Oscilloscope traces.

The first photo is a trace of the coil current in start mode,1 cm = 2 amps and the horizontal scale is 1 cm = 10 milli seconds. At 1000 rpm (engine) the peak coil current is about 7 amps giving a very high energy spark. The average current is only about 300 milli amps. The slower it gets the more energy it gets so at cranking speed there is a monster spark.

Next is the trace with the system running at about 4000 rpm (engine) , horizontal scale is 1 cm = 2 ms ,The time between pulses is slightly less than 15 ms which equals 1 revolution for the engine so 15 ms per rev is 1000/15 revs per second , times 60 equals revs per minute. (1000x60)/15 = 4000 RPM. The vertical scale is unchanged. The coil is peaking at about 5 amps , still quite a nice spark and the average current is less than 1/2 amp

Here are some traces showing the coil primary voltage with and without a 0.22 uf capacitor connected accross the switching transistor , the same as a points system. The first two photos are with the capacitor and the second two without. In the first photo the time base is set for 200us per division and the voltage is 50v per division. The ov point is one division from the bottom. Therefore with the capacitor connected the initial pulse is about 250 + volts , the spark duration is just over 1000us or 1 ms and from the second photo where the time base is set to 20 us per division the rise time for the initial pulse can be seen to be about 30 us to over 300v.

In the second pair of photos without the capacitor the initial pulse disappears off the screen so probably about 400v , the rise time can be seen in the 4th photo which is about 3 us ,there is less ringing after the initial pulse but the spark duration is about the same which really surprised me.The big difference is the rise time is about 10 times faster.

With the open air spark gap set to 0.3" (10 times the plug gap) to compensate for the increased pressure in the cylinder. I found in both modes , start and run that the rpm could go much higher without the capacitor.

A couple of other interisting traces. The first one is the primary coil voltage using the last system I built with the darlington transistor. It has 4 75v zener diodes connected in series accross the colector and emitter to limit the voltage to 300v. The second trace is the coil primary current during starting. With the scope set to 2 amps per divison the current almost gets to 12 amps so as you can imagine produces a hefty spark. Once the engine starts and the RPM increases to 900-1000 the maximum current falls to about 5-6 amps. The average current at idle is only 300 ma.