Q. I agree with the writer who warned about the danger of those old time Tesla coil projects. I've often wondered why you chose to include them in the newsletter.

A. Take a look at the cover of any current issue of the popular technical magazines and you'll probably see bold headlines urging readers to build a certain project being featured in that particular issue. The old time TC projects seen in TCBA News are carried from an historical perspective rather than from the “you must build it” point of view. True, some were both dangerous and inefficient. As the Corums have inferred, many of the early TC projects were nothing more than ignition coils.

Of course, it is possible to redesign some of the old coils using updated information and modern materials. But this is not the basis for including them. As a long time Tesla coil enthusiast, it has been my desire to reprint every Tesla coil project article ever published (an impossible task).

Lastly, I have a great respect for those old time tinkers who worked into the wee hours of the morning in a damp basement winding their resonators with unabated enthusiasm. Although the modern TC builder is better equipped and has the advantage of modern technology, I would hope that future TC builders will look back on us with the same deference.

Q. It has been my experience that the equation given for finding capacitance in power factor correction for neon sign transformers (Volume 12, #3) can be off by 20-30 mfds.

A. You are not the first to bring this discrepancy to my attention. The condition may be due to the fact that neon sign transformers are designed for discharge lamps and not LCR circuits. Evidently, the neon sign transformer used for Tesla coils sees inductive and capacitive loads not seen in discharge lamps. Even so, the equation not only is more understandable and easier to use than others I've seen but it does place you in the ball park.

The best way to resolve power factor correction would be to use a power factor meter. Because power factor meters are difficult to find, the next best method would be to employ an ammeter, voltmeter, and wattmeter. The power factor could then be determined by reading the wattmeter and dividing its value by the product of amperes and volts (PF = W/E x I).

Where a wattmeter is unavailable, run an ammeter in series with the transformer input and keep and eye on the pointer as capacitance is added to the transformer terminals. At some point, the critical capacitance will be reached and the ammeter pointer will no longer drop. That is the point at which power factor correction has been attained.

Q. In “Tips for Tuning Tesla Oscillators,” D.C. Cox states that the spark gap should be across the high voltage terminals and the capacitor in series. Isn't that contrary to what you advocated in earlier issues?

A. The information I provided came from RADIO ENGINEERING PRINCIPLES by Lauer & Brown. The authors preferred placing the capacitor across the high voltage transformer because “any high frequency currents taking place in the oscillatory circuit will pass through the condenser instead of through the transformer winding, on account of the lower reactance of the former.”

Their reasoning made sense to me and as far as I know Lauer & Brown are the only engineers to have specifically favored this arrangement. They also stated that radio frequency chokes should be employed if the alternate method was used. In my opinion, radio frequency chokes should be used regardless of which circuit is employed.

As you know, Tesla used a variety of arrangements. Figure 134 (page 210) of the Martin book shows a Tesla circuit that is identical to the one suggested by Lauer & Brown. Anyway, I don't see it as a big deal. Two chokes and a safety gap should be the basic ingredients of any Tesla coil protective circuit. Of course, additional safety components as suggested in past articles on this topic may be used.

Some time ago, two of our readers got into a heavy debate about capacitor/spark gap placement theory. They were going to contribute an article on their experimental work to substantiate each theory. As of this writing, no such article has been received.