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.

Return to the index of Vital Spark articles.

G3MGQ’s Month on the Air

Prepared by the clubs RSGB trainer, G3MGQ, you will find the latest DX contests including the ones to shoot for as well as ones to give a wide berth. Why not download the latest edition of Month on the Air and enjoy your DX just that little bit more.

Become a member of HERC

Join the Hastings Electronics and Radio Club.

Why not join one of the largest and most established Radio clubs in the South East of England? Very low joining cost, and free for a year to new licencees.

Vital Spark Archive

Vital Spark newsletter articles

Take a look through a large selection of articles written by club members over the years which have been published in the monthly Vital Spark newsletter .

Used Ham Radio Equipment

View HERC's Used Ham Radio Equipment for sale list..

Every four weeks, HERC's Used Ham Radio Equipment for sale list is updated on the site. Bookmark the gear for sale page to re-visit easily and take advantage of the used equipment on sale through the club.

Club Photographs

HERC Image Galleries.

Here is the official HERC photograph archive which contains multiple image galleries spanning several decades since the club was formed many years ago. Enjoy the images!

UK Amateur Radio Repeaters

UK repeaters

Click button above for full list, or a local repeater callsign below for info.

GB3EB 2m in Uckfield- Active
MB6EB 2m DStar Node in Eastbourne - Active
MB6RY Wires-X DigiGate in Broad Oak - Active
GB3HE 70cm in Hastings - Active
GB7HE 70cm DStar in Hastings - Coming soon
GB3ZX 70cm in Eastbourne - Low Power
GB3JT 23cm ATV in Hastings - NoV cleared
GB7RY 70cm X-Wires Repeater Rye - Active
GB7ES Eastbourne - DSTAR Rpt. - Active
GB3ES 2m in Hastings - Active

For a complete list of repeaters, head over to
the UK Amateur Radio repeaters list.

Popular pages

Get your amateur radio licence - Find out more about amateur radio licence training.
Month on the Air - G3MGQ's popular monthly DX contest/expedition list.
Wilf Gaye Memorial Cup - The clubs annual operating event in the memory of Wilf Gaye M0GYE.
St. Richard's College Buildathon/STEM/ARISS - HERC attends St. Richard's Catholic College for their various events surrounding the Tim Peake ARISS contact.
G3BDQ - John Hey's Rare QSL Cards.
Sussex Electronics Radio Fair - SERF Sussex Electronics Radio Fair 2016.
Vital Spark - A selection of articles re-published from the Vital Spark.
RSGB News - Find out how to get RSGB news on your mobile or PC.
Experimenters Corner - A selection of Proteus projects by Bob Gornal (G7DME)
BBADL - Bath Based Distance Learning Course.
Conquest Hospital Radio - Presented by HERC member Antony (G4CUS).
Radio Rallies 2016 - An up to date list of radio rallies scheduled for 2016.
Club QSL Cards - A selection of QSL cards the club has received over the years.
Other Newsletters - Excellent newsletters and magazines from other clubs.
TX Factor episodes - Take a look at the TX Factors YouTube videos.
John Taplin - A bio of the late John Taplin.

Amateur Radio Resources

Other Radio Clubs & RAYNET

BSARS - Brede Steam Amateur Radio Society

RAYNET - The Hastings and Rother RAYNET Group.

HERC members sites

Sigord - Gordon Sweet
Hastings Radio Comms - Andrew Haas-Campbell
Hoofbags - Liz Costa