The Tesla coil is one of Nikola Tesla's most famous
inventions. It is essentially a high-frequency air-core transformer. It takes
the output from a 120v AC to several kilovolt transformer & driver circuit
and steps it up to an extremely high voltage. Voltages can get to be well above
1,000,000 volts and are discharged in the form of electrical arcs. Tesla
himself got arcs up to 100,000,000 volts. Tesla coils are unique in the fact
that they create extremely powerful electrical fields. Large coils have been
known to wirelessly light up florescent lights up to 50 feet away, and because
of the fact that it is an electric field that goes directly into the light and
doesn't use the electrodes, even burned-out florescent lights will glow.
A Tesla coil transformer operates in a significantly
different fashion from a conventional (i.e., iron core) transformer. In a
conventional transformer, the windings are very tightly coupled and voltage
gain is determined by the ratio of the numbers of turns in the windings. This
works well at normal voltages but, at high voltages, the insulation between the
two sets of windings is easily broken down and this prevents iron cored
transformers from running at extremely high voltages without damage.
Unlike those of a conventional transformer (which may couple
97%+ of the fields between windings), a Tesla coil's windings are
"loosely" coupled, with a large air gap, and thus the primary and
secondary typically share only 10–20%
of their respective magnetic fields. Instead of a tight coupling, the coil
transfers energy (via loose coupling) from one oscillating resonant circuit
(the primary) to the other (the secondary) over a number of RF cycles.
In each 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 charged tank capacitor to discharge into the
primary winding.
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 the primary energy transfers to the
secondary, the secondary's output voltage increases until all of the available
primary energy has been transferred to the secondary (less losses). Even with
significant spark gap losses, a well designed Tesla coil can transfer over 85%
of the energy initially stored in the primary capacitor to the secondary
circuit.
As the secondary coil'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 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.
