Contactless ignition system for internal combustion engine

A contactless ignition system for an internal combustion engine comprising:

• an ignition coil including a primary winding and a secondary winding;
• semiconductor switching means connected to the primary winding of said ignition coil for controlling the primary current supply through the primary winding of said ignition coil;
• current detecting means connected to said switching means for detecting the level of primary current supplied to the primary winding of said ignition coil;
• a rising time detecting circuit connected to said current detecting means for detecting the rising period of time of the primary current until the level of the primary current attains a predetermined setting after the primary current starts to be supplied to the primary winding of said ignition coil, thereby generating a signal indicative of the rising time of the primary current;
• means for generating a rectangular waveform signal having a period synchronized with the rotational period of the engine;
• means for generating a current proportional to the rotational speed of the engine;
• an off-time control circuit producing, on the basis of said current proportional to the rotational speed of the engine and said rising-time indicative signal, an off-time control signal for controlling the off-time of said ignition coil and to provide dwell angle control;
• a control switch-over circuit gating said off-time control signal only when the engine rotation period indicated by the level of said current proportional to the rotational speed of the engine is shorter than a predetermined setting;
• and
• an OR circuit controlling the off-time of said switching means on the basis of the logical sum of the output signal from said control switch-over circuit and said rectangular waveform signal.