The Tesla Coil was invented by Nikola Tesla near the end of the 19th century. It is a pulsed air-core resonant transformer capable of creating enormous voltages and huge discharges. It achieves its high voltages through resonant rise as opposed to merely primary/secondary turns ratio as in conventional transformers. Tesla eventually evolved his design into a three coil "magnifying transmitter" which he hoped to use to transmit power and information throughout the world without wires. In addition to the Tesla Coil, Nikola Tesla was responsible for numerous inventions, some of the more notable being radio, the AC induction motor, and the multiple phase power distribution system we use today.
How does a Tesla coil work? Short simplified answer: In the illustration to the left you can see a primary circuit consisting of a capacitor and inductor. First the capacitor is charged to a high voltage...typically over 10,000 volts by means of a high voltage transformer. Eventually the voltage reaches the breakdown voltage of the spark gap, at which point the sparkgap begins conducting. The spark gap acts like a switch and once it starts conducting, it connects the capacitor in parallel with the primary inductor. The capacitor then discharges into the inductor. This creates a magnetic field around the primary inductor (part of which couples into the secondary). For a brief instant most of the energy in the circuit is stored in the magnetic field around the inductor. The field then collapses causing the inductor to discharge back in to the capacitor.
This exchange swings back and forth several times a little like a wave in a tank of water. This happens hundreds of thousands of times per second in a medium sized coil. Since the magnetic field around the primary inductor is loosely coupled to the secondary (i.e the secondary is inside the primary's magnetic field). Some energy is transferred into the secondary with each "swing". Since the primary circuit has been designed to resonate at the same frequency as the secondary (by choosing specific capacitor and inductor values), the secondary begins to resonate as well and a huge voltage potential is created at the discharge terminal.

The above is intended as a simplified non-technical explanation. A web search can turn up many sites with in-depth technical discussions and explanations.


12" Diameter Tesla Coil
This is our newest coil and it uses a 12" diameter PVC secondary wound with #18 double-Formvar motor wire. It is powered by a 19.9kV pole transformer using an arc welder in series for current limiting. The transformer is conservatively rated at 15kVA though this coil has only been run at 3-4kVA at most.


12" Tesla Coil operating at approximately 3 to 4kW


Here the arc is being directed to the left using a breakout point This photo also caught strikes to the door behind the system, as well as a couple of strikes to the capacitors which will need some protection in the future. Output of this coil is presently limited by the size of the room and the coil's proximity to other objects.


12" Tesla Coil operating at approximately 3 to 4kW

Here no breakout point was used, and the majority of the arcs are striking the door directly behind the system since this was the closest object. The arc in the the upper right of the image can be seen striking an object, and then leaving the object and striking something above it.

In this photo you can also see a "racing spark" and the secondary beginning to arc brightly at the bottom near the base. Some racing sparks burned the insulation off of a couple of turns and caused them to short out near the base. This marked the end of the photo shoot. Sparks like these which travel up and down the secondary surface itself are often caused by over coupling. After the windings are repaired raising the secondary should help rectify this in the future.

6" Tesla Coil

The photo above is of a 6" diameter coil circa 1997. This coil initially used an air blown spark gap powered by a leaf blower, ductwork terminal, and milk crate base. Not much to look at but it produced some terriffic sparks, and it actually destroyed a pole transformer at one point. The coil was 27" tall and double wound on a plexiglass tube with #22 double Formvar magnet wire. Double wound means that the tube was wound once, and then a second winding was wound over the first and connected to the first in parallel. The idea is that resistance is reduced, "Q" factor increased, and current capacity increased. Why not just use thicker wire?? Well on a 6"x27" tube we could get a good 800+ turns of wire using double-wound #22 wire. This gave the secondary a specific inductance. Using thicker wire would mean fewer turns, less inductance, and higher frequency. It is unclear if double winding makes anoticeable difference in real world performance. Accurate testing of identical secondaries would be need to know if it makes a difference. At any rate with the addition of a rotary spark gap and various other improvements (image below) this coil eventually produced arcs well over six feet in length which is not bad for such a small coil.


Custom series rotary gap built by Ed Wingate.
G10 rotor with tungsten electrodes

Coil stats:
  • Secondary: 6" diameter plexiglass double-wound (2 layers) to 27" with #22 double Formvar magnet wire.
  • Supply: 19.9kV distribution transformer.
  • Capacitors: FCI .01uF 100kV DC (several in parallel) or Maxwell .05uF 50kV pulse (two in series).
  • Current limiting: Miller adjustable arc welder
  • Max arc length: 77"

Movies of the 6" coil
   A 790k Quicktime movie of the 6" diameter coil shot back in '96. The coil was using an air blown gap. Even though the arcs are not as long as in the movie below, this one is more dramatic since it was darker and the arcs have a real "bolt like" quality. The angle is better as well. This movie was digitized on a professional frame-by-frame system, before being compressed to a Quicktime file ('96 was back before you could digitize high quality video in realtime on any cheap computer!). It is interesting to look at this video by stepping through a frame at a time.
  640k Quicktime movie of the 6" diameter coil running in Fall '97. Longest arcs are between six and seven feet, and the coil is running at about 4kVA. The character of the discharge is very different than the movie below since a single steady arc is breaking out from the toroid seam. There was a little too much sunlight coming in the door however for the sparks to have that fat look in the movie below.
 A 400k Quicktime movie of the series rotary gap operatingat at about 3kVA. It's not terribly exciting, but it is interesting. The electrodes appear almost stationary since they are being illuminated by the strobing light coming from the four series gaps.


Mini 6" Coil



This was a fun little (or not so little really!) coil that was powered by a 14.4kV potential transformer current limited by a shorted uWave oven transformer. It used a fan-blown multiple spark gap and performed very well sending deafening white hot bolts up to the ceiling during short runs.

To the right is one of the gap modules. These were mounted in a plenum to which two high speed muffin fans were attached.



Resource Pages

Tesla Coil Mailing List Home page
The mailing list is simply the single best resource on the net
for anyone interested in the construction of Tesla coils.

Bill Beaty's Page
a fantastic resource for the electronics experimenter
Tons of links!

Tesla Coil Web Ring
Huge collection of links

Matt Behrend's Tesla Page
Diagrams, equations....pretty much everything
you need to learn about or build a Tesla coil.

Bert Pool's Tesla Page
Lots of images of conventional coils as well as
magnifiers. Also two good articles explaining
operation and construction.

Check out our Web Links page
for more of our favorite Tesla links!


The Museum Of Electricity
ElectricMuseum.com
 Copyright © 2005 Charles Brush
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