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A Tesla coil is a category of disruptive discharge coils, named after their
inventor, Nikola Tesla. Tesla actually experimented with a large variety of
coils and configurations, so it is difficult to describe a specific mode of
construction that will meet the wants of those who ask about "Tesla" coils.
Tesla coils are composed of coupled resonant
circuits.
Introduction
Most oil insulated
transformers need large and long insulations at their connections to prevent
discharge in air. Tesla coils spread their electric field over a large distance
to prevent high field strength in the first place.
Early
coils
The American Electrician gives a description of one wherein
a glass battery jar, six inches by eight inches, is wound with 60 to 80 turns of
American wire gauge No. 18 B & S magnet wire. Into this is slipped a primary
consisting of eight to ten turns of AWG No. 6 B & S wire, and the whole
combination immersed in a vessel containing linseed or mineral
oil.
Disruptive "Tesla" coils
Following
the initial research of voltage and frequency by William Crookes, Tesla
developed a series of coils that produced high-voltage, high-frequency currents.
In the majority of Tesla's experiments, he used machinery of his own design to
produce the Tesla effect. These early coils would use the "disruptive" action of
a spark gap in their operation. The setup can be duplicated by a Ruhmkorff coil,
two condensers, and a second, specially constructed, disruptive
coil.
The Ruhmkorff coil, being fed from a main source, is wired to
condensers (now called capacitors) on both ends in series. A spark gap is placed
in parallel to the Ruhmkorff coil before the condensers. The discharge tips
usually preferably metal balls under one inch in diameter, though Tesla used
various forms of dischargers. The condensers were of a special design, small
with high insulation. These condensers consisted of plates in oil that were
movable. The smaller the plates, the more frequent the discharge of this early
coil apparatus. The plates also help nullify the high self inductance of the
secondary coil by adding capacity to it. Mica plates were placed in the spark
gap to establish an air current jet to go up through the gap. This made the
discharge more abrupt. An air blast was also used for this objective.
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The condensers are connected to a double primary (each coil in
series with a condenser). These are part of the second specially constructed
disruptive coil. The primaries each have twenty turns of No. 16 B & S rubber
covered wire and are wound separately on rubber tubes not less than a 1/8th inch
thick. The secondary has 300 turns of No. 30 B & S silk-covered magnet wire,
wound on rubber tube or rod, and the ends encased in glass or rubber tubes. The
primaries must be large enough to be loose when the secondary coil is place
between the coils. The primaries must cover around two inches of the secondary.
A hard rubber division must be placed between these primary coils. The ends of
the primaries not connected with the condensers are lead to a spark gap.
(Norrie, pg. 35-36)
Tesla's later coils were considerably larger and
operated at much higher power levels. These later systems were powered from
large high voltage transformers, used banks of glass bottle capacitors immersed
in oil to reduce corona losses, and used rotating spark gaps to handle the
higher power levels. Tesla also dispensed with using oil to insulate the
transformer coils, relying instead on the insulating properties of air. Tesla
coils achieve great gain in voltage by loosely coupling two resonant LC circuits
together, using an air-core (ironless) transformer. Unlike a conventional
transformer, whose gain is limited to the ratio of the numbers of turns in the
windings, Tesla coils' voltage gain is proportional to the square root of the
ratio of secondary and primary inductances.
Later
coils
When Tesla patented a later device (US1119732 — Apparatus for Transmitting
Electrical Energy), he called it a high-voltage, air-core, self-regenerative
resonant transformer that generates very high voltages at high frequency.
However, this phrase is no longer in conventional use.
This later coil
type is the usual device built by modern enthusiasts. It is an air-core,
dual-tuned resonant transformer that generates very high voltages at radio
frequencies. The coil achieves a great gain in voltage by means of a resonant
circuit, unlike a conventional transformer whose gain is limited to the ratio of
the numbers of turns in the windings.
Although modern Tesla Coils are
designed to generate long sparks, Tesla's original system were designed for
wireless communication and power transmission, so he used large radii of
curvature to prevent corona and streamer losses. Tesla coils' outer conducting
surfaces, which are charged to a high potential, have large radii of curvature
to minimise leakage of the oscillating charges through corona discharges or
sparks. The intensity of the voltage gain of the circuit with a free, or
elevated, toroid is proportional to the quantity of charge displaced, which is
determined by the product of the capacitance of the circuit, the voltage (which
Tesla called "pressure"), and the frequency of the currents employed.
