Light dimmer for solid state ballast

Abstract

A system for controlling the current in a solid state, high frequency ballast energizing a gaseous discharge lamp for providing light dimming therein is disclosed. The ballast includes a thyristor/capacitor inverter bridge for generating the lamp current which is regulated by a commutation circuit synchronized with zero crossings of the thyristor current. Lamp current and voltage level signals are fed back to the commutation circuit for proper lamp current regulation. A dimming signal is added to either the lamp current feedback or voltage feedback signal and this composite signal is provided to the lamp current regulator in varying the bias thereof for selectively controlling light intensity. Dimming may be accomplished either manually by means of a rotatable switch having an output which varies linearly with shaft rotation or automatically by means of an ambient light level sensor to provide a predetermined output light level which may be manually adjusted. A common isolated control line facilitates photosensing control in a multiple ballast/light system with inter-ballast and ground isolation provided by optoisolators. In a preferred embodiment, a user adjustable balanced bridge is responsive to either manual or light level-dependent automatic control inputs for varying the bias of an input amplifier of the lamp current regulator.