Single ballast for powering high intensity discharge lamps

Details Single ballast for powering high intensity discharge lamps Single ballast for powering high intensity discharge lamps

2000-07-18

2001-11-13

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Current and/or voltage regulation

C315S20900T, C315S224000

Reexamination Certificate

active

06316885

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to ballast circuits for powering high pressure gas discharge lamps, and more particularly to a single ballast circuit for powering plural high pressure gas discharge lamps connected in series.
BACKGROUND OF THE INVENTION
A high pressure discharge lamp, such as a metal halide, mercury or high pressure sodium lamp, is typically powered by an electromagnetic ballast circuit incorporating an iron core. The electromagnetic ballast arrangement receives voltage from a power source, and outputs a ballast voltage for driving the lamp. The ballast circuit, which uses the iron core to achieve the necessary voltage adjustment, represents a major component of ballast cost, as well as bulk. The foregoing type of ballast circuit typically suffers the problem of powering only a single high pressure lamp. Existing solutions to more efficiently utilizing a ballast circuit so that it simultaneously powers plural (e.g. dual) high pressure gas discharge lamps sometimes suffer difficulties in starting a wide range of lamps available from various manufacturers. Another problem has been that high pressure discharge lamps undergo physical changes that alter their starting characteristics as they age causing similar difficulties in starting. It would be desirable to utilize a ballast transformer so that it powers plural high pressure discharge lamps, and realizes a considerably reduced per-lamp ballast cost and improved ballast efficiency, while minimizing the above mentioned starting difficulties.
SUMMARY OF THE INVENTION
The invention overcomes the foregoing problem in an exemplary embodiment comprising a ballast circuit for a plurality of serially connected, high pressure gas discharge lamps. The circuit 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 voltage, 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 ionize the non-shunted lamp, starting a breakdown process, 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. The mentioned capacitance has a value to provide current flow in the non-shunted lamp of sufficient magnitude as to avoid premature lamp degradation due to sputtering of its electrodes when the shunted lamp is not on. The value of the shunting capacitance is selected such that the phase of the 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 during warmup so that the reignition voltages do not add in real time and, therefore, the sustaining voltage does not need to be doubled and the ballast maximum OCV parameter does not need to be doubled.


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Electric Discharge Lamps by John F. Waymouth (1971), The Massachusetts Institute of Technology.

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