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How do I determine the voltage of a tesla coil?
I built a medium size Tesla coil with a 12,000 volt 60 Ma NST. I get 3 to 4 foot streamers. I want to know the approximate output voltage.
It is notoriously difficult to measure the voltage. It is of course an RF voltage, of the order of hundreds of thousands of volts, with a frequency most likely well under 1 MHz. It is often said that a spark jumps about 1cm per 20Kv. However this is not the case with tesla coils, where the RF voltage builds over a number of cycles pumped via the primary excitation till it generates hot plasma leaders which push out from the "terminal". That terminal is usually called a "toroid" because of its shape, and is the capacitive part of the resonant secondary. It is intended to make the leaders travel outwards, away from the coil, and minimise corona losses as the voltage builds. The leaders travel away (as highly conductive threads of plasma) until the energy from the coil and the energy lost through the plasma are balanced. The leader is a highly conductive streanm of plasma, more like an arc, and behaves more or less as a piece of wire. The streamers are the "feathery tendrils" that branch off the leader, so they may have some sort of "volts/cm" concept, but I don't know.
The first link has some discussion of voltages, and the second link shows calculations, probably a reasonably practical approach. This builder mentions that an 8 foot leader is about 500KV. It does not necessarily follow that a 4 foot leader is 250KV however, though it does suggest something about the energy available.
It seems to me that the length of the leader is a practical way of describing the power of a Tesla coil, as it does represent the energy provided. It seems a handy way to compare them, or determine whether adjustments are effective.
The Wikipedia link below has as good a discussion on the topics as I have seen. The terminology associated with these things is from the era in the 1890s when not much understanding of radio things was around, and explanations from the time are not easily understood. Tesla and his coils were instrumental in the early development of understanding, but perhaps the coils were usually more heavily loaded by antennas, rather than being intended to give forth displays.
16 Foot Arcs from a 10" Tesla Coil (Secondary is 10 x 54" !)
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