repeatable. Switched or Solid State Tesla Coil (SSTC): These use power semiconductor devices, usually thyristors or transistors such as MOSFETs or IGBTs, triggered by a solid state oscillator circuit to switch pulses of voltage from a DC power supply through the primary winding. They provide pulsed excitation without the disadvantages of a spark gap: the loud noise, high temperatures, and poor efficiency. The voltage, frequency, and excitation waveform can be finely controllable. SSTCs are used in most commercial, industrial, and research applications as well as higher quality entertainment coils. Single resonant solid state Tesla coil (SRSSTC): In this circuit the primary does not have a resonant capacitor and so is not a double-tuned circuit; only the secondary is. The current to the primary from the switching transistors excites resonance in the secondary tuned circuit. Single tuned SSTCs are simpler, but the resonant circuit has the total Q factor depends only on the secondary side resonant. Dual Resonant Solid State Tesla Coil (DRSSTC): The circuit is similar to the double tuned spark excited circuit, except in place of the AC supply transformer (T) in the primary circuit a DC power supply charges the capacitor, and in place of the spark gap semiconductor switches complete the circuit between the capacitor and the primary coil. Singing Tesla coil or musical Tesla coil: This is not a separate type of excitation, but a modification to the solid state primary circuit to create a Tesla coil which can be played like a musical instrument, with its high-voltage discharges reproducing simple musical tones. The drive voltage pulses applied to the primary are modulated at an audio rate by a solid state "interrupter" circuit, causing the arc discharge from the high-voltage terminal to emit sounds. Only tones and simple chords have been produced so far; the coil cannot function as a loudspeaker, reproducing complex music or voice sounds. The sound output is controlled by a keyboard or MIDI file applied to the circuit through a MIDI interface. Two modulation techniques have been used: AM (amplitude modulation of the exciting voltage) and PFM (pulse-frequency modulation). These are mainly built as novelties for entertainment. Continuous wave: In these the transformer is driven by a feedback oscillator, which applies a pulse of current to the primary winding each cycle of the RF current, exciting a continuous oscillation. The primary tuned circuit serves as the tank circuit of the oscillator, and