| 摘要 |
PURPOSE:To reduce the generation of a noise by turning on and off a semiconductor switching element, connected in series to a heater, at the zero-cross point in time of supplied AC power. CONSTITUTION:Transistors (TR) 16 and 19 connected to an AC power source through a couple of lines 11 and 12 turn on when an AC voltage level is higher than a positive or negative constant level. Then, high-level signals outputted by one-shot pulse generating circuits 20 and 21 are inputted to FFs 36 and 37 respectively, and a timer circuit 38 and a delay circuit 54 operate by their outputs to drive a semiconductor switching element 61 which controls the feeding of electricity to a heater 7. In this case, the output of the circuit 54 is so delayed that its leading and trailing edge coincide with the point in time when the AC voltage of the element 61 is zero. Consequently, the element 61 is turned on and off at the zero-cross point of the AC voltage, thereby reducing the generation of a noise. |
