AOH :: THEORY2.TXT|
Tesla Coil Theory
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 circuts: When the spark gap arcs, the momentary short circut
created dumps the capacitor's charge into the coil's primary. The
resulting l-c circut 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 refferred 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 circut resistance (capacitor, coil and intermediate
wiring) limits the overall efficiency of the tank circut. 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 fine tuning.
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 circut. 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 arching between the primary and secondary and generates the
desired high voltage at the top end.
The "Oudin 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 oudin 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).
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