Single ballast for powering high intensity discharge lamps

Abstract

A ballast circuit for a plurality of serially connected, high pressure gas discharge lamps comprises an electromagnetic ballast arrangement receptive of an input power signal, providing an output ballast voltage for driving the plurality of lamps, and providing an open circuit ballast voltage (OCV) when the lamps are disconnected from the arrangement. An ignitor circuit is connected between the ballast arrangement and the first lamp, and produces at least one ignitor pulse, per each half cycle of the ballast voltage, of high voltage and high frequency compared to the open circuit ballast voltages to initiate starting of the first lamp. A capacitance shunts one of the lamps providing a sufficiently low impedance to a high frequency ignitor pulse so that a substantial portion of the pulse first appears across a non-shunted lamp during lamp starting so as to start the non-shunted lamp and then, when the voltage across the non-shunted lamp falls, to impress a substantial portion of the OCV across the shunted lamp to initiate its starting. When the number of serially connected lamps extends to more than two, the value of each shunting capacitance is selected such that the phase of ignitor pulse is not equal at any two lamps in real time. Furthermore, the shunting capacitance acts to provide a small reignition voltage spike phase shift so that the reignition spikes do not add in real time so as to reduce the required sustaining voltage.