JPS5736770A - Metal vapor discharge lamp - Google Patents

Abstract

PURPOSE:To improve the flux maintaining rate of a lamp and the service life, by providing a current limiting impedance set to short between both ends during stable lighting in a circuit connecting with the main electrode thereby preventing the over current at the start of the lamp. CONSTITUTION:When connecting a metal halide lamp 65 to a power source 63 through a ballast 62 for a mercury lamp, a heater 41 is heated by the discharge between the main electrode 32 and the auxiliary electrode 42 to open a normally closed bimetal switch 54 thus to start the main discharge between the electrodes 32 and 33. Consequently the current at the start of lighting will flow into the circuit through a current limiting impedance 40 connecting with the electrode 33. Upon reaching to stable lighting state, a normally open bimetal switch 58 provided at both ends of the impedance 40 is closed by the radiation heat from the heater 41 to short between both ends. Consequently the over current at the start of the lighting is prevented to reduce the evaporation of the electrode material, thereby a metal halide lamp having high start voltage can be used for long term while employing a cheap ballast for a mercury lamp.