Zero crossing circuit for electronic ignition system

Claim 1

An electronic ignition system comprising an ignition coil for providing a high voltage output, ignition circuit means connected to said coil, and ignition timing generator means having first and second output terminals and operable to provide an output voltage varying in amplitude from some positive to some negative voltage, said generator means being connected at the output terminals thereof to said ignition circuit means for initiation of the operation thereof, said ignition circuit means including temperature compensation means for detecting the zero crossing point of said generator means and including a first transistor having predetermined operating characteristics and input, output and common electrodes, the input of said transistor being connected to the first output terminal of said generator means, the common electrode of said first transistor being directly connected to a fixed reference potential, and biasing means having predetermined values connected to said first transistor for establishing therein a first predetermined current density when the generator output at the first terminal thereof is zero, and cooperating with the first transistor characteristic to establish at that current density the switching voltage at the input of said first transistor, said transistor being rendered conductive upon application at said input electrode of said switching voltage by said generator means to produce an output between said output and common electrodes of said first transistor for operating said ignition circuit means, said biasing means including a second transistor having operating characteristics similar to that of said first transistor and having input, output and common electrodes, said input and output electrodes being coupled together causing said transistor to operate effectively as a diode and being coupled to the second output terminal of said generator means, the common electrode of said second transistor being connected to the reference potential, said biasing means biasing said second transistor into conduction to establish therein a second predetermined current density substantially equal to the first predetermined current density when the generator output at the first terminal thereof is zero in order to maintain the second output terminal of said generator means at a voltage level substantially the same as the switching voltage established at the input of said first transistor and to maintain said first transistor in a partial state of conduction when the output voltage of said generator means at the first terminal thereof is zero, the switching voltage at the input of said first transistor changing with temperature or supply voltage variations and the voltage level at the second output terminal of said generator means being maintained in accordance therewith, said switching voltage of said first transistor substantially corresponding to the zero crossing point at the first output terminal of said generator means when said generator is going from a negative to a positive output to ensure said first transistor is turned on upon said generator means reaching its zero crossing point despite said temperature changes and said supply voltage variations, said second transistor having a lower dynamic impedance than a conventional diode to maintain a low impedance at the input of said first transistor and further comprising said biasing means having predetermined values establishing the switching voltage at a level substantially corresponding to the zero crossing point at the first output terminal of said generator means when said generator is going from a positive to a negative output to ensure said first transistor is turned off upon said generator means reaching its zero crossing point despite said temperature changes and said supply voltage variations.