Lighting circuit for vehicular discharge lamp

摘要

Disclosed is a lighting circuit for a vehicular discharge lamp, which can accurately and promptly detect an abnormal status to thereby improve the reliability and safety usage. When a lighting switch in the lighting circuit is set on, a batter/voltage is boosted by a DC booster circuit and then converted into an AC voltage by a DC/AC converter before being supplied to a metal halide lamp. When power supply continues with the lamp in an open-circuit state or a short-circuited state, a fire may be set or an electric-shock originated accident may occur. An abnormality detector for detecting an abnormal status is provided to cope with such a problem. The abnormality detector detects the output voltage of the DC/AC converter, very close to the lamp voltage, determines that the lamp in the open-circuit state when the detected level is equal to or above a predetermined value and determines that the lamp in the short-circuited state when the detected level is equal to or below a predetermined value. The abnormality detector sends an abnormality detection signal to apower cutoff relay circuit to open its relay contact on the power line to cut off power supply to the DC booster circuit. The short-circuited state/of the lamp can also be determined by monitoring individual voltages applied to both terminals of the lamp and comparing those voltages with each other.