Download The Tesla coil

The Tesla coil: A Frankenstein folly or a
brilliant Nikola Tesla creation?
Steve Taranovich - January 02, 2014
The ancient Greek god Zeus was said to be able to throw lightning bolts; Nikola Tesla emulated this
with his Tesla coil design on a grand scale. But is this device a great invention, a novelty or just a
novelty for entertainment purposes?
Before I get to the enhancements of the Tesla coil over time and discuss its many important uses in
today’s electronics world, I want to point out the importance of this design vs. the amusing aspects
of it. Then it’s on to femto second, laser-induced plasma filaments in air, a mini-Tesla coil with a DC
input and the construction of the world’s largest Tesla coil.
First some fun facts.
Frankenstein
Well, Mary Shelley created the monster, but Universal Studios special effects expert Ken Strickfaden
thought enough of the Tesla coil to use it as an instrument to bring life to her monster. Later,
Strickfaden created the Science on Parade show (Later named Kenstric Space Age Science show).
Strickfaden was an electrician, film set designer, and electrical effects creator. His Tesla coils were
used in more than 100 motion picture films and TV programs, including Mary Shelley’s 1931
Frankenstein, The Wizard of Oz and TV’s The Munsters as well as his last work---Young
Frankenstein.
The 1931 movie Frankenstein showed Ken Strickfaden’s Tesla coil (just left of center)
(Image courtesy of Jerry Ohlinger’s Movie Material Store, Inc.)
Nikola Tesla
Nikola Tesla, on the other hand, the creator of the device named after him, the Tesla coil, built his
first oscillator as a tool to study high frequencies using electricity. Tesla also made high-frequency
electrical generators which helped lead to high-frequency electrical lighting. After all, these devices
were a sort of first high-frequency, efficient lighting ballasts. Tesla experimented with fluorescent
and incandescent lighting and even high-frequency arc lighting. This was the 1890s!
Tesla also discovered an important principle in electricity through the Tesla coil technology--electrical resonance. This principle made it possible to eliminate one of the two conductors that
carry current from a power supply to a load. From this he created the single-wire electric motor.
Electrostatic induction was born which is called capacitive coupling in our modern times.
Much of this work eventually helped science to develop high-energy particle accelerators, wireless
telegraphy and telephony and radio.
Tesla coil design
I am a believer in using short cuts in designs as long as they are technically solid and robust. In
designing a Tesla coil, there are numerous equations and formulas that must be calculated. As a
short cut, Kevin Wilson recommends the usage of the TeslaMap program. This is probably the fastest
and easiest road to designing and building the Tesla coil. Wilson has a really extensive paper that is
a great help in understanding and building a Tesla coil. There are potentially dangerous areas in
building a Tesla coil, even for seasoned EEs, so please pay attention to warnings and details if you
are not familiar with high voltage. We’re all professionals for the most part, but caution and safe
measures are always prudent in any design effort.
The world’s largest Tesla coil
The world’s largest Tesla coil6
Tesla invented the Tesla coil in 1891. He resumed his development and improvements to that device
in 1899 when he built a large lab in Colorado Springs. He built the secondary that was a 51 foot
diameter design inside a wooden building that contained no iron materials. The facility included an
80 foot wooden tower that was topped by a 200 foot mast with a large copper sphere on top that
served as an antenna. The device produced bolts of lightning over 100 feet long.
Figure 1: An old image of the Wardenclyffe Tower in partial stages of completion. Work on
the 55 foot cupola had not yet begun. (Image courtesy of Wikipedia Commons)
Figure 2: An artistic representation of what the finished tower would have looked like if it
had been completed (Image courtesy of Wikipedia Commons)
Since then no one has ever built such a large Tesla coil except on Long Island, NY in Tesla’s other
lab at Wardenclyffe near Shoreham, NY, very close to my former home. The Wardenclyffe Tower, as
it was called, existed from 1901 to 1917 when it was demolished. Tesla had intended it as a wireless
transmission device that would serve as a trans-Atlantic wireless telephony, broadcasting and a
massive demonstration of wireless power transmission.
A classic Tesla coil design
Gary L. Johnson from the Electrical and Computer Engineering Department at Kansas State
University has one of the most complete analyses I have seen in his IEEE article “BUILDING THE
WORLD'S LARGEST TESLA COIL” and I summarize his work in the following section.
Tesla’s design had two stages of voltage increase. The first stage contained a standard iron core
transformer with high per unit impedance which stepped up the input line voltage to between 12 to
50 kV at 60 Hz. The second stage was a resonant air core transformer (Essentially this was the Tesla
coil) that was able to step up the voltage to between 200 kV to 1 MV at a frequency of 500 kHz for
smaller designs and 80 kHz for larger designs.
