Tachometer driven from a capacitor discharge ignition system and

Abstract

A tachometer circuit for an engine having an alternator driven capacitive discharge ignition system functions as a frequency-to-current converter converting each ignition capacitor discharge into a precisely controlled amount of electric charge, which is then passed thru an indicating microammeter movement graduated in RPM. The precisely controlled electric charge is developed in a charge transfer capacitor, with the peak voltage appearing across the capacitor being regulated by a programmable transistorized voltage regulator. The temperature coefficient of the charge transfer capacitance is neutralized by the temperature coefficient of the voltage regulator, such that the product of capacitance multiplied by voltage--which is equivalent to charge--remains constant. The precisely controlled charge is initially established by machine trimming of a voltage--determining resistor in the voltage regulator, thereby allowing practical large volume manufacture of the circuit from unmatched components.