Tuning a Tesla coil

There are normally two places to tune a standard SGTC. The primary coil tapping point and the spark gap distance. For a DRSSTC of course only the primary coil tapping point is used for tuning. The primary coil tuning is used to synchronize the primary resonance circuit with the given secondary resonance circuit. The secondary inductance and capacitance are constant values which determine a secondary resonance frequency. In theory, the energy transmission from primary circuit to secondary circuit is maximized when the resonance frequency is equal in both circuits. However, an arc from the topload adds additional capacitance to the secondary circuit which in turn lowers the ideal synchronized resonance frequency. The capacitance change varies depending on the length of the arc and if the arc strikes anything. Thus, ideally the primary circuit should then be tuned a bit lower then the secondary circuit. This results in an initially out of tune tesla coil with increasing performance after the arc forms. The primary coil can be tuned in two ways.

Try and error

Run the coil and measure the arc length. Turn off the coil and change tapping point, then turn the coil on again. If the arc length increase the changes are in the correct direction. Make smaller steps as you get closer to the optimal tapping point until satisfied.

Resonance measurement

Connect a signal generator to the bottom of the secondary coil (disconnect ground first). Measure the frequency with a flying probe from an oscilloscope with a constant distance (ca. 0.5m) to the topload. Set a sinus signal with ~30kHz lower calculated frequency at the signal generator. Now gradually increase the frequency until the maximum amplitude is observed with the flying probe. Keep in mind that there could be harmonics (multiple peaks). Your resonance frequency is at the most dominant peak. It’s possible to mount a loose wire to the topload to simulating an arc for lower secondary frequency.

This resonance frequency can now be set on the primary coil. To do this disconnect everything from the primary coil and the primary capacitor. Now connect the primary coil and primary capacitor in parallel. One end connects to GND of a signal generator. The other end connects to the signal generator probe through a 1 kOhm resistor. Now feed the previously measured secondary resonance frequency into the circuit and measure the amplitude between resonance circuit and 1 kOhm resistor with an oscilloscope. Parallel resonance is achieved where impedance is at its highest. Therefore, adjust primary coil tap until amplitude is at its lowest.