| 摘要 |
PROBLEM TO BE SOLVED: To accurately find out a tube in the last stage of its service life, and cut off the operation of an inverter circuit by comparing divided tube voltage and mixed reference voltage with each other, reversing output of a voltage comparator when the tube voltage exceeds the mixed reference voltage, and cutting off the inverter output. SOLUTION: A DC power source is converted into a high voltage AC power source by an inverter circuit 10. Inverter output voltage is impressed on a hot cathode fluorescent tube 20 through a ballast capacitor Cb of a current limiting element. Voltage V to be impressed is divided by resistors Ra and Rb in parallel to the hot cathode fluorescent tube 20, and is divided by the resistor Ra' and Rb' after half-wave rectification is performed by a diode Da, and is filtered by the capacitor Cb. Its voltage VD is impressed on a minus side input terminal of a voltage comparator 30. Constant reference voltage Vs and voltage by reversely amplifying a light dimmer signal are mixed together by a mixer circuit 40, and are inputted to a plus side input terminal of the voltage comparator 30. |
