Lamp ballast circuit with overload detection and ballast operability

Abstract

A gas discharge lamp ballast with an indicator of operability of the ballast is disclosed. The ballast circuit comprises circuitry for providing a d.c. bus voltage on a bus conductor with respect to a ground, and a resonant load circuit. The resonant load circuit includes lamp terminals for connecting to a removable gas discharge lamp, a resonant inductor, and a resonant capacitor. The resonant inductor and resonant capacitor are selected to set a magnitude, and resonant frequency, of a bidirectional current in the lamp. Further included is a converter circuitry, including first and second serially connected switches coupled between the bus conductor and the ground, and providing to the resonant load circuit, at a node coupled between the first and second switches, a voltage that alternates between first and second voltage levels. Circuitry for generating first and second switch-control signals for alternately switching into conduction the first and second switches, including circuitry responsive to a feedback signal representing a current in the resonant load circuit, are further included. The ballast circuit, moreover, includes overload detection circuitry for detecting if voltage applied to the lamp terminals exceeds an overload level; and circuitry for indicating, in response to such an overload condition being detected by the overload detection circuitry, probable operability of the ballast circuit.