Abnormality detecting circuit for discharge lamp with delayed interruption for undervoltage

Claim 1

A lighting circuit for a discharge lamp, comprising: abnormality detection means for detecting an input voltage and/or an input current to be input to a lighting circuit from a power supply, said abnormality detection means having a first reference value or reference range for comparison and a second reference value or reference range both to be set for said input voltage and/or said input current; a power supply circuit for supplying a stable supply voltage and/or reference voltage necessary for an operation of a circuit section associated with lighting control of said discharge lamp based on said input voltage to said lighting circuit; said abnormality detection means comprising an input-voltage abnormality detector for detecting an abnormality in a battery voltage and stopping an operation of said power supply circuit upon detection of an abnormality in said battery voltage, thereby inhibiting power supply to said discharge lamp, said input-voltage abnormality detector comprising a first circuit associated with detection of a voltage drop, a second circuit associated with detection of an overvoltage and a first OR gate for receiving output signals of said first and second circuits; wherein said first circuit includes a first comparator, a first delay circuit and an AND gate having one input terminal directly supplied with an output signal of said first comparator and an other input terminal supplied via said first delay circuit with said output signal of said first comparator, an output of said AND gate being supplied to said first OR gate; and said second circuit includes a second comparator, a second delay circuit and a second OR gate having one input terminal directly supplied with an output signal of said second comparator and an other input terminal supplied via said second delay circuit with said output signal of said second comparator, an output of said second OR gate being supplied to said first OR gate; whereby said abnormality detection means stops power supply to said discharge lamp when a state where a value of said input voltage and/or a value of said input current is equal to or smaller than said first reference value or said first reference range continues for a predetermined time or longer, and permits power supply to said discharge lamp when a state where said value of said input voltage and/or said value of said input current is equal to or greater than said second reference value or said second reference range continues for a predetermined time or longer.