Practical Tesla Coil System by Rodney King

Practical Tesla Coil System   (2009 Constructor’s Award Winner)

The author first made a working model in his teens nearly fifty years ago. Since taking early retirement the challenge has been to build an improved version aided by a better understanding of the operating principles.

A Tesla Coil is effectively a voltage step-up high frequency transformer usually driven by a spark transmitter. Whilst the true Tesla Coil has a floating secondary winding with two output connections it is more usual to ground one end making an “Oudin” configuration. For optimum efficiency both primary and secondary windings should resonate at the same frequency with a close inductive coupling between them. In its simplest form, the secondary winding has a very large number of turns wound on a long former. In the absence of a discrete tuning capacitor this is effectively a “normal helical resonator” behaving like a miniature quarter-wave transmission line. Mainly due to the high inductance per unit length a self-resonant frequency of 1MHz is achieved with a coil only 25cm (10 inches) long! It is interesting that the total length of wire required for such a component is usually about half a wavelength.

Power from a 12 volt dryfit lead-acid battery is interrupted at a frequency of about 100 Hz by a vibrator from an old WW2 PSU and applied to the primary of a conventional car spark coil. An automotive 0.25 uF capacitor across this winding generates a 3kHz “ring” each time the vibrator contacts open.

practical-tesla-coil-system-rodney-kingThe “spark transmitter” circuitry consists of a spark-gap across the spark coil’s secondary winding shunted by a series resonant circuit consisting of another (smaller value) high voltage capacitor pair and the Tesla coil’s primary winding. On each half cycle of the 3 kHz drive the capacitor charges until the voltage becomes high enough for the spark-gap to strike. This immediately triggers a 1 MHz “ring” in the Tesla’s primary circuit.

As previously described, this high frequency signal is inductively coupled into the Tesla’s secondary coil capable of producing a 25mm (1”) high frequency spark. Unfortunately the voltage has to be restricted by closing the spark-gap to about 0.8 mm to prevent inter-turn sparking on the Tesla’s output coil.

High voltages at high frequency have a number of interesting characteristics:

An impressive streaky purple spark can be drawn to a hand-held metal object without the risk of electric shock as muscles are unable to contract a million times per second! Sparks directly onto the skin can, however, cause burns.

Flaring discharges emanate from any sharp surface connected to the output.
There is a noticeable smell of ozone gas associated with any spark.
A plasma effect is produced in a clear incandescent light bulb connected to the output.
A fluorescent tube or low energy bulb glows when brought close to the high voltage coil.

Past applications have included “quackery” medical treatments. Glass gas-filled attachments (similar to the incandescent electric light bulb above) produce safer sparks to stimulate parts of a person’s body. This so-called “violet wand” equipment was once claimed to improve hair growth when used on a patient’s scalp. From the Internet it appears that the equipment is currently popular as a form of torture in the BDSM sex scene!

More compact commercial units use encapsulated multi-layer secondary coils similar to early TV line output transformers. Furthermore, the “spark transmitter” section is replaced by a heavy-duty buzzer whose highly inductive solenoid is driven directly from mains voltage and the contacts serve as a built-in moving spark-gap.

The “competition” model is relatively small compared with those built by enthusiasts in the USA using high voltage mains transformers intended for driving large neon displays. Such units commonly produce lethal sparks well over 30 cm (1 foot) long.

Rodney King – Vital Spark July 2009.

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