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Description
Later coils consist of a primary tank
circuit, which is a series LC circuit composed of a high voltage capacitor,
spark gap, and primary coil; and the secondary LC circuit, a series resonant
circuit consisting of the secondary coil and the toroid. In Tesla's original
plans, the secondary LC circuit is composed of a loaded secondary coil which is
then placed in series with a large helical coil. The helical coil is then
connected to the toroid. The toroid actually forms one terminal of a capacitor,
the other terminal being the Earth (or "ground"). The primary LC circuit is
"tuned" so that it will resonate at the same frequency as the secondary LC
circuit. The primary and secondary coils are magnetically coupled, creating a
dual-tuned resonant air-core transformer.
The terminal consists of a
metallic frame, sometimes a toroid covered with smooth half spherical metal
plates (constituting a very large conducting surface). The terminal has
relatively small capacitance, charged to as high a voltage as practicable. The
outer surface of the elevated conductor is where the electrical charge chiefly
accumulates. It has a large radius of curvature, or is composed of separate
elements which, irrespective of their own radii of curvature, are arranged close
to each other so that the outside ideal surface enveloping them has a large
radius.
The frame is carried by a strong platform and rests on insulating
supports. The circuit consists of a coil in close inductive relation with a
primary, and one end of which is connected to a ground-plate, while its other
end is led through a separate self-induction coil (whose connection should
always be made at, or near, the center in order to secure a symmetrical
distribution of the current) and a metallic cylinder to the terminal. The
primary coil may be excited by any desired source. The important requirement is
that a resonant condition be established. A high frequency alternator or a
capacitor discharge can be used to excite the primary coil.
The conductor
of the shaft to the terminal is in the form of a cylinder with smooth surface of
a radius much larger than that of the spherical metal plates, and widens out at
the bottom into a hood (which is slotted to avoid loss by eddy currents and for
safety). The secondary coil is wound on a drum of insulating material, with its
turns close together. When the effect of the small radius of curvature of the
wire itself is overcome, the lower secondary coil behaves as a conductor of
large radius of curvature, corresponding to that of the drum (this effect is
applicable elsewhere). The lower end of the upper secondary coil, if desired,
may be extended up to the terminal and should be somewhat below the uppermost
turn of the primary coil. This lessens the tendency of the charge to break out
from the wire connecting both and to pass along the
support.
Utilization and production
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Transmission
A large Tesla coil of more modern design
can operate at very high peak power levels, up to many megawatts (a million
watts; hundreds of thousands of horsepower). It should therefore be adjusted and
operated carefully, not only for efficiency and economy, but also for safety.
If, due to improper tuning, the maximum voltage point occurs below the terminal,
along the secondary coil, a discharge (spark), or possibly a ball of plasma, may
break out and damage or destroy the coil wire, supports, nearby objects, or
anything else in the way.
Safety
and precautions
It is advisable to begin the tuning, in which the
primary coil's resonant frequency is set to the
same value of the secondary coil's, using low-power oscillations, then increasing the power
until the apparatus has been brought under control.
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| Typical
Tesla Coil Schematic This example circuit is designed to be driven by alternating currents. |
While tuning, a small projection (called a "breakout bump") may be added to
the top terminal in order to stimulate corona and spark discharges (sometimes
called streamers) into the surrounding air. One or more elements or plates of
somewhat smaller radius of curvature or protruding more or less beyond the
others (in which case they may be of larger radius of curvature) so that, should
the voltage rise too high, the powerful discharge may escape to the air. Tuning
can then be adjusted so as to get the longest streamers at a given power level,
corresponding to a frequency match between the primary and secondary coil. For a
variety of technical reasons, toroids provide the best overall shape for top
terminals of modern Tesla coils.
Since Tesla coils can produce currents
or discharges of very high frequency and voltage, they are useful for various
purposes including classroom demonstration, theater and movie special-effects,
and product/technology safety testing. In typical operation, long, branching
high-voltage sparks may strike out in all directions from the toroid into the
air, producing a dangerous, yet strangely beautiful, lightning-like display of
electricity "in action".
