This invention relates to lighting systems which control the level of illumination of one or more lamps, such as in a stage lighting system or in other lighting applications where varying intensities of lighting is desired. In particular, this invention relates to controlling the intensity of light from high pressure lamps rather than incandescent lamps or low pressure discharge (fluorescent) lamps.
Solid-state electronic dimmers have been used to control the level of illumination from incandescent lamps for a number of years. While some of these solid-state dimmers are of an open loop type, others sense load voltage and feedback a signal proportional to load voltage (to stabilize the lamp control system and to compensate for line voltage variations). Some dimmers also sense load current in order to prevent damage to the dimmer from overcurrent due to the connection of excessive wattage of incandescent lamps to the dimmer output or due to inadvertent short circuits. U.S. Pat. No. 3,821,601 issued to Kappenhagen et al. on June 28, 1974, describes an incandescent dimmer utilizing voltage feedback and overcurrent protection.
Utilizing high pressure discharge lamps with a dimmer not specifically designed for such lamps is generally unsatisfactory. Solid-state dimmers generally control the portions of each half cycle during which voltage from an AC voltage source is supplied to a load. The high pressure discharge lamps tend to extinguish when the voltage remains off for a significant portion of a half cycle and the normal ballasting used will typically not reestablish the arc. Further, the arc voltage is not proportional to lamp intensity and voltage feedback does not provide a satisfactory method of stabilizing operation.