Measuring Tesla Coil Secondary Parameters
Most coilers probably don’t measure their secondary coil parameters. However, I’ve been doing a lot of experimenting with different types of counterpoises, top loads, and distances of the top load from the top of the coil, etc. I make secondary coil measurements quite often to see the effect different things have on R, L, and C. Since the top of the secondary is at a very high impedance and low capacitance it is difficult to make any kind of measurements there. I measure everything at the base of the coil with methods that have no appreciable effect on the circuit.
Now, I know the secondary coil has distributed capacitance, and distributed inductance for that matter, but for simplicity purposes I treat the secondary coil as a series RLC circuit with lumped components. As a matter of fact, I believe the ideal secondary coil would be a simple series circuit with lumped components. The current and resonant voltage rise along the length of the entire coil would be linear and maximum.
Here is my technique (all measurements made at the ¼ wave resonant frequency):
- 1. Measure resonant frequency, Fr (Fr needs to be measured accurately with a frequency counter)
- 2. Measure input impedance, Zin = R = Vin/In (measured with oscilloscope voltage probe and current transformer)
- 3. Measure half-power bandwidth, BW (bandwidth is the frequency range between the two half-power points, Fu – Fl. The half-power points are the frequencies where the input current is .7071 times the maximum current at resonance. This is a tricky measurement because the load will change on the generator and the frequency and output voltage will have to be adjusted at the same time to get the current you want and to keep the output voltage constant. Also, it is difficult to make accurate measurements with an oscilloscope).
- 4. Calculate Q: Q = Fr/BW
- 5. Calculate X: X = Q*Zin
- 6. Calculate L: L = X/2*pi*Fr
- 7. Calculate C: C = 1/2*pi*Fr*X
That’s about it. All measurements were made at the base of the coil where the impedance is low and with methods that do not appreciably effect the circuit. This “equivelant” circuit should act the same as the secondary coil from small-signal up to 10’s of kilowatts and higher (until sparks break out).
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