Tesla coil

Technical data:

Secondary coil: 10.5cm diam., 32cm high, 1000 turns 0.3mm enamelled wire on PVC tube, coated with polyurethan varnish.
Primary coil: 20cm diam., 16 turns 1mm copper wire, spacing between turns 1cm, on wooden form.
Primary cap: 8nF, plate stack type (4 in parallel), PE dielectric.
Spark gap: 5 copper tubes (2.5cm diam, 10cm long), spacing 1mm, transformer is connected to the outer tubes.
Primary transformer: 10kV, 100mA oil burner ignition transformers (5 in parallel).
Streamer length: something around 50cm (20"), arcs to ground more.

tesla coil running

Last changes

Building two more caps and so increasing the primary capacitance to 8nF made it possible to use a larger top terminal (metal hot-water bottle). This has increased the streamer length dramatically (from about 15cm to 50cm), although the coil is probably not tuned properly (best output with maximum primary turns).

More pictures

Click on the images for larger verisons.

Circuit diagram

What is a tesla coil?

The tesla coil is a resonant transformer, used to produce extremely high voltages (several million volts). It was invented, as the name suggests, by Nikola Tesla around 1900. It differs from a usual transformer in that it a) operates at a much higher frequency (20kHz-400kHz, as opposed to usual 50 or 60Hz) and b) does not have an iron core ("air core transformer").

In a simple model, it can be described as two inductively coupled resonance circuits. The primary cap is charged by the primary transformer. When the spark gap fires, the primary resonant circuit is closed and starts to oscillate. If the circuits are tuned to the same resonance frequency, energy is transferred from the primary to the secondary circuit (and back and so on periodically) in the form of increasing oscillations. The maximum voltage in the two circuits depends on the relative values of capacity and inductance. In the case of a tesla coil, the secondary inductance is high (many turns) compared to the primary inductance (few turns), causing the secondary voltage to be higher than the primary voltage by a factor of maybe 20.

More theory

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Jochen Kronjaeger