Protection circuit for fluorescent lamps operating at failure mode

Abstract

A circuit for protecting fluorescent lamps connected to high frequency electronic ballasts has a detecting bridge connected to the fluorescent lamps, an output of the detecting bridge varying linearly with the highest voltage across the fluorescent lamps, a number of shunt capacitors equal to the number of fluorescent lamps, each shunt capacitor being connected between the detecting bridge and a corresponding fluorescent lamp, a timer connected to the detecting bridge output, a trigger connected to an output of the timer, the trigger being responsive to an over-voltage condition of a fluorescent lamp operating at failure mode, and a controllable switch connected across the fluorescent lamps, the controllable switch being controlled by an output of the trigger. The controllable switch, when closed, couples the shunt capacitors across the terminals of a corresponding fluorescent lamp and shuts down the fluorescent lamp which is operating at failure mode or at the end of its operating life. This causes the fluorescent lamp or lamps operating at failure mode or at the end of its operating life to be individually shut down, without affecting the operation of the remaining fluorescent lamps.