摘要 |
PURPOSE:To increase safety and to improve reliability by inputting a zero-cross pulse which triggers a power control element synchronously with the set voltage of a variable resistor for temperature control. CONSTITUTION:The temperature of a heater 6 is detected by sensors 8 and 9 and the conduction of the power control element 7 is controlled on the basis of their detection results. This temperature control is performed by turning on and off the power control element 7 by a trigger circuit synchronously with the zero crossing of an AC power source 1 through a zero cross circuit E, temperature setting circuit C, pulse superposing circuit F, heater temperature detecting circuit B, and comparing circuit 17. The comparator circuit 17 consists of PNP transistors (TR) 35 and 36 having a constant current circuit 37 interposed, and pulses of the zero cross pulse E are superposed F upon the variable resistance 16 for temperature control to constitute a temperature control output signal system into a pulse system entirely. Consequently, even if the setting circuit C enters an open fault state, the power control element 7 turns off, increasing the safety. |