If I have an input of ac current 50/60 hertz into a capacitor of 40nanofarads and after that a rotary spark gap(2000-5000 discharge per second) , what would be the waveform of output? And how will the waveform change in case of static spark gap? I am trying to understand tesla coil.

  • \$\begingroup\$ Very noisy. You might want to simulate your actual setup, there are probably some variables \$\endgroup\$
    – PlasmaHH
    Jul 4 '16 at 12:17
  • \$\begingroup\$ Ac current ----> Capacitor------->Rotary spark Gap/Static Spark Gap----> What would be the waveform of output? A general idea of waveform would be fine \$\endgroup\$
    – NOob94
    Jul 4 '16 at 12:42
  • \$\begingroup\$ Your input current frequency is exact: 50 or 60Hz, the spark gap has to synced with this frequency, so 50 or 60Hz. The output is a rectified 50/60 Hz, the spark gap shall conduct only at peak value of sine wave. \$\endgroup\$ Jul 4 '16 at 12:56
  • \$\begingroup\$ Would it become a square wave dc ? \$\endgroup\$
    – NOob94
    Jul 5 '16 at 2:50
  • \$\begingroup\$ and would the frequency remain 50/60 hz inspite of 5000 rotations per second? \$\endgroup\$
    – NOob94
    Jul 5 '16 at 8:41

The waveform from the spark gap has little effect on the operation of the Tesla coil. Think of it as just a high voltage switch turning on and off 100-120 times per second. Theoretically it would be a square wave with a sine wave laid over it, but in reality it is very noisy. I just think of it as a "burst" of current flowing into the primary tank circuit consisting of the resonant capacitor and primary coil. This "burst" excites the LC circuit causing it to oscillate at a frequency determined by the formula,

enter image description here

where F is the frequency in Hertz, L is the inductance of the primary coil in Henrys, and C is the capacitance of the primary capacitor in Farads. This oscillation on the primary coil induces a current in the secondary coil, which excites the secondary tank circuit (made up of the secondary coil - 'L' - and the capacitive topload - 'C'). If both the primary tank circuit and the secondary tank circuit resonate at the same frequency, the voltage induced on the secondary coil adds up every cycle (constructive interference of the waveforms) and that's how you get such a high voltage on the output.

The spark gap is only used to energize the tank circuit and excite it repeatedly. The waveform is not terribly critical, but just picture it as a square wave (on - off - on - off). In reality there is an oscillation laid over this "square" wave and it is very noisy.

  • \$\begingroup\$ So as per the formula, number of rotations of rotary spark gap wouldnt affect the waveform right? \$\endgroup\$
    – NOob94
    Jul 8 '16 at 14:13
  • \$\begingroup\$ In theory it shouldn't affect the primary coil waveform much, but in practice it can cause destructive interference, can discharge the capacitor too soon leading to less efficient energy transfer, excessive heating of the spark gap contacts, etc leading to poor operation. \$\endgroup\$
    – DerStrom8
    Jul 8 '16 at 17:54
  • \$\begingroup\$ Right. I don't even use a rotary mechanism. A static spark gap works fine. The interuptions are basically broad band noise. The LC tank acts as a bandpass filter amplifying at the resonant frequency across that band. \$\endgroup\$
    – docscience
    Jul 30 '16 at 0:56

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