Operating the Ignition Transitor Within Safe Limits

Transistorized ignition, like any engineering design, is controlled by a balance between cost and performance. Theoretically, a transistor can be placed in a current production ignition circuit with the necessary arrangements for triggering. Practically, however, this requires a transistor with voltage capabilities that are not economically feasible. This paper examines some circuit designs which permit the use of low voltage commercial transistors and some of the protective requirements involved. A detailed presentation of the transient phenomena at the time of turnoff of these circuit arrangements is given. Keeping transistor operation within safe regions is also discussed, including relationships between load lines, coil frequency, and switching time.