten
welding rods, normally used for tungsten-inert-gas (TIG)
The welding rods are wrapped around a 3/4 inch copper tube.
The copper tube stops about 1/2 inch shy of the hot end.
The rods are held around the copper tube with three heavy
duty hose clamps per electrode.
Two sets of electrodes on one side of the box are retracted
to a hard stop by a vacuum diaphragm (made with 1/8th inch
neoprene, 50A durometer).
The electrode retraction gives a smooth transition from start-up
to full power.
A vacuum leak is introduced with a valve to set the rate at
which retraction occurs, and the vacuum is, of course,
provided by the blower.
Tungsten wear is negligible with our system.
rods never get past being warm to the touch, except very
near the end with the sparks.
Filters and Tuners
The large tube structures are a pair of magnetically isolated
one-to-one current baluns. The two one-to-one baluns are wired in
series on the tank capacitor side, and in parallel on the primary
side to produce a four-to-one current balun.
The baluns provide an impedance match that keeps counter-EMF
from returning to the spark box (allowing the spark box to perform
it's task efficiently). Each bifilar balun is made with a total
of 240 feet of 1/4 inch type L tubing with 1.125 inch between
tube centers. The balun diameter is 31.5 inch (diameter chosen
to fit through a 32 inch wide door).
All of our tube structures are joined with compression fittings so
we can easily assemble/disassemble.
There are a total of 5 half wave inductors in our system. Each
half wave uses 10 lb of 23 AWG wire around a 12.625 inch by 4 foot
cardboard concrete form (approximately 1.9 Km wire each).
Each one-to-one balun has a half wave inside and a half wave
outside. These half waves are wired across both the front
end and the back end of the primary feed line.
Since the half waves operate at their natural resonant frequency
they provide extreme signal selectivity.
Neither the baluns nor the half waves climb more than a few
degrees above ambient temperature. We use an infrared
thermometer. If a component is liberating heat, it likely needs
re-engineering. Clearly the energy that goes into heating
components will not be available for making sparks.
The main structure supporting the primary is 6 inch PVC pipe and
is made conductive using a mix of HVAC AF 100 metal tape and
aluminum foil held to the pipe with 3M #77 spray adhesive. We keep
the metal foil to a minimum as magnetic fields induce eddy
currents into the aluminum (less so when the aluminum is thin).
Top-end Capacitors (toroids)
Our top end half wave has a consistent load; this allows us to
choose a top end capacitance for maximum power through. The
frequency is set by the array of half wave coils.
The top end
capacitance is chosen entirely for minimizing the impedance for a
given spark jump. We picked an opening of 52 inches and kept
raising the top end capacitance until the unit no longer fired.
Then we closed the jump to 48 inches (the length of the 1/2 wave
inductor). This method ensures that voltage is the minimum
required for reliable firing and the current is as large as
possible (notice that our sparks are white hot).
We use Styrofoam for supporting the primary and for blocking the
top end half wave, Styrofoam has excellent RF qualities. The top
end disks are also aluminum foil covered Styrofoam.
Primary and coupling
The primary needs a lot of turns and would normally be considered
very over-coupled. However k between the magnetically isolated
baluns lowers the overall system k to the point where tight
coupling on the primary becomes a necessity.
A limit switch actuated by the vacuum diaphragm operates an
industrial control relay to provide proper sequencing.
The high powered circuit for the transformer is switched by
an additional heavy-duty relay.
We tried many different circuit topologies over the last few years
and so far this is the best we have found.
Why we use English (imperial) units
Our apologies for the non-metric units; everything in the U.S.A is
in inches and feet.
Let's start with the half wave inductors. You will need five
medium sized furniture casters and a board that's about five feet
long. Mount four of the casters upside down with one caster on the
end oriented at 90 degrees. You will be using the fifth caster to
keep the form from rolling off as you wind.
Your spool of wire should be mounted in a box with bearings so
that it rolls easily. We always wind from left to right. It
takes two people, one to spin the form and the other to guide the
wire with their fingernail (or stick). If your form is slightly
out of round (typical) you can make four or five Styrofoam circles
to stick inside the form to straighten it. We typically use the
same Styrofoam disks as hubs for the P.V.C. pipe that supports the
It takes us about two hours to wind an inductor, but we have a lot
of practice (25 miles of wire in the last 6 years). It is rather
Varnish the inductor with a coat immediately after winding to
avoid problems. You could put on a zillion coats of high gloss
and make it look like a piece of candy, but we don't do this
anymore since the polyurethane varnish is a dielectric and gets
polarized. Five coats varnish is plenty, make them thin coats or
they will run under the wires, pool on the underside, and look
Forming tubes is a bit tricky. Get spools of tubing that come in a
large diameter box as they are less work hardened. Inspect the box
before purchasing; some boxes have banged up tube. Unless you are
Mr. Muscle, I would suggest handling no more than 60 or 70 ft. of
tubing before sweating in a splice. A pair of normal mortals will
get pretty wiped out after winding 240 ft of type L tube.
It is best to have a very smooth form. If anything with a
minuscule amount of taper as this makes life easier when removing
the tube from the form.
Secure one end of the tube to the form with screws or packing
tape. Wind the coils around the drum carefully. When you are
finished winding, put a Bungee cord on the loose end of the tube
and pull the Bungee cord tight. Now get a rubber mallet and start
(lightly) tapping the tube to get out any slack. Place the tubes
shoulder to shoulder and tap on the side of the tube (gently) with
a soft contoured wood block to get rid of any gaps. It takes an
hour or two of (gentle) tapping per balun. If you ding up some of
the tube, stick in a splice.
Now move your bungee cord so that you can put pre-cut spacers
between the tubes while still keeping the tube under tension.
Once you have the spacers in place you tape the tube with big X's
between the legs to keep the tube from shifting when you remove
the Bungee cord. Now you can glue the top half of the legs on the
spacers and work the tube off the form. Do not glue the spacers
to the tube or glue the tube to the form. Once removed from the
form you can glue the inside leg parts and remove the tape. You
will need solvent to remove any tape residue. Best to wear gloves
as the acids from your hands leave nasty prints on the copper.
Varnish the tube or it will lose its shine rather quickly.
Building the spark box
When you build your box, make sure to leave a few inches
space between electrodes. In fact make your box just a bit
taller and deeper (but not wider), than the box shown as
this makes adjusting the stops easier.
A real nice trick (borrowed from manufacture of electric motors)
is to drill the electrode holes over-sized in the box partitions.
Then use flanges of the correct size on both sides of the
partitions. You use a long copper pipe that goes all the way
across the box surrounded by electrodes and clamps. Then glue the
flanges. When the glue hardens you get perfect electrode
The baluns are very dangerous, keep away from them when the coil
is running. Get hit with this level of power from either end of
the coil, and you are dead! We have no control over your safety;
it would break our heart if you got hurt, so please be careful!
Good luck from Lawrence Morris and Jared Dwarshuis March, 2010.
Other things we have done
Here are some other Tesla coil related things we have done: