23 October 2000
GENERAL: The Secondary circuit includes a large air core inductor and a Terminal which behaves as a virtual capacitor. Together this arrangement forms a resonant circuit which must be at the same frequency as the Primary circuit. RF electrical energy Is inductively coupled from the Primary to the Secondary circuit where it oscillates in sympathy in the Primary circuit. As a consequence extremely high RF voltages develop on the Terminal. (in excess of 200kV) Spherical and toroidal shaped terminals will develop large electrical sparks as a consequence, however other shaped Terminals will do this also but less dramatically.
INDUCTOR: Inductors range in values from approximately 10mH to about 100mH with Q's in excess of 100. The size under consideration here will be 50mH and it has a Q of 150. The Primary Circuit in this discussion operates at 117kHz. At this frequency a 50mH inductor requires 37pF of capacitance to be resonant. Since self capacitance of such a large inductor runs about 20pF, a Terminal capacitance of 17pF will be required. Terminals of reasonable size can privide such a value. Terminal and coil construction details details are covered in other pages.
As with most Tesla Coils, the coefficient of coupling (k) between the primary and secondary winding of the RF transformer, T2, is usually between 0.10 and 0.20. One would initially think that only 10 to 20% of the RF energy would find its way to the secondary circuit. Actually, all of the primary circuit energy, less circuit resistance losses, reaches the secondary, even with k's of 0.10.
The energy does not transfer from the primary to the secondary circuit instantly, but during several cycles of RF oscillation. This process can be regarded as ring down in the primary and ring up in the secondary circuits. When all of the energy reaches the secondary circuit, there will be none left in the primary circuit. (remember conservation of energy) In the secondary, the energy will continue to ring and start making its way back to the primary circuit. Thus it rings down in the secondary and rings up in the primary circuit. In other words, the energy bounces back and forth between the primary and secondary circuits until it is converted to heat by circuit losses or sparks, corona, etc.
The oscilloscope patterns were obtained by powering a Tesla Coil with 12 Volts dc and replacing its spark gap with contacts of a vibrating reed relay. The probe for the Primary trace was connected directly to the primary circuit while the other probe was connected to a metre long piece of wire held about a metre away from the secondary coil.
The primary and secondary waveforms look very much like an amplitude modulated RF signal with the carrier suppressed. In this case, ths modulating signal works out to about 25kHz. This happened to be the frequency spread between the high and low frequency peaks when doing measurements to obtain k. In this case, k calculated to 0.20. Details are covered in the MEASUREMENTS page.
The following expressions describe the Secondary and Primary Circuit waveforms above.T
FL and FU are frequencies obtained during coefficient of coupling measurements described in the MEASUREMENTS PAGE.
Return to home page