Ignition-coil voltage measurement using sphere gap and resistive divider methods

In the previous video, I verified that my simple sphere gap measurement setup is quite satisfying. In this video I use the method to measure the pulsed voltage generated by my ignition coil. Actually, to measure a high voltage pulse we can use a high-frequency high voltage o’scope probe, like North Star PVM series. By using oscope, we can obtain the whole waveform, instead of a peak voltage as we get from the sphere gap method. Unfortunately, I don’t have the probe at the moment. That’s why I tried the sphere gap method.

The coil is supplied by a 11.6v voltage source, and driven by 555 timer and MOSFET transistor. In other words, it is configured as inductive discharge ignition. It turns out that the sparks start to happen at the distance of 0.4 cm. This gap means that the voltage between the balls is around 17.4 kV.

However, some people say that the voltage generated by an ignition coil is normally around 30 kV. I need to investigate this further, probably the ignition coil itself is not good enough. Or maybe some unknown sources of error creep in..


For some reasons, the high voltage generator had been modified to be capacitive discharge system. The capacitor bank was charged up to 310 Vdc. It seemed that the coil output voltage had become higher. In effect, when the coil output was not loaded, sparks forcefully jumped across the coil terminals. In the previous system this behavior was not observed.

A as substitution to the North Star high voltage probe, I tried to build myself a resistive voltage divider. I used a 10 megaohms high voltage resistor and a  10 kohms regular carbon resistor. They are connected in series, and thus forms a 1:1000 voltage divider. Measurement using oscilloscope showed that the coil output voltage reached 17.5kV when the output was loaded with 110 kohms, and peaked at 21 kV when loaded with 280kohms.