Switching circuit for a fluorescent lamp with heated filaments

Abstract

Abstract A ballast for a rapid start fluorescent lamp with heated filaments has a secondary winding supplying lamp current, tertiary windings for supplying heater current to the filaments, and solid state switching circuits in secondary circuits through the filaments. Each switching circuit, which may be integrated with the ballast circuit or connected to a filament within the lamp envelope, comprises two voltage divider resistors having a common junction coupled to the gate of a triac whose primary electrodes are connected in parallel with the voltage divider. The triac is in series with the filament and supplies heating current to the filaments prior to lamp starting. When the lamp starts and conducts rated operating current the voltage divider reduces the voltage at its junction and at the triac gate below triac breakdown level thereby substantially eliminating heater current. BACKGROUND OF THE INVENTION Commonly used fluorescent lamp ballast are of the rapid start type which includes a transformer primary winding and a secondary winding supplying operating current to the lamp, and also one or more heater windings which supply current through the filaments at each end of the fluorescent lamp. The heating current is supplied when power is switched on to the ballast and resistively heats the filaments to emissive state. In this state a relatively low voltage, and hence a smaller and less expensive ballast, is needed to ignite an arc across the lamp. Heating the filaments protects them from damage which would occur with cold ignition, and prolongs lamp life. The rapid start ballast circuit has the disadvantage that heater current continues to flow when not needed after the lamp starts, and accordingly it is an object of the invention to eliminate the waste of continuous heater current while retaining the advantages of rapid starting. STATEMENT OF INVENTION According to the invention an electronic circuit for a fluorescent lamp with heated filaments comprises primary lamp terminals for carrying lamp current, secondary lamp terminals for supplying heating current to the lamp filaments, a transformer including a primary winding for alternating line current, a secondary winding with connections to the primary lamp terminals, and a tertiary winding in a secondary circuit through the secondary lamp terminals, switching means in the secondary circuit including a voltage divider with a junction intermediate the divider, an electronic valve connected in parallel with the voltage divider, the valve having a control coupled to the junction for causing conduction on each half cycle of aternating applied across the valve when a break down voltage is applied to the control, and the voltage divider and valve respectively being in series with the lamp terminals such that rated lamp current through the voltage divider applies a voltage at the control lower than the breakdown voltage of the valve, thereby to stop conduction of the valve and limit current through the lamp filaments.

Inventors

• 👤 Carlo S. Bessone
• 👤 Frank M. Latassa

Bibliographic Details