Tesla Coil


Tesla coils are the amatuer experimenter's easiest way of generating output voltages in the hundreds of thousands of volts, if not millions of volts. Commonly showcased in movies, Tesla coils - invented by Nikola Tesla - are also used in professional environments to simulate lightning and destructively test a range of objects for insulation quality.

Typical schematic of a conventional Tesla Coil

Tesla coils are air cored resonant transformers. Resonant, basically meaning 'in tune', and transformer, meaning the things you find in almost every household appliance used to step up or down the voltage. In the case of a tesla coil, the voltage is stepped up. Tesla coils are nothing to joke about - they can generate very dangerous voltages, so one must be careful when working with them.

To hit an electric circuit into resonance, two basic components are required. A capacitor (stores energy in the form of an electrostatic field) and an inductor (stores energy in form of a magnetic field).

The capacitor in the tesla coil is used for storing the energy, while the inductor (or primary coil) is used to provide mutual inductance to the secondary coil. In order to discharge the capacitor's energy into the primary coil to initiate power transfer, we need some form of an interruptor or switch. The difference in interruptor design separates the many different types of tesla coils.

A 'beer bottle' capacitor bank used in my Tesla coil. Each bottle is filled with salt water then topped with mineral oil for HV insulation.

My good old fashioned spark gap made from a piece of tile and a series of aluminium bars.

The old, traditional way of tesla coiling was to use a spark gap for an interruptor. A spark gap is simply a gap between two electrodes, connected to certain points in the circuit. When the voltage over these electrodes exceeds a pre-set amount (usually a couple thousand volts), the electricity arcs across and discharges the capacitor's energy straight into the primary coil. This energy dump provides a voltage spike across the secondary coil through mutual inductance. This voltage spike is introduced across the toroid (top of the secondary coil), and is discharged from there.

I have to admit I didn't put much effot into designing and constructing this tesla coil, so please accept my dodgy handywork.

Anyway, onto my coil... the first thing I did was to make a beautiful secondary coil. This bugger took about 4 hours just to wind the wire on. Though the feeling one gets upon completion is a great one!

This coil is 30cm long and made from 1200 turns of 22 gauge wire. The final step was to coat it in polyutherane varnish.

Next come the supports for the primary coil. The primary coil wire is quite thick, since it has to carry hundreds of amps from the capacitor discharge.

1. A long piece of wood is filed down at specially marked sections

Here's a closeup of the filed down bits

2. It's then cut into four sections. Theres a 30cm ruler for scale.

3. All four sections all glued onto a ceramic board base.

4. Time to wind the wire. The thick 10 gauge wire is very hard to work with, and screws are used to secure it to the base.

5. The completed primary coil

A closeup of the screw in method of mounting the wire.

Those are the hard bits done. The toroid came next on the list.... I took a joy trip to the local ventilation ducting store and found myself a meter of 4 inch ducting.

The ducting is coiled around into a doughnut shape, and covered in aluminium foil.

The primary and secondary coils mounted in position on a thick plaster base

The setup with toroid, primary coil and secondary coil all secured together.

The high voltage transformer, used to supply power to the primary circuit, took a bit of scrounging to find.

These aren't your little square transformers you find in your VCR player, these are big things that are used in the powering of large neon tubes. So I called up the local neon sign manufacturing company and they gave me a used transformer for $20. Not bad.

This particular unit takes in 240V and outputs 15kV at 30mA.

Coronal discharge from toroid

Here is the first light!

Complete dismal performance for its size, I must say. But hey, it still makes 25cm topload to ground arcs, and 7cm corona discharges.

And this is a long exposure with me swinging a grounded rod around the topload. Notice the ground arcs visible this time. The corona discharge is there again because I forgot to take the discharge rod off the topload.

This setup is completely ridiculous in terms of spark length, however. It should be capable of up to 40cm arcs to ground, but I guess you get what you work for!

Overall the performance was not satisfying, but it was fun to build nevertheless. What else can I say?





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