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Air discharges
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| A small later-type "Tesla coil" in operation. The output is giving 17 inches sparks. The diameter of the secondary is 3 inches. The power source is a 10000 VAC 60 Hz current limited supply. |
A small later-type "Tesla coil" in operation. The output is giving 17 inches
sparks. The diameter of the secondary is 3 inches. The power source is a 10000
VAC 60 Hz current limited supply.
While generating discharges, electrical
energy from the secondary and toroid is transferred to the surrounding air as
electrical charge, heat, light, and sound. The electric currents that flow
through these discharges are actually due to the rapid shifting of quantities of
charge from one place (the top terminal) to other places (nearby regions of
air). The process is similar to charging or discharging a capacitor. The current
that arises from shifting charges within a capacitor is called a displacement
current. Tesla Coil discharges are formed as a result of displacement currents
as pulses of electrical charge are rapidly transferred between the high voltage
toroid and nearby regions within the air (called space charge regions). Although
the space charge regions around the toroid are invisible, they play a profound
role in the appearance and location of Tesla Coil discharges.
When the
spark gap fires, the charged capacitor discharges into the primary winding,
causing the primary circuit to oscillate. The oscillating primary current
creates a magnetic field that couples to the secondary winding, transferring
energy into the secondary side of the transformer and causing it to oscillate
with the toroid capacitance. The energy transfer occurs over a number of cycles,
and most of the energy that was originally in the primary side is transferred
into the secondary side. The greater the magnetic coupling between windings, the
shorter the time required to complete the energy transfer. As energy builds
within the oscillating secondary circuit, the amplitude of the toroid's RF
voltage rapidly increases, and the air surrounding toroid begins to undergo
dielectric breakdown, forming a corona discharge.
As the secondary's
energy (and output voltage) continue to increase, larger pulses of displacement
current further ionize and heat the air at the point of initial breakdown. This
forms a very conductive "root" of hotter plasma, called a leader, that projects
outward from the toroid. The plasma within the leader is considerably hotter
than a corona discharge, and is considerably more conductive. In fact, it has
properties that are similar to an electric arc. The leader tapers and branches
into thousands of thinner, cooler, hairlike discharges (called streamers). The
streamers look like a bluish "haze" at the ends of the more luminous leaders,
and it's the streamers that actually transfer charge between the leaders and
toroid to nearby space charge regions. The displacement currents from countless
streamers all feed into the leader, helping to keep it hot and electrically
conductive.
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In a spark gap Tesla Coil the primary-to-secondary energy transfer
process happens repetitively at typical pulsing rates of 50–500 times/second,
and previously formed leader channels don't get a chance to fully cool down
between pulses. So, on successive pulses, newer discharges can build upon the
hot pathways left by their predecessors. This causes incremental growth of the
leader from one pulse to the next, lengthening the entire discharge on each
successive pulse. Repetitive pulsing causes the discharges to grow until the
average energy that's available from the Tesla Coil during each pulse balances
the average energy being lost in the discharges (mostly as heat). At this point,
dynamic equilibrium is reached, and the discharges have reached their maximum
length for the Tesla Coil's output power level. The unique combination of a
rising high voltage Radio Frequency envelope and repetitive pulsing seem to be
ideally suited to creating long, branching discharges that are considerably
longer than would otherwise be expected by output voltage considerations alone.
However, even 100 years later, there are many aspects of Tesla Coil discharges
and the energy transfer process that are still not completely
understood.
Reception
The secondary coil and its
capacitor can be used in receive mode to utilize atmospheric electricity.
Generally, though, Tesla coils are not used for these purposes. Theoretically, a
variation of the Tesla coil could utilize the phantom loop effect to form a
circuit to induct energy from the earth's magnetic field and other radiant
energy. This concept is part of Tesla's wireless transmission of electric power
distribution system (US1119732 — Apparatus for Transmitting Electrical Energy —
1902 January 18).
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The skin effect myth
The
dangers of high frequency electrical current are sometimes perceived as being
less than at lower frequencies. This is often, but mistakenly, interpreted as
being due to skin effect, a phenomenon that tends to inhibit alternating current
from flowing inside conducting media. Although skin effect is applicable to good
electrical conductors (i.e., metals), the skin depth of human flesh at typical
Tesla Coil frequencies is still of the order of 60 inches or more. This means
that high frequency currents will still preferentially flow through deeper,
better conducting, portions of an experimenter's body such as the circulatory
and nervous systems. In reality, a human being's nervous system does not
directly sense the flow of potentially dangerous electrical currents above 15–20
kHz; essentially, in order for nerves to be activated, a significant number of
ions must cross their membrane before the current (and hence voltage) reverses.
