[ischmitz]

Loren is talking about the input signal to pin 7 of the CDI box. I ran a quick SPICE simulation and it confirms his statements: The Schmitt-Trigger does not seem to care about dV/dt. In the simulation even a low-frequency sine wave works perfectly all right. The SCR gets triggered slightly after the zero crossing. This puts the level to about -200mV. Changing R6 shifts that point a little bit but not to zero. And it is amazing how accurate the simulation shows waveforms at the gate of the SCR that match real scope traces.

When I use my little square wave generator (555) I can trigger the 6-pin units down to 60Hz with no issue. 2V p-p symmetric around 0V. The 3-4kHz DC/DC converter noise results in some interesting harmonics. Tiny changes in supply voltage change the time when the DC/DC kicks in after the SCR turns off. This causes a slight instability of some scope traces in continuous mode. When I tweak the supply voltage there are audible changes in the spark noise for lack of better description. However, even a simple digital storage scope can capture waveforms with no problem. And with smart trigger options on better scopes things get even easier since you can discriminate the trigger conditions by pulse width. I posted some data from my 4-channel Tek TDS2014 on another thread.

As a hint: The frequency of the DC/DC converter is a function of load and supply voltage and the load is not constant between sparks. You can use the SCR gate signal as trigger source even when looking at the DC/DC converter.

Ingo