two years to build, I finally managed to finish this
monster. You might ask, what do TESLA coils do?
The short answer is: They
make big sparks! At 243 Kilohertz no less. This
particular one is capable of 30" sparks when running at full
power. The one in the picture, close to my head, is
about 20" long. The blue-violet discharges are very
entertaining and you might say even spooky. Most of what
you see is corona discharges. The power companies hate
them because they waste energy.
The concept was developed by Nikola
Tesla the famous inventor who lived till 1943.
It was his belief that power could be transmitted through the
ether by contraptions such as this. He wanted to be able
to send power to distant locations and not have to string up
wires. These were the days when electricity for
consumers was still in its infancy. Although small
amounts of power can be transmitted this way, it would never
approach an engineering break even. The basic design has
changed little since his work of the late 1800's. It relies on
two resonant circuits and the "Q" involved in each to boost
the voltage to very high levels. It's a great
educational tool for students of electrical engineering as the
calculations are better understood now than years ago when it
was more of an art. When you wish to step up the voltage
most people think of transformers. This doesn't work so
well as you increase the frequency. You reach a point
where the combination of a transformer and tuned circuits work
best. EE students learn that high "Q" circuits can boost
voltage and impedance considerably if care is maintained.
Mr. Tesla was very instrumental in proposing AC or Alternating
Current as a means of transporting power. His most
notable invention was the AC motor and had over 700
patents. At the time, certain competitors were all
behind DC transmission which turned out not to be the way to
go. The U.S. Supreme Court finally credited him with the
invention of radio which everyone thinks belongs to
Marconi. Equipment from Tesla's lab was used for the
first radio transmissions by Marconi. It was only after his
death that he became recognized for his great vision even
though some of his ideas didn't work. He died
relatively poor and many call him "the most forgotten inventor
of technology". More on Mr. Tesla can be found at: http://www.pbs.org/tesla/ll/index.html.
reading if you're interested in Radio and early Electrical
Engineering. At least three dozen books have been
written about him and I'm sure you'll find them interesting
reading. Overall, this was a man that deserved much more
credit. Did you know that he built the first
Robot? He built the first remotely controlled
vehicle? He held the patent on the Hypoid gear found in
My Coil Specifications:
The coil is of conventional
design using double resonant circuits driven by a 9,000 volt
neon sign transformer. Closing (firing) of the
primary winding is by a double "spark gap" . It's
entirely home built with most of the critical parts obtained
on e-bay. Cost to build: $160
|Maximum Secondary Voltage
at full power
|Theoretical maximum spark
|KVA of primary
|Primary drive voltage
|Primary charging current
|Primary storage capacitor
|0.03uf @ 35KV
|Primary spark gap
spacing, 2 each
|0.193 inches combined
|Height of Secondary
|Coupling between coils, K
It goes without saying that
building a device of this sort doesn't come without some
serious risks to your health if not careful. The
voltages and currents in the primary circuit are
lethal. A current passing through your heart of
about 0.02 amps is usually fatal. In most of our day to
day lives, the voltage is not high enough to force that much
current. Here, your dealing with 9,000 volts pushing
more electrons through the body so the chance of reaching
serious currents is almost guaranteed! Not good
odds. With the primary driver running at 60 hertz, these
high currents and voltages become a serious safety
issue. Never attempt to operate a device of this type unless
you understand the risks and how to manage them.
about the 400,000 volts that the coil can generate in the
secondary? Surprisingly, not near as much of a problem
because of "skin effect" and much lower currents. In
electrical engineering we learn that high frequency signals
travel on the surface of conductors. At 243 khz, only
about 5 mils of depth is involved. Our internal organs
are not exposed to these currents. Hence, working with
the secondary is much less hazardous. As a young man, I
built a lower powered coil and would let it arc to my hand,
often. It would only give a tingle and maybe a mild
burn. Since my heart has been working some 60 years
since then, ie, has aged a little and wearing out, (I've
been told I have Congestive Heart Failure, CHF). I'm not
going to pull that trick again. Most Tesla Coilers
now advise against direct contact. The photo on the
right was taken in 1988 at the International Tesla
Symposium. These stunts are common for the movie
Another hazard, even
when the coil is turned off, is the primary storage
capacitor. It can be charged to 12,700 volts and, like all
capacitors, can retain that charge if not properly bled off.
The capacitor is of very high quality and has little leakage.
Safety circuits are included in the design but if they were to
fail you could be "done in" even when the coil was unplugged.
Standard safety proceedures must be followed when adjusting or
working on the device to ensure no energy remains in the
don't work with these devices unless you completely
understand how to remove the risk. Three people have
died working with Tesla Coils.
One more consideration is the generation of Ozone. Large
amounts are created in the vicinity of big electrical
discharges. The Ozone molecule, O3 is both a
good guy and bad. High in the atmosphere, its desireable
because it filters short wave ionizing UV rays from reaching
the surface and helps prevent skin cancer. When Ozone is
released near the surface, it can create an allergic reaction
in some people. This often happens in smoggy areas such as Los
Angeles because its a byproduct of pollution and is
corrosive. More on Ozone.
The primary spark gaps generate high levels of short wave UV
radiation but this can be easily blocked by filters.
Since large amounts of RF energy are created, people with pacemakers should
not be in the vicinity of an operating tesla coil.
Because of the RF, it should only be operated in a grounded
metal building such as my shop. This shielding helps
prevent the operator from becoming a high powered illegal
The largest Tesla Coil ever built still seems to be by Nikola
Tesla himself. He demonstarted 60+ foot long
arc's. The Tesla Coil was inspiration for the "particle
accelerator" which is in common use today in nuclear physics.
The Tesla coil is sometimes confused with the Van De Graff
generator which is a DC device with low duty cycle.
Picture at right
shows both breakdown sparks and corona discharges. In a
darkened room, corona can be seen extending for several feet.
An ion propulsion motor has been added to the experiments
(photo not available) which involves spinning a disc by
throwing off ionized gas molecules. Looks allot like a
rotating fireworks display. Ion propulsion is an
important concept for long distance space travel and is now
being used in some of our interplanetary explorations.
Photo at left
shows a neon lamp that is taped to an insulated post.
It's just floating.... no wires connected to it. The RF
energy is lighting the bulb brightly by ionizing the gas
inside. This was Mr. Tesla's inspiration for generation
of lighting w/o wires. He felt that you could illuminate
an entire city without having to string wires. This type
experiment also led to the discovery of the fluorescent
lamp which he's been credited. Various gases glow
different colors when excited. It takes quantum
mechanics to explain how this works which ultimately led to
the discovery of the laser. Did you know that the gas
content of a bulb can be determined by just observing the
"line spectra" when it's ionized? The invention of the
spectrograph was the result of this observation. By use
of this tool, Helium was discovered on the sun before it was
ever found here on earth. Science has provided some
amazing tools to work with.
Can I see it in operation?
The unit has been sold in mid 2016 and is now
longer available. However, I would be willing
to answer any questions you have about the design
Where can I learn more?
Many books have been written about Tesla and his
inventions. The local library has several plus a
new one seems to hit the market every couple of
years. You can find considerable material on the
Internet, specific to the construction of Tesla Coils.