| 摘要 |
PROBLEM TO BE SOLVED: To prevent the flow-in of surge current into a transistor installed in an after stage of a driver circuit to eliminate the need of a large capacity for the transistor by installing a surge protective circuit in an illumination control circuit. SOLUTION: When surge voltage gets on a power line, a Zener diode ZD3 of a surge protective circuit 5 turns on and then a sixth transistor TR6 is applied with the base potential VBE divided by resistors R13, R14 and thereby turns on, and consequently the base potential of a fifth transistor TR5 of a driver circuit 4 is cut off. When surge voltage is applied, a fourth transistor TR4 becomes an on-state due to a Zener diode ZD2, so even if the fifth transistor TR5 is turned off, a current path to a lamp is maintained through a Zener diode ZD4 and the lamp 6 does not light off. Due to this structure, there is no need of using a transistor of a large capacity in an after stage. Since a transistor of a small capacitor can be used in the after stage, the entire equipment can have a compact outer shape.
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