Abstract: The voltage in the secondary of an ignition coil is prevented from reaching an amplitude at which a spark might be created by a Zener diode connected in parallel with the series transistor acting as a switch which interrupts the current through the primary winding of the coil. Upon breakdown of the Zener diode, the series transistor is switched to the conductive state briefly, thereby preventing further voltage build-up in the secondary. When the engine is in operation, a transistor which forms part of a voltage divider is switched to the conductive state, connecting the voltage divider between the secondary winding and the Zener diode. The voltage divider is so dimensioned that the Zener diode will respond to a voltage slightly less than the breakdown voltage of the emitter-collector circuit of the series transistor, but higher than the voltage required for creating a spark.