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 4 - Issue 1

Page 4 of 18

Input - Output

Q. Is it better to have the spark gap in series with the transformer or across it?

A. We all have asked that question at one time or another. My own experience has been that it makes no difference in regard to output potential or spark length. However, I choose to place the spark gap in series as shown at the right. A spark gap placed across the transformer secondary short circuits the windings and, therefore, draws a heavy current. This could burn out the secondary windings. The problem is greatly reduced when the spark gap is in series because the high frequency currents will pass through the capacitor rather than the transformer on account of the capacitor's lower reactance. If you insist upon placing the spark gap across the high voltage secondary, rf chokes MUST be used to prevent the high frequency surges from reaching the transformer. Even when using current limiting transformers such as a neon sign or furnace ignition units, I place the spark gap in series and use rf chokes for added protection.

Q. On more than one occasion you have talked about current limiting transformers. Please explain how this is done?

A. Power transformers have their primary and secondary windings coupled very tightly. Sometimes the two windings are wound on the same leg side by side. Another method used is to wind the primary and secondary right over one another (shell type). These are efficient arrangements as the windings have a high mutual inductance. The current limiting (neon sign) transformer primary and secondary windings are loosely coupled. Generally, they are placed at each end of the core and wound on the short leg. A short core is placed across the main core to act as a magnetic shunt. This is akin to a magnetic short circuit. A small air gap somewhere along the core helps to prevent magnetic saturation. The loose coupling and magnetic shunt help to control the total current that can be drawn by the windings.

Q. Some time ago you suggested that the final turns of a coil should be double or triple spaced. Is there any need for this practice if strict attention is taken in regards to proper coupling?

A. I think so. Look at a photo of Tesla's Magnifying Transmitter and you will see a greater spacing at the upper turns where the potential is highest. Even if there was no advantage, spacing the final 5-10 turns will do no harm.

Q. You made a reference to the fact that the professional coil builders use meters to tune their coils. I'd like to set up a meter panel for this purpose but am having a difficult time finding the right units. My volt meter is appropriate but my ammeter is too small. Is there a way to convert it to higher values?

A. Yes. The range of your ammeter can be increased by connecting a suitable resistance across the meter terminals. To find the proper resistance value, multiply the internal resistance of the ammeter by the actual maximum reading of the scale. Now divide this answer by the difference between the desired maximum value and the actual maximum value of the original scale. A factor must now be used to convert the readings presently on the scale to the new range. This can be obtained by dividing the new maximum scale by the actual maximum scale reading. Incidentally, a kilowatt meter is also handy. However, they are difficult to find and quite expensive. Your best bet is to search the surplus market.

Q. I've been using mica capacitors and a spark gap made from two screws. They took considerable abuse in my off resonance tuning experiments. And opening the spark gap so no spark occurred did not seem to leather them. Recently, I hooked up a rotary gap and destroyed both caps. I even had a spark gap to protect the caps?

A. I'm not at all sure the rotary gap is the culprit. First of all, you concede that the capacitors took considerable abuse. Chances are the strain from consistent abuse rendered them ready for a break down. Secondly, you did not mention the voltage ratings or the circuit voltage being applied. If borderline, the breakdown was just a matter of time. The voltage rating of the capacitors should be at least twice that of the circuit voltage. The fact that the gap did not protect them may mean that the gap was set incorrectly or that the failure of the capacitors was NOT due to any circuit surges. Many forces come into play with circuits carrying high voltages at high frequencies and it is not always easy to explain why such things occur.