Lighting circuit for discharge lamp

A lighting circuit for a discharge lamp having a DC power supply section to be supplied with a voltage from a DC power supply and supplies an output voltage, based on the output voltage of said DC power supply section, to said discharge lamp, said lighting circuit comprising:

• (1) lighting detection means for detecting if said discharge lamp is lighted;
• (2) input-voltage monitor means for detecting an input voltage to said DC power supply section and checking if said input voltage lies within an allowable range, said input-voltage monitor means having input-voltage comparing means for detecting said input voltage to said DC power supply section and comparing said input voltage with a first threshold value or a second threshold value, and threshold changing means for changing said first or second threshold value;
• and
• (3) power-supply allowing/inhibiting means for allowing or inhibiting power supply to said discharge lamp in accordance with a signal from said input-voltage monitor means,
• whereby when said input voltage to said DC power supply section is less than said first threshold value at a time said input-voltage monitor means receives a signal indicative of a light-OFF state of said discharge lamp from said lighting detection means, said input-voltage monitor means sends a signal to said power-supply allowing/inhibiting means to cut off power supply to said discharge lamp and when said input voltage to said DC power supply section exceeds said second threshold value, said input-voltage monitor means sends a signal to said power-supply allowing/inhibiting means to supply power to said discharge lamp, and
• whereby when said signal from said lighting detection means indicates said light-OFF state of said discharge lamp, said threshold changing means sets said first threshold value in said input-voltage monitor means smaller than said first threshold value in a light-ON state of said discharge lamp and/or sets said second threshold value in said input-voltage monitor means smaller than said second threshold value in said light-ON state of said discharge lamp.