TCBA founder, Harry Goldman and the TCBA logo

TCBA - Tesla Coil Builders Association

Devoted to the construction, operation and theoretical analysis of the Tesla coil

TCBA Volume 17 - Issue 1

Page 4 of 18

Input - Output

Q. What happened to “Mad-Man” Marcum (Volume 16, #1, p.2)?

A. Marcum was involved in an experiment in which he planned to walk through a high voltage electromagnetic field. It paralleled the Philadelphia Experiment in which a U.S. Naval vessel is alleged to have passed through a time warp. Some claim the ship became invisible when huge cables wound around its exterior were energized (see “Tessler Did That” in the news notes section). This is what Michael Marcum expected would happen to him. During a preliminary test, a metal object disappeared when thrown through the magnetic field. At present, no one seems to know what happened to Marcum. Talk show host Art Bell offered to make a photographic record of the experiment but he has been unable to make contact either by phone or correspondence. I can think of three possible answers to your question. (1) The experiment worked and Marcum is now in another dimension of time and space. (2) Marcum was fried when he entered the intense magnetic field. And (3), he's back in jail (Mr. Marcum was once incarcerated for taking a half-dozen pole pigs without the owner's consent). Stay tuned.

Q. Anyone designing a Tesla coil needs a reference curve showing the approximate relationships between the three quantities of greatest practical interest: (1) Main capacitor size; (2) volt-amperes available from the high voltage transformer; (3) resulting spark, arc, or streamer length?

A. The graph you propose would be useful. But as you say, the relationships would be “approximate.” That's because the streamer length given off by two coils using identical volt-ampere and capacitor parameters would differ if there was a dissimilarity in their characteristics. That is, the coils may differ in size, efficiency (Q), coupling or mutual inductance, terminal capacitance, and so on. Even so, a graph of this nature would be useful in providing a guide by which to make predictions and/or comparisons.

Q. Is it possible for a Tesla coil secondary to be too long for a given power supply?

A. Tesla coils work best when components are properly matched. That includes a proper relationship to the size of the secondary and the power source driving the circuit. A secondary can become too large both in length and diameter for the driver to enable full secondary potential. On the other hand, it is possible to overpower a secondary system. The results in the latter case would become immediately apparent.

There are numerous letters in my files representing cases where an overenthusiastic experimenter reported disappointment (or failure) because their power source and coil system were not properly matched. For example, one letter states “I have built a coil with a secondary 10' tall. The Tesla coil is powered by a 15 kv, 30 ma. neon sign transformer. The most I can get out of this coil is 15" discharges.” One can conclude that either the circuit is oversized or under powered.

The same situation can happen in regard to terminal capacitances. The larger the capacitance, the more power that is needed to drive the system. Several letters in my files declare disappointment when applying more terminal capacitance than the system can power. Of course, some failed to realize that the coil system required retuning. Even with retuning, there comes a time when adding terminal capacitances requires more power.

The best rule of thumb is to use moderation in assembling a TC unit. That applies in the choice of power input, secondary size, wire and number of turns, terminal capacitance, and so on.

Q. I am interested in increasing the power of my current project. At present, it is powered by a 15 kv, 30 ma. neon sign transformer. Can I add a 15 kv, 60 ma. transformer? Or should I buy another 15 kv, 30 ma. unit? In either case, will I need to increase the capacitor? By how much?

A. Yes, it is possible to add a 15 kv, 60 ma. transformer to your 15 kv. 30 ma. unit. However, it has always been my preference to connect units of identical ratings. That is, to match a 15 kv, 30 ma. transformer to another 15 kv, 30 ma. transformer.

If you add more power to drive your Tesla coil, you should increase the size of your capacitor. A good guide to follow is to use a .005 uf capacitor for every 30 ma. provided by the transformer. Therefore, a 60 ma. transformer would call for a .01 uf capacitor. And a .015 uf capacitor would be required if you employ a 90 ma. source.


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