Detection and protection circuit for fluorescent lamps operating at

Abstract

A circuit for protecting fluorescent lamps connected to a high frequency electronic ballast has a detecting bridge connected to the fluorescent lamps, an output of the detecting bridge varying linearly with a difference between a highest and a lowest voltage across the fluorescent lamps. A timer is connected to the detecting bridge output, and a trigger is connected to an output of the timer, the trigger being responsive to an over-voltage condition of a fluorescent lamp operating at failure mode. A controllable switch is connected across the fluorescent lamps, the controllable switch being controlled by an output of the trigger. The controllable switch, when closed, shorts the fluorescent lamps between their terminals, to protect the circuit when one or more fluorescent lamps are operating at failure mode or at the end of their operating life. A controller may be connected to a voltage-divided output of the detecting bridge, the output of the controller being fed back, and connected to, the high frequency electronic ballast. The ballast may then be shut down, or the output of the controller may be connected to a dimming circuit to dim the fluorescent lamps.