Turn-off protected ignition system for internal combustion engines

Abstract

A transistorized ignition system is protected against interruption of power supply by, for example, turn-off of the main or ignition switch while the transistor is in conductive condition so that, upon sudden interruption, an ignition event could be triggered at a time unsuitable for the remainder of the engine. A turn-off protection circuit is provided which includes a coupling resistor connected to the junction of the primary of the ignition coil and the main ignition current supply transistor thereto. The coupling resistor is connected in a circuit which holds the main current supply transistor in conductive condition, supplied from the energy stored in the ignition coil even after the main, ignition switch has been opened. A closed circuit for current flow due to this stored energy is provided within the overall ignition circuit. Preferably, the system includes a coupling transistor, connected across the power supply, which is blocked when the main transistor is conductive, so that the main transistor is controlled by power supply voltage appearing at the collector of the coupling transistor; the coupling resistor of the turn-off protected circuit then applies the inductive voltage from the coil to the collector terminal of the coupling transistor, and hence simulates continued power supply to the main current supply transistor to continue to hold it conductive and thus prevent abrupt turn-off and initiation of an undesired ignition event.