120. Tesla Coils

A Tesla coil is a high frequency, very high voltage transformer. The operation and circuit are described below. Our smaller Tesla coil radiates enough RF to illuminate a fluorescent tube a foot away. You can draw a one foot arc into your body, using a metal rod; the frequency is so high that the ions in your body do not have time to move far enough to do damage.

The Giant Tesla Coil generates a four foot discharge that will illuminate fluorescent tubes many feet away, and will generally terrify anyone anywhere near it. People with pace makers are supposed to move to the back of the room.

Tesla Circuit Operation

A preliminary step-up transformer boosts the line voltage to ten or twenty thousand volts. (A neon sign transformer is used for this purpose in the giant Tesla coil.) This voltage arcs across the spark gap F at 60 Hz, ringing the tuned circuit consisting of the capacitor C and the Tesla primary (a few turns) at 1-3 MHz. The Tesla secondary (many fine turns) has its own resonant frequency determined by its inductance and the internally distributed capacitance between its windings. The primary LC circuit is tuned to this resonant frequency for maximum coupling.

A spectacular demonstration with the small Tesla coil is to hold a fluorescent bulb in one hand and a metal rod in the other. When you draw an arc with the metal rod from the Tesla coil, the current passes through your body and lights the bulb. The voltage is over 1 million volts; why doesn't it shock or kill you? There are a number of hypotheses on this: a. The skin effect -- the current travels in the outer dead layer of skin. b. Human nerve circuits do not respond to high frequencies like 1 MHz, perhaps because of the reason mentioned above; there isn't time for the sodium and potassium ions to move far in the nerve cells. c. The impedance of the Tesla coil is very high, and it therefore induces only very small currents in humans.

A skin depth calculation using the conductivity of sea water, sigma = 4 mhos/m, gives a depth of 0.25cm, so this effect would not play a large role in protecting you, at least from pain stimulation on the skin surface. The answer is probably a combination of effects b and c from above. The current through the body is too small to generate enough heat to injure the person, and the high frequency does not stimulate pain nerves, or induce muscle contraction.

Our giant Tesla coil does sting you if you get into its circuit path to the ground. Tesla coil builders claim that the "gentleness" of the particular Tesla coil depends on the cleanness of the separation of 60 Hz and the high frequency at the spark gap.

A nice description of how the tesla coil circuit really works is here.