A Tesla coil is an electrical resonant transformer circuit designed by inventor Nikola Tesla in 1891.[1] It is used to produce high-voltage, low-current, high frequency alternating-current electricity.[2] Tesla experimented with a number of different configurations consisting of two, or sometimes three, coupled resonant electric circuits.Tesla used these circuits to conduct innovative experiments in electrical lighting, phosphorescence, X-ray generation, high frequency alternating current phenomena, electrotheropy, and the transmission of electrical energy without wires. Tesla coil circuits were used commercially in spark gap radio transmitters for wireless telegraphy until the 1920s,[1][3] and in medical equipment such as electrotheropy and violet ray devices. Today, their main usage is for entertainment and educational displays, although small coils are still used as leak detectors for high vacuum systems.[4][5]
Originally, Tesla coils used fixed spark gaps or rotary spark gaps to provide intermittent excitation of the resonant circuit; more recently,electronic devices are used to provide the switching action required.
Excitation
Tesla coil circuits can be classified by the type of "excitation" they use, what type of circuit is used to apply current to the primary winding of the resonant transformer:[5][21][22]
Spark-excited or Spark Gap Tesla Coil (SGTC): This type uses a spark gap to close the primary circuit, exciting oscillations in the resonant transformer. Spark gaps have disadvantages due to the high primary currents they must handle. They produce a very loud noise while operating, noxious ozone gas, and high temperatures which may require a cooling system. The energy dissipated in the spark also reduces the Q factor and the output voltage. Tesla's coils were all spark-excited.
Static spark gap: This is the most common type, which was described in detail in the previous section. It is used in most entertainment coils. An AC voltage from a high voltage supply transformer charges a capacitor, which discharges through the spark gap. The spark rate is not adjustable , but is determined by the 50 or 60 Hz line frequency. Multiple sparks may occur on each half-cycle, so the pulses of output voltage may not be equally-spaced.
Static triggered spark gap: Commercial and industrial circuits often apply a DC voltage from a power supply to charge the capacitor, and use high voltage pulses generated by an oscillator applied to a triggering electrode to trigger the spark.[6][22] This allows control of the spark rate and exciting voltage. Commercial spark gaps are often enclosed in an insulating gas atmosphere such as sulfur hexafluoride, reducing the length and thus the energy loss in the spark.
Rotary spark gap: These use a spark gap consisting of electrodes around the periphery of a wheel rotated at high speed by a motor, which create sparks when they pass by a stationary electrode.[22] Tesla used this type on his big coils, and they are used today on large entertainment coils. The rapid separation speed of the electrodes quenches the spark quickly, allowing "first notch" quenching, making possible higher voltages. The wheel is usually driven by a synchronous motor, so the sparks are synchronized with the AC line frequency, the spark occurring at the same point on the AC waveform on each cycle, so the primary pulses are repeatable.
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