"Tesla Coil Theory"
Line voltage is stepped up by the high-voltage transformer to a
magnitude sufficient to charge the capacitor and force the spark gap to
break down. This action is repeated at the 60 hertz line frequency.
Efficient energy transfer is enhanced by choosing a capacitor whose
impedance is equal to that of the secondary winding.
Essentially, the capacitor and tesla coil primary form one pair of tank
circuits: When the spark gap arcs, the momentary short circuit created
dumps the capacitor's charge into the coil's primary. The resulting
L-C circuit rings as the charge oscillates back and forth between the
capacitor and the coil. The oscillations decrease in amplitude as the
charge is dissipated by resistance. The ringing generates what is
referred to as a "damped wave". The next half-cycle of 60-hz recharges
the capacitor, starting the process anew. a train of hi-frequency
waves, spaced at 60-hz intervals, is the product.
Resonance must be achieved to effect ringing: This demands the
capacitor and primary have the same reactance: x(L) = x(C) the circuit
resistance (capacitor, coil and intermediate wiring) limits the overall
efficiency of the tank circuit. efficiency is rated in terms of
"quality factor", represented as 'Q': Q = w(L)/r = x(C)/r
Increasing resistance reduces Q, thereby reducing efficiency. The
effect is minimized by winding the primary with a few turns of heavy
conductor, well-spaced, over a large radius form - the larger the
better. The capacitor size and construction also bear upon Q. The
physical size and area should be minimized. This, however, works
against large capacitance values and dictates the coil be wound with
more turns. The goal, then, is to achieve a workable range of
component specifications rather than to strive for overly close
tolerances. Coils often are wound with tapped primaries to facilitate
The secondary coil is situated both electrically and physically within
the primary. The secondary and its discharge electrode, generally a
sphere or toroid, make up the second tuned circuit. The winding
supplies inductance; The capacitor is created by the electrode and the
(earth) ground-plane. Air is the dielectric. Most tesla coils employ
secondaries of 1/4-wavelength at their operating frequencies. This
establishes a configuration with maximum current at zero voltage with
minimum current at the electrode. this situation proves to be
beneficial, in that it enhances coupling with the high current primary,
avoids arcing between the primary and secondary and generates the
desired high voltage at the top end.
The "Bi-polar Tesla Coil is a 1/2-wave variation on the Tesla Coil. It
possesses the essential tesla coil operational characteristics, but it
employs a horizontally placed secondary with discharge electrodes at
both ends and a primary at the center. The Bi-polar primary sometimes
is merely a pair of taps on the secondary (an autotransformer). The
1/2- wave arrangement affords a coil with maximum discharge at each end
and a virtual ground at the midpoint of the winding(s).
The Oudin coil is similar to the Tesla coil except that the inner most
turn of the primary and the bottom of the secondary are both connected
to ground. Oudin coils usually have a characteristic cone shape with
the large end sitting on the ground. This configuration almost
eliminates the chance of primary to secondary arcing and the loss of
energy by corona at the top windings of the secondary coil. However,
many neon sign transformers simply can not be used in this way, unless
you use only half the output (ie use only one of the two High-voltage
outlets) thus reducing spark length considerably.