The air core resonant transformer is similar to a conventional transformer with a different mode of
operation. Instead of using firm coupling between the primary and secondary windings where the
voltage transformation ratio is dependent upon the turns ratio, the Tesla coil employs a loose
primary to secondary coupling. The voltage gain in a Tesla coil is realized via resonance. Plus a
conventional transformer usually uses an iron core operating at low frequency but the Tesla coil
uses an air core that operates more efficiently at high frequency.
The really large Tesla coils like the one in Colorado Springs, had a third stage of voltage boost which
increased the final output to between 10 to 25 MV!
Figure 3: The classic Tesla coil schematic circuit contained a fixed spark gap (G) in smaller
coils and a rotary in larger coils. C1 is a low loss primary capacitor made of mica and rated
at 20 kV. L1 was the primary coil of 2 to 5 turns for the smaller coils and 1 to 2 turns for
the larger ones. L2 was the secondary coil made up of 25 turns for the large coils and 400
turns for the smaller ones. C2 is actually the distributed capacitance between the L2
windings and the voltage grading structure at the top of the coil (a toroid or sphere) and
ground.
C2 in Figure 3 would change as the volume charge density around the secondary, increasing a bit
when sparking started. Its capacitance would increase as it would get closer to the surrounding
metal wall.
When the gap was not conducting then C1 charges (See Figure 4 showing only the central part of
Figure 3) Since the inductive reactance is smaller than the capacitive reactance at 60 Hz, L1 looks
like a short circuit at that frequency and the capacitor gets charged by the secondary iron core
transformer.
Figure 4: C1 gets charged with the gap open
Here is how it worked
Here is how it worked: The voltage across the capacitor and gap would reach a certain value and the
gap would arc over creating the circuit in Figure 5.
Figure 5: The gap is shorted when the arc occurs and effectively splits the circuit into two
parts in which the iron core transformer operates at 60 Hz and the circuit on the right side
of the gap operating at a frequency determined by C1, L1, L2, and C2.
The iron core transformer output voltage will drop to zero while the input remains the same as long
as the arc is occurring. The transformer current becomes limited by the equivalent series
impedance of RS + jXS shown in Figure 5.
Now let’s look at the lumped circuit model of the shorted Tesla coil for analysis (See Figure 6)
Figure 6: The lumped circuit model of the shorted Tesla coil where R1 and R2 are the
effective resistances of the air-cored transformer primary and secondary respectively.
Figure 6 shows M as the mutual inductance between the primary and the secondary of the
transformer.
When the gap arcs over, C1 stores all of the energy at first, but as time goes on C1, L1, C2, L2 and M
share the total energy, which will decrease due to R1 and R2 resistances. If the design is done
properly, by the setting of particular values of C1, L1, C2, L2 and M then all of the energy in C1 will be
transferred to the secondary at once at time t1. At t1 there is no voltage on C1 and no current in L1. If
the gap opens at t1 then energy cannot get back to the primary which causes no current storage in L1
and hence the capacitor will not be charged.
Subsequently the secondary becomes an RLC circuit and C2 and L2 will have non-zero initial
conditions. The circuit will now resonate at a frequency set by C2 and L2 values. S with an open gap,
the secondary of the Tesla coil is a classic RLC circuit and as we would expect, has a damped
sinusoid output.
If we set the gap to arc over at a voltage near the peak of the 60 Hz sinusoid, then there will be only
one pulse each half cycle (120 pulses/second) being supplied to the air-cored transformer. So if the
secondary is designed for a 240 kHz resonant frequency then each pulse will need to supply enough
energy to sustain an oscillation in the secondary for 2,000 cycles. Dr. Johnson found that the arc only
lasts for 10 us which is only 2 or 3 cycles of the output waveform.
The genius of Nikola Tesla is realized by those who try to complete Tesla’s experiment in Colorado
Springs or Wardenclyffe on Long Island, NY. However, that will not stop engineers from trying. I’m
looking forward to more efforts like Dr. Johnson’s and his students.
A mini Tesla coil using a DC voltage input
A mini Tesla coil using a DC voltage input8
The electrical engineering faculty members at UTeM Malaysia, put together a pretty neat design for
a Tesla coil with better mobility, being less bulky than typical designs. Their goal was to generate a
high frequency current with a medium voltage, that is, 2.5kV on the secondary with an unusual
24VDC at the primary input side instead of an AC voltage as is typical with most designs.
