JPS56138767A - Temperature control device for heat fixing device - Google Patents

Abstract

PURPOSE:To enable to prevent effectively a heat fixing device from being overheated by providing a circuit for controlling power supply to a heat fixing device based on the output signal of a temp. sensing means and a temp. detecting means, and controlling the temp. of the heat fixing device in correspondence to the moisture absorption state of image receiving paper. CONSTITUTION:A thermistor 51 of negative characteristics of which the