My latest gadget-A medium size Tesla Coil

My Tesla coilTaking over 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:  Great 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 cars today?

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 400,000 volts
Theoretical maximum spark length
39 inches
Resonant Frequency
243 khz
KVA of primary
Primary drive voltage
9,000 volts
Primary charging current
0.060 amps
Primary storage capacitor
0.03uf @ 35KV
Primary spark gap spacing, 2 each
0.193 inches combined
Height of Secondary
Secondary turns
L/C primary
L/C secondary
Coupling between coils, K
0.15,  Considered light
Secondary measured operating "Q"


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.

Man sitting on teslaBut what 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 industry.

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 capacitor. Again, 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 transmitter.

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.

Breakdown & Corona dischargesPicture 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.

Lighting neon lamp with no wiresPhoto 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 at:

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.

Revised: 11/15/16