Dimming controller for a fluorescent lamp

Abstract

A control module controls the illumination intensity of a fluorescent lamp. A controller controls a switch to conduct current through cathodes of the lamp, and the switch ceases conducting current when commutated into non-conduction. The switch is triggered into conduction at a predetermined extinguishing point within and prior to the end of each half-cycle of applied AC current. The switch is thereafter commutated into non-conduction at a predetermined ignition point when the applied AC current reaches a predetermined level near the zero crossing point of each half-cycle of applied AC current. The current level at commutation creates a sufficiently high di/dt effect to cause an ignition pulse of voltage from the ballast across the lamp cathodes sufficient to ignite the ionized medium into an illumination plasma. The switch extinguishes the plasma. The illumination intensity of the lamp is related to the time between the ignition point and the extinguishing point. The extinguishing point is variable and selected by control signals, such as momentary power interruptions.