Over voltage detection circuit for use in electronic ignition systems

#1. Field of the Invention

#The present invention relates to electronic ignition systems and more particularly to an over voltage detection circuit for use therein to protect the active components during load dump conditions.

#2. Description of the Prior Art

#FIG. 1 is a schematic diagram of the embodiment of the present invention; and

#FIG. 2 is a waveform diagram useful for explaining the operation embodiment of FIG. 1.

#Turning to FIG. 1 there is shown a series driver type electronic ignition system 10 including the over voltage detection circuit of the present invention. Electronic ignition system 10 as generally known in the art comprises a dwell control circuit portion and an output power drive portion. In response to timing ignition signals generated in timed relationship with the operation of an automobile engine there are generated, in the dwell control portion of the circuit, dwell control signals which are then supplied at terminal 12 of electronic ignition system 10. As shown, the circuitry included within the dashed outline is suitable for manufacture in monolithic integrated circuit form with the components and devices shown outside thereof being external components being coupled to respective pins of the integrated circuit.

#The dwell control signal supplied at terminal 12 may be generated by any well known dwell control circuitry in response to the timing signals which would be supplied at the inputs of the integrated circuit. For instance, U.S. Pat. No. 3,871,347 to Howard Weber, assigned to Motorola Inc., shows such an ignition system.

#Electronic ignition system 10 is recognized as being of the series driver type in that the dwell control signals supplied at terminal 12 cause the series power device transistor 14 to be rendered conductive or nonconductive during the correct portion of a particular firing cycle of engine operation to render external Darlington amplifier, comprising transistors 16 and 18, conductive and then nonconductive accordingly. Generally, during most of a particular firing cycle, series driver transistor 14, as well as the external Darlington amplifier, are maintained in a nonconductive condition. In response to dwell control signals rendering the Darlington amplifier conductive current flows through the primary of ignition coil 22 via resistor 24 to ground. Thus, in this condition, current begins ramping upward through the ignition coil as shown by waveform 50 of FIG. 2. At a predetermined current level, typically near 6 amps, the current through ignition coil 22 is limited by the current sense regulator 26 limiting the current supply through series driver transistor 14 to the Darlington amplifier. The current limit time interval is shown by the flat upper portion of waveform 50 of FIG. 2. In response to dwell control signals generated at terminal 12, at the end of a particular firing cycle, transistors 27 and 28 which are shown connected in a Darlington configuration and coupled to the base of series driver 14 are rendered conductive to shunt base current drive supplied from a power supply conductor 30 via resistor 31 away from the base of series driver 14. This renders both the series driver device 14 as well as the external Darlington amplifier nonconductive to abruptly stop the current flow through ignition coil 22. Hence, the field about ignition coil 22 collapses to generate a spark to cause firing in the engine as is well understood.