PURPOSE:To detect which lamp has a broken filament among many lamps by comparing the output voltage wave-form of an AC power source due to the short circuit release set to the different value for each lamp and the short circuit release time set for each lamp, and judging which lamp has a broken filament. CONSTITUTION:The change time of the output voltage wave-form of the AC power source 2 due to the short circuit release set to the different value for each of lamps L1-Ln and the short circuit release time set for each lamp are compared to judge which lamp has a broken filament. When a broken filament is detected by a broken filament occurrence detecting section, the secondary side of transformers CT1-CTn is short-circuited by a short circuit controller, this short circuit is released for the short circuit release time, thereby the change time of the output voltage wave-form of the AC power source 2 is measured by a wave-form change time measuring section 6. This time and the short circuit release time set differently for each lamp are compared. Which lamp has a broken filament can be detected accordingly.
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