Control circuit providing constant power source

Abstract

A circuit for starting and operating an arc lamp includes a full wave rectifier having an input connected to a source of alternating current line voltage and a pair of output terminals having a filter capacitor connected thereacross for providing a source of direct current voltage. Connected to one of the direct current terminals is a resonant charging circuit comprising a controlled rectifier switch, an inductor and a capacitor serially connected in a circuit loop. Another inductor, which may comprise the secondary winding of a starting pulse transformer, is connected between the output of the resonant circuit and one of the supply terminals for the arc lamp to provide the dual function of both averaging filter and lamp ballast. The reference terminal of the direct current source is connected to the other arc lamp supply terminal and through a back swing diode to the output of the resonant circuit. A trigger pulse generator is coupled to the resonant charging circuit for intermittently operating the controlled rectifier switch, and a voltage divider is connected across the direct current source for sensing changes in the line voltage and adjusting the pulse rate of the trigger generator in response thereto. In this manner, once the arc lamp is started and operating, the resonant charging circuit maintains a source of constant power to the arc lamp regardless of line voltage variations. Protective sensing circuitry is also provided for controlling the trigger generator output in the event the line voltage is too low, the load current is too high, or the resonant circuit capacitor has not discharged to zero.