Ballast circuit for low pressure gas discharge lamp

Abstract

An improved ballast circuit for a low pressure gas discharge lamp having two filament type electrodes at opposite ends of the lamp is disclosed. The ballast circuit includes a first device for providing a high AC voltage for application across opposite electrodes of the lamp; a solid state controlled current source for providing a controlled current to the first device for enabling the first device to apply the AC voltage across the lamp electrodes to illuminate the lamp, wherein the voltage placed across said electrodes is non-sinusoidal; and a second device coupled to the first device for inhibiting the non-sinusoidal voltage from causing mercury migration in the lamp. The second device includes circuit connections for enabling current flow through only one filament of the lamp; and for biasing the voltage applied across the lamp about a DC level for balancing the effect upon mercury migration within the lamp caused by enabling current to flow through only the one filament. The controlled current source is adapted for providing an initially high current for enabling lamp warm-up following ignition and subsequently a lower constant current for enabling steady state operation of the lamp at a uniform illumination.