Just some points about automobile/motorcycle coil ignition systems and voltages.
I is quite true that with a traditional Kettering ignition system, either with mechanical points or semiconductor switch, that the voltage applied to the primary of the coil is 12V (or 6V) during the dwell period when the points are closed. At low engine RPM this voltage causes the current through the coil primary to ramp up until the current is limited by the resistance of the coil primary winding and possibly an external ballast resistor. The maximum primary current is normally in the region of 3A to 6A.
When the points open, the primary of the coil jumps up to typically 400V which is transformed to 15KV to 30KV (depending on the particular coil), on the secondary. The typical turns ratio of a coil is not as high as you might expect at only around 40 to 1.
With a capacitor discharge ignition (CDI) a 1uF to 4uF capacitor is charged up to between 100V and 400V by an inverter. At the ignition point the charged capacitor is connected to the primary of the coil which then increases the voltage by around 40 on the secondary.
The thing is that, it is the spark plug that ultimately limits the secondary voltage by arcing over.
About making a transformer to double the output voltage from the secondary of a standard coil, my feeling is that the efficiency would be low and building a 50KV transformer would be quite difficult.
Off the top of my head, I would have thought that you would get much better results by choosing a high performance coil and over-driving the primary to generate the 50KV that you want. Of course, you will be over-volting the coil but you would probably get away with it. I do vaguely remember reading that some ignition coils do in fact produce secondary voltages around the 50KV mark. I would be inclined to go for an automobile coil rather than a motorcycle coil, as an automobile coil tends to be more sturdy.
In any event, you would need to make sure that the coil did not overheat.
By the way, remember that most automobile ignition leads are restive for electrical noise suppression and you may get a fatter spark by using high tension leads without built-in resistors.
In terms of driving the coil, you would get much better results and give the coil an easier life by driving the primary from a custom circuit rather than a mains dimmer.
I made a fantastic spark generator, running at around 10KHz, from a TV line output transformer (LOPT). It was an evil hissing thing, because of the high frequency, and it was great fun, until the LOPT burned out.
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