| 摘要 |
PURPOSE: To accurately judge the lifetime of a fluorescent tube even in the dimming condition by providing a minus voltage detecting circuit, which detects the smoothed waveform of the minus side voltage of the high frequency current of an electrode of a fluorescent tube and which outputs a detecting signal. CONSTITUTION: When the control signal from a timer circuit 11 is sent to a minus voltage detecting circuit 12, the circuit 12 is operated on the basis of this control signal. The end of lifetime of a fluorescent tube 6 is judged on the basis of a judgment whether the minus side voltage is generated after the predetermined time passes or not. Namely, in the case where the minus side voltage is disappeared after the predetermined time passes, a judgment that the lifetime of the fluorescent lamp 6 is left is performed, and in the case where the minus side voltage is generated, the end of the lifetime of the fluorescent tube 6 is judged. This judgment is performed by detecting the minus side voltage with the circuit 12, and the detecting signal is sent to a fluorescent tube lifetime informing device 14 through a protecting circuit 13. Even in the light dimming condition, a detection whether the lifetime of the fluorescent tube 6 reaches the end or not can be accurately performed. |
