So I was reading about the special properties of radio frequencies. Specifically I read this: "In contrast, RF current can be blocked by a coil of wire, or even a single turn or bend in a wire. This is because the inductive reactance of a circuit increases with frequency."

from here: https://en.wikipedia.org/wiki/Radio_frequency#Special_properties_of_RF_current

And it occurred to me that tesla coils had something to do with coils and radio frequency. So I looked up this: https://en.wikipedia.org/wiki/Tesla_coil#Operation

In the tesla coil operation section we immediately find this: "A Tesla coil is a radio frequency oscillator"

So basically tesla coils resonate at very high radio frequencies, and if you look at one they are all coil, which is supposed to block RF.

As a last ditch effort to solving my own questions I tried to read the tesla coil Wikipedia article, but there wasn't much about coils blocking RF.

However like half way through or something I found this:

"The supply transformer (T) secondary winding is connected across the primary tuned circuit. It might seem that the transformer would be a leakage path for the RF current, damping the oscillations. However its large inductance gives it a very high impedance at the resonant frequency, so it acts as an open circuit to the oscillating current. If the supply transformer has inadequate leakage inductance, radio frequency chokes are placed in its secondary leads to block the RF current."

So like one of the coils is blocking the RF but not the others? Weird.


2 Answers 2


Inductors can be used to block RF, but that's not the only thing they can do in circuits. Look at what the inductor in an RLC tank circuit does for example.

Same with capacitors. They can be used to block DC current in a circuit, but that's not the only thing they're used for.

Like capacitors, inductors are often used in circuits to store energy. Where capacitors store energy in the electric field, inductors store energy in their magnetic field.


Notice "can block". Not "always will block". And "block" really means "attenuate", not nullify. If energy source is strong enough, then even after attenuation, RF current effects may be noticeable even in circuit with loops.

In Tesla coil, because it is open secondary winding of a transformer with much higher number of turns than the primary, voltage on secondary terminals is very high (thousands of volts), but RF current is relatively small, because 1) secondary's terminals are not connected into a well conducting circuit 2) the product of voltage times current is limited by available power, so assuming constant power (for simplicity of explanation), the higher the voltage(number of turns), the lower the current. In reality available power will depend on details of the coil and so parameters of the coil and circuit are tuned so that the power is as big as possible.

The spectacular effects of Tesla coil are due to strong electric field due to high voltage between terminals of the coil, which can cause electric break down of the air and then current increases. But current is still relatively small, which is why small battery-powered Tesla coil shooting sparks isn't super dangerous.


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