Optimization of Tesla Resonator Coil Performance Through the Use of Massive Terminal Loadings

by: Richard Hull, Tesla Coil Builders of Richmond

One of the big advantages given to the Tesla coil system, which we have noted in our work over the years, is the seemingly endless amount of terminal capacitance which may be added to the Tesla resonator. This has always resulted in improved performance from a number of standpoints. The spark length has always increased for a given resonator with increased terminal size. (Of course, a requisite increase in power is required to break out of the large terminal capacitance.) The current in the arc is increased dramatically as well.

In days of yore, the classic little Tesla oscillator was adorned with, at most, a drawer pull or brass bed post ball of 1 or 2 inches in diameter. The old lower powered systems just used a pointed bolt or stud projecting from the resonator! Ugh! The performance of all these systems was less than stellar. Even the giant Kaufman classic coil system in Pittsburgh was adorned with a shamefully small terminal ball. (about 6-10 inches) Considering the massive dimensions of its resonator, this ball was almost non-existent!

Modern pioneering efforts by Dick Aurandt and Bill Wysock have favored the toroidal form of terminal capacitance and due to their efforts, the performance of Tesla systems improved in the 1970s. Bill's efforts, in particular, set a new standard of performance and also set a new look for the proper outfitted Tesla coil system. Grandpa's needle point or drawer pull became obsolete. Performance was up. This performance increase was not totally due to the toroidal terminal capacitance. It also was due to the better quenching of the spark in more efficient gap systems which allowed tighter coupling. This increase in coupling also increased performance. Ultimately, though, the terminal capacitance had demanded all of these modifications for its synergistic addition to the top of the resonator.

By the late 1980s, we all seemed to have an idea of the way a modern Tesla coil should look and perform. This ultimately stagnated the design at the new level originally established some 15 years prior by Wysock. What was needed was fresh basic research into Tesla resonant systems similar to the Wysockian revolution of the 70s and 80s.

Our group started our investigations in 1988. We started by refusing to accept any conventions which we could not prove or see work through empirical experiment! We set as our goal, the attainment of maximum spark length with a minimum of input energy and resonator size. Basically, we seemed on a Quixotic quest to re-invent the wheel. We would wind up tilting at a few sacred windmills.

We found numerous ways to improve the Tesla coil system even further. Many techniques were really known already, in theory, but had never been applied to any extent by the coil building community. We broke certain molds on small systems by using 13-20 turn primaries, using vanishingly small capacitances in the resonant tank of the oscillators to produce longer sparks than one might have thought possible. We designed and perfected numerous special quench systems for commutating the arc. We also proved that excellent performance could be had up to 3 KW without the previously “demanded” rotary spark gap. Perhaps our biggest discovery was that terminal capacitance could be increased to seemingly absurd limits. This has forever changed the look of “plus ultra” performance Tesla coil systems. Our video report tapes have spread “the word” to all parts of the land. If it's performance you want, you better secure the largest terminal capacitance you can effectively power up!

The Theory

The resonator in any Tesla system is actually a quarter wave helix. The fact that the wire is wound into a helix produces both self-inductance and internal capacitance which is, ultimately, part of a series resonant circuit which determines the helix's resonant frequency. This combination of inductance and capacitance reduces the wire length, for a given frequency, below that required by the formal equation for frequency and wavelength (λ=c/f). The inductance mounts much faster in small, single layer coils than the capacitance. The inductance in a medium Tesla resonator is some tens of millihenries while the capacitance is only a few picofarads. Classically, the peak voltage produced by the resonator is a function of the rapidity and magnitude of the base current applied to the resonator and the inductance contained within it. The actual spark length in air is, of course, related to this voltage. Using only a pointed rod on the coil's output terminal, this voltage may never be reached as the air will breakdown early, before the internal capacitance of the resonator can charge to its peak value. This, ultimately, limits the spark length in air. In this respect, we must regard the actual output terminal of our systems in a similar, but not equivalent, manner to a static machine, such as the Van de graff generator. The difference is that our coils pump much more energy per unit time to the terminal capacitance than is the case with the static machine! Thus, we need not worry about dust particles or tiny irregularities on our terminals to the degree required with the low power, low duty cycle, static devices. The crux of this discussion is that we must utilize a large, regular formed, terminal capacitance just as in the static machine.