| 摘要 |
PURPOSE: To prevent the unstable drive of a transistor for switching and to stabilize the circuit operation by stopping the oscillation after lowering the high-frequency wave output once as an abnormality preventive operation. CONSTITUTION: The point (b) of an abnormality detecting circuit 15 has a timer function to operate after a necessary preheating time elapses from the time when the voltage at the point (a) is detected. When the service life of a discharge lamp 20 in the lighting condition comes to the final period, the voltage at the point (a) is raised, and its abnormal condition is detected. As a result, the point (b) is made to the level L, a converting transistor Tr13 is turned off, so as to transfer from a lighting frequency to a preheating frequency and the output fed to the discharge lamp 20 is lowered. After a specific time elapses, the point (c) of the circuit 15 is made to the level H, an oscillation stopping transistor Tr19 is turned on and the oscillation of a lighting circuit is stopped. Since the oscillation is stopped after the output is lowered once, an excessive stress is never applied to the parts used in the circuit and a protective operation can be carried out at a low cost. |
