| 摘要 |
PURPOSE:To detect an abnormality of lamps without being affected by the fluctuation of the lamp voltage due to causes other than the abnormality of the discharge lamps by comparing the potential difference between both ends of a potential difference detecting impedance element with a threshold value. CONSTITUTION:Voltage dividing impedance elements 3-6 are connected to discharge lamps LP1 and LP2 to form voltage dividing circuits BA1 and BA2. A potential difference detecting impedance element 7 is connected between mid-points of the voltage dividing circuits BA1 and BA2, and the potential difference between both ends of the element 7 is compared with a threshold value by a comparing circuit 8. An abnormality of lamps can be detected based on the change of the lamp voltage due to the abnormality of the lamps LP1 and LP2 at the terminal stage of life without being affected by the fluctuation of the lamp voltage due to causes other than the abnormality of the lamps LP1 and LP2, e.g., the change of the lamp voltage due to the change of the ambient temperature and power source voltage or dimming. |