And, since the body no longer provides a warning "shock", novices may touch the
output streamers of small Tesla Coils without feeling painful shocks. However,
there is anecdotal evidence among Tesla Coil experimenters that temporary tissue
damage may still occur as muscle, joint pain, or tingling for hours or even days
afterwards. This is believed to be caused by the damaging effects of internal
current flow, and is especially common with continuous wave (CW) solid state or
vacuum tube type Tesla Coils.
Large Tesla Coils and Magnifiers can
deliver dangerous levels of high frequency current, and they can also develop
significantly higher voltages (often 250,000–500,000 volts, or more). Because of
the higher voltages, large systems can deliver higher energy, potentially
lethal, repetitive high voltage capacitor discharges from their top terminals.
Doubling the output voltage quadruples the electrostatic energy stored in a
given top terminal capacitance. If an unwary experimenter accidentally places
himself in path of the high voltage capacitor discharge to ground, the high
current electric shock can cause involuntary spasms of major muscle groups, and
may induce life-threatening ventricular fibrillation and cardiac arrest. Even
lower power vacuum tube or solid state Tesla Coils can deliver RF currents that
are capable of causing temporary internal tissue, nerve, or joint damage through
Joule heating. In addition, an RF arc can carbonize flesh, causing a painful and
dangerous bone-deep RF burn that may take months to heal. Because of these
risks, knowledgeable experimenters avoid contact with streamers from all but the
smallest systems. Professionals usually use other means of protection such as a
Faraday Cage or a chainmail suit to prevent dangerous currents from entering
their body.
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Instances and devices
Tesla's
Colorado Springs laboratory possessed one of the largest Tesla coils ever built,
known as the "Magnifying Transmitter". The Magnifying Transmitter is somewhat
different from classic 2-coil Tesla coils. A Magnifier uses a 2-coil "driver" to
excite the base of a third coil ("Resonator") that is located some distance from
the driver. The operating principles of both systems are similar. The world's
largest currently existing 2-coil Tesla coil was made by Greg Leyh. It is a
130,000 watt unit, part of a 38 foot tall sculpture. It is owned by Alan Gibb
and currently resides in a private sculpture park at Kakanui Point near
Auckland, New Zealand. [1]
The Tesla coil is an early predecessor (along
with the induction coil) of a more modern device called a flyback transformer,
which provides the voltage needed to power the cathode ray tube used in some
televisions and computer monitors. (CRT displays are now slowly being replaced
by liquid crystal displays and other technologies.) The disruptive discharge
coil remains in common use as the "ignition coil" or "spark coil" in the
ignition system of an internal combustion engine. A modern variant of the Tesla
coil is also used to power plasma globe sculptures and similar
devices.
Popularity
Tesla coils are very popular devices
among certain electrical engineers and electronics enthusiasts. Someone who
builds Tesla coils as a hobby is called a "Tesla coiler", or simply "coiler".
There are even "coiling" conventions where people attend with their home made
Tesla coils and other electrical devices of interest.
'It should be noted
that there are rather significant safety issues regarding coil assembly and
operation by hobbyists (including professional engineers)
Mag. Erwin
Kohaut, a physics teacher at the Austrian high school BGRG 12 Rosasgasse in
Vienna, Austria, and some students built one of the biggest Tesla coils of
Europe. It stands in the cellar of that school.
Low power Tesla Coils are
also sometimes used as a high voltage source for Kirlian photography. [2] Tesla
coils are featured in the Command & Conquer series of strategy computer
games as a weapon on the Soviet side and in the Destroy All Humans! video game
as an anti-air emplacement.
The Jim Jarmusch film 'Coffee and Cigarettes'
(2003) featured a segment starring the couple Jack and Meg White from the band
The White Stripes entitled 'Jack shows Meg his Tesla coil'. In the segment, the
pair are having a coffee. Jack explains the work of Nikola Tesla to Meg and
demonstrates the coil he has by his side.
.