Jochen's High Voltage Page

Tesla coils

Tesla coils are widely discussed on the net (and in the literature). There is a vast number of web sites on this topic. I am not concentrating on tesla coils - this page is the only one of my site dealing with them. Also, the coil I´m presenting here is far from being perfect - in fact, it is mostly makeshift and many devoted coilers would close their eyes in horror being confronted with it ;-)

Tesla coils rely on electromagnetic induction just like ordinary transformers (see Obits or ignition coils). There are, however, a few important differences.

Schematic tesla coil circuit. The primary resonance circuit is made up of the spark gap F, the primary capacitor C1 and the primary winding L1, the secondary resoance circuit consists of the inductance L2 of the secondary winding and the capacity C2 of toroid and winding together towards ground. The 10kV source charges C1 with low frequency AC or DC.

Let´s first have a look at how the thing works in principle (compare schematic above). You might want to have a look at this calculated scenario while reading (this link will open another window).

At the beginning, the spark-gap F is non-conducting, and the primary cap C1 is charged from the 10kV source. As soon as its voltage reaches the breakdown voltage of the gap, F fires and becomes conducting. Now the primary resonance circuit is closed and starts to oscillate at a high frequency determined by C1 and L1 (usually a few 100kHz). Through the small inductive coupling, energy is (comparatively) slowly transferred to the secondary resonance circuit, which in turn oscillates with increasing amplitude. At the same time, the amplitude in the primary circuit decreases due to conservation of energy. Ideally, when the energy transfer is complete the primary amplitude is zero and the spark gap quenches, opening the resonance circuit. In this case the secondary circuit performs a damped oscillation. If for some reason the gap does not quench, the energy is tranferred back to the primary circuit and so forth. This periodic exchange of energy between two resonace circuits is called a beat, and it is a basically unwanted phenomenon in tesla coils - one would prefer the whole energy to stay in the secondary circuit until it is used up by sparks or other losses.

All this (charging, energy transfer, decay), though slow compared to the oscillation itself, may happen within a fraction of a second. Actually, the 10kV source is often a 50Hz (resp. 60Hz) transformer, implying that the whole process is completed in 10ms and repeated 100 times per second.

The two resonance circuits must be tuned to the same frequency. This is done by choosing a suitable primary capacity C1 for coarse tuning, and then varying the number of turns of the primary coil for fine tuning. A calculation shows that in this case the maximum voltage in the secondary circuit is higher than the primary peak voltage by a factor of Sqrt(C1/C2), or, which is the same, Sqrt(L2/L1). Basically, this is due to conservation of energy, as the energy E stored in a capcitor C at voltage U is E=1/2*C*U^2.

Schematic and actual construction of a tesla coil, with primary and secondary winding. The toroid can be replaced by any conducting object of comparable size and roundness. The smaller photos to the right show the primary capacitor (top) and the spark gap (bottom). For details see text.

The above photos show my own coil, which is probably a typical first-time model. Some techincal data:

Being hit by a tesla coil streamer is not harmless. The high voltage output is not pure high frequency, but contains a 100Hz component that may still kill you. Also, skin effekt in human tissue is a myth, as its conductivity is by far too low. Arcs between secondary and primary or to the mains wiring may put you in direct connection to primary or mains voltage, with the same result as touching primary or mains voltage.

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