| 摘要 |
PURPOSE:To prevent brakage of the parts of a high voltage circuit by detecting the switching of video signals by means of a detection part, and holding an illuminance control part in a low illuminance state for a prescribed time by means of the output of said detection. CONSTITUTION:If a switch to switch plural video signal is operated, a signal signifying this operation is inputted to the detection part 1, and the base voltage of a transistor TR 4 is lifted for a time set preliminarily by a time constant circuit 2. Accordingly, the collector and the emitter of the TR 4 come in conductive state to each other, and the control terminal voltage of a contrast control part 3 is made swiftly droop. Therefore, no abnormal current will flow through the high voltage circuit consisting of a flyback transformer 11, a switching power source 10, and a horizontal output TR 13. After the swift dropping of the control voltage of the control part 3, the voltage is gradually returned to regular state after the lapse of a time preliminarily set by the time constant circuit 2.
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