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 electric
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 (now called capacitors), and a
second, specially constructed, disruptive coil. (Norrie, pg. 228)
These capacitors 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 helped
to extinguish the arc, making the discharge more abrupt. An air
blast was also used for this objective. (Norrie, pg. 230-231)
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
capacitors are lead to a spark gap. (Norrie, pg. 35-36)
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
When Tesla patented a later device (U.S. Patent 1,119,732 — 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.
Tesla Coil operation is significantly
from a conventional transformer whose gain is limited to the ratio
of the numbers of turns in the windings.
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.
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. However, unlike a conventional transformer which may
couple 97%+ of the magnetic fields between windings, a Tesla Coil’s
windings are "loosely" coupled, typically sharing only 10-20% of
their respective magnetic fields. Most oil insulated transformers
need large and long insulations at their connections to prevent
discharge in air. Many later version Tesla Coils spread their
electric field over a large distance to prevent high electrical
stresses in the first place, thereby allowing operation in free air.
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
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.
Tesla experimented with these, and many other, circuit configurations. In either circuit, the AC supply transformer charges the tank capacitor until its voltage is sufficient to break down the spark gap. The gap suddenly fires, allowing the fully charged tank capacitor to discharge into the primary winding.
And, in either circuit, the Tesla Coil
primary winding, spark gap, and tank capacitor are all connected in
series. Once the gap fires, the electrical behavior of either
circuit is identical. Experiments have shown that neither circuit
offers any marked performance advantage versus the other.
Experienced Tesla coil builders almost exclusively use the top circuit, often augmenting it with low pass filters (resistor and capacitor (RC) networks) between the supply transformer and spark gap.
This is especially important when using transformers with fragile high voltage windings, such as Neon-sign transformers (NST’s). Regardless of which configuration is used, the HV transformer must be of a type that self-limits its secondary current by means of internal leakage inductance. A normal (low leakage inductance) high voltage transformer must use an external limiter (sometimes called a ballast) to limit current.
designed to have high leakage inductance to limit their short
circuit current to a safe level.
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. Capacitive "loading" by the streamers tends to lower
the resonant frequency of a Tesla Coil operating under full power.
For a variety of technical reasons, toroids provide the best overall
shape for top terminals of modern Tesla coils.
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
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
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.
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.
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.
This concept is part of Tesla’s wireless transmission of
electric power distribution system (US1119732 — Apparatus for
Transmitting Electrical Energy — 1902 January 18).
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
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.
Instances and devices
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.
A modern variant of the Tesla coil is also used
to power plasma globe sculptures and similar devices.