I'm currently working on a project designing an experimental engine control unit, which ignites a flammable mixture with a standard spark plug and standard ignition coil (the old style https://en.wikipedia.org/wiki/Ignition_coil). The primary is first charged to a nominal voltage (12V I think) then, to generate the spark, the primary voltage is quickly dropped to zero and the collapsing magnetic field in the secondary inductor causes a voltage spike across the spark plug. I'm controlling the primary voltage with a microprocessor and MOSFET and charge the primary when the engine is "armed" and turn off the coil when "arm" is switched off.
I'd like a way of making the spark coil fail-safe, i.e. if there is a sudden power cut, no spark will be created due to the sudden voltage drop, but be able to generate a spark by intentionally dropping the voltage whenever needed. This needs to be 100% reliable, as it could be safety-critical if there is a build up of explosive gasses and an emergency shutdown is needed. I understand usually you'd connect a flyback diode (plus Zener/resistor) in reverse bias across the coil to stop the voltage spike, but the voltage spike is needed to generate a spark...
I looked at make-before-break relays to connect the flyback diode before the primary current is switched off, but couldn't really find any available and I'm not sure how they could be connected to stop the spark in the event of a power cut or when the "arm" switch is turned off. Likewise with using solid-state switches to include a diode in the loop, I'm not sure if it could be connected quickly enough to beat the voltage spike.
Thanks in advance for any input, this has been puzzling me for months now. I'm a mechanical engineer at heart, so please be patient if I've misunderstood anything in the above!