Miniature Tesla Coil Produces a Big Spark

Robert Svangren
Northwest Tesla Coil Builders
112 E 16th St.
Kennewick, WA 99337

This little Tesla coil is named Penatubo after the fiery volcano in the Philippine Islands.

Hundreds of hours have been spent with this Penatubo project. After researching numerous books and technical papers on how to build Tesla coils, I came to the conclusion that TCBA is the only real reliable source for good sound information.

I fully expected to get 35 foot arcs from this little coil but finally came to accept that a 13.5" arc from a coil of this size was a mark of success.

One sphere, one toroid, and two oblates were built and tested. A 3X8 oblate was chosen as the best terminal capacitance.

Twenty-one secondary coils were wound and tested as well as five different dielectrics as secondary forms. In addition, many different sizes of wire and turns were wound before determining the best combination. The final coil chosen was 594 turns of #28 AWG formvar on a polypropelene dielectric tube measuring 1&7/8" in diameter and eight-and-a-quarter inches long. The specs follow: 19.4 ohms, 3.40 millihenry inductance, and a frequency of 844,816 Hz with an 8" oblate.

Six different primary coils were tested and the final primary was a copper-berillium half-inch banded coil with five and one-half turns.

Four types of capacitor banks were tested with the final capacitor being custom built in my lab. It took about 50 hours to design and construct the final model. This is a plate capacitor with several hundred metal and dielectric plates. It has a very high Q, and a low inductance. The capacitor rating is .0054 uf at over 50 KVAC breakdown voltage. However, I rate it at a safe 30 KVAC. The capacitor is installed in a 1/4" Lexan case.

The main spark switching system consists of four tig welder electrodes with tungsten tips. Two sets are used to make up a series of two gaps.

A 12 KV, 30 mA transformer was used to power the unit. Filters, safety gaps, metered variacs, were incorporated into the system.

The final result is a smooth operating coil with an arc length averaging 10.5" with about every fifth arc reaching out to 13.5". A segmented arc-line meter was used to test arc length and gave a constant 13.5 inch discharge (see TCBA News, Volume 13, #2, p.16).

The Durlin Cox tank circuit suggested in a previous TCBA issue really works well. After hundreds of radical tests, not one transformer or capacitor was destroyed.