The proposed mini Tesla coil uses all the same components as a traditional Tesla coil such as the
toroid, primary and secondary coils, spark gap and capacitor. On big difference is that the minidesign does not use a rotary of sphere gap, but instead a contactor of a relay since conventional
spark gaps would not work with a 24 VDC input.
The 24 VDC is not enough to fire the gap across the relay contactor since a one millimeter gap needs
3 kV to generate an arc. Instead, the arc is created by opening and closing the gap very quickly.
During the opening and closing of the relay, the 24V will charge the tank capacitor which will in
turn transfer energy to the primary of the transformer.
Guiding high voltage arcs in the Tesla coil
Finally, let’s take a look at a technique guiding the high voltage arcs of the Tesla coil by using laserinduced plasma filaments in air5.
A terawatt femtosecond laser was used to guide high-voltage arcs in the air using plasma filaments.
A high-voltage electrode, set at the top of a Tesla coil voltage elevator, emanates arcs to a grounded
node.
Extremely short, pulsed laser beams, having energy in the mJ region, have non-linear propagation in
air. Long plasma filaments, also known as Birkeland currents (Much of Nikola Tesla’s research and
experimentation was in the plasma region of guided energy and power) having electron density of
1016 – 1018 cm-3 are able to be generated due to dynamic competition between the non-linear Kerr
self-focusing (which eventually collapses the beam) and plasma de-focusing (which tends to halt the
collapse of a beam). The laser ionizes the air and causes this effect.
The plasma filaments in this experiment5 with radii of 200 um can exist over relatively long distances
carrying high-voltage energy across the air gap. The high voltage Tesla coil in the experiment
produced bursts of pulses up to 350 kV at tens of us rise times. By modifying the Tesla coil to give it
the capability of triggering the discharge pulses with sub-nanosecond jitter, perfectly straight laserguided arcs were generated between the electrodes.
Figure 7: Diagram of coupling device for creation of a plasma antenna (Courtesy of
reference 10)
Virtual plasma antennas10 in the RF region might be a practical application using this technique. This
phenomenon gives passive radar a tremendous advantage in using a wideband diverse pulse train
that improves detection probability of the radar with stealthy fluctuated targets which traditional
antennas have difficulty in generating. Plasma antennas vs. copper antennas have great success with
wideband frequencies enabling them to transmit a more diverse radar waveform.
Other practical uses of Tesla coil technology
When Tesla created his Tesla coil, he actually made an antenna out of the high-voltage secondary
which acted as a very powerful radio transmitter. A great many early radios used Tesla coils in their
transmission antennas. After creating the Tesla coil, the inventor used it to investigate and
experiment with fluorescence, biological effects, x-rays, wireless power, radio and ultimately the
Earth’s electromagnetic nature related to the atmosphere.
Tesla coil circuits were used in early wireless telegraphy in spark gap radio transmitters until the
1920s.
The Tesla coil technology was used in an early 20th century, but now obsolete, medical therapy---electrotherapy. A violet ray was the old medical appliance that would apply a high-voltage, low
current, high-frequency current to the human body. This technique was said to cure many ailments.
Unfortunately, this was a misguided use of Tesla’s technology which caused more problems to
people than it cured.
An early particle accelerator design in 1928 by Rolf Wideroe, generated its high voltage using a
Tesla coil. Even the flyback transformer is a form of Tesla coil which was first used to generate the
high voltage in a TV tube.
Please share with our readers your Tesla coil experiments, experiences and designs.
References
1 Tesla coil theory on Terry Blake home page
2 Richie’s Tesla coil web page
3 Tesla coil design web page
4 The Zeus Tesla coil on the Hazardous Physics webpage
5 TRIGGERING AND GUIDING OF HIGH-VOLTAGE TESLA COIL DISCHARGES BY FEMTOSECOND
LASER-INDUCED PLASMA FILAMENTS IN AIR, Yohann Brelet, Aurélien Houard, Bernard Prade,
Jérôme Carbonnel, Yves-Bernard André and André Mysyrowicz
6 BUILDING THE WORLD'S LARGEST TESLA COIL HISTORY AND THEORY, Gary L. Johnson 1990
7 SOLID STATE TESLA COIL, Dr. Gary L. Johnson 2001
8 A Simple Design of a Mini Tesla Coil With DC Voltage Input, M. B. Farriz, A. Din, A.A. Rahman,
M.S. Yahaya, J.M. Herman
9 Solid State Tesla Coils and Their Uses, Sean Soleyman submitted in 2012 as a research project for
his Master of Science degree at U of C Berkeley, CA
10 Reconfigurable Plasma Antenna Produced in Air by Laser-induced Filaments: Passive Radar
Application, Mostafa Alshershby and Jingquan Lin
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