| 摘要 |
PURPOSE:To prevent a large current from flowing in a main circuit even in half- wave firing state produced during the period of time while the predetermined period of time after a discharge lamp is started by shortcircuiting the output terminal of a lamp voltage detecting circuit for the predetermined period of time. CONSTITUTION:When a discharge lamp L is started, a voltage is produced across a current limiting inductance CH2. The voltage is applied to the base of a transistor Tr3 via a resistor R18. Thus, the transistor Tr3 is conducted. Accordingly, the output of a lamp voltage detecting circuit becomes zero. The conducting phase angle of an AC controlling element S1 is delayed and fixed to operate to reduce the lamp current. On the other hand, a capacitor C8 is charged via a resistor R20 from the starting time. When the capacitor becomes higher than the prescribed voltage, the transistor Tr3 is interrupted via a diac S4 and a thyristor S3. As a result, the lamp voltage detecting circuit produces a DC voltage corresponding to the lamp voltage. In this manner the lamp current is reduced during the glowing period of time over several seconds after the lamp is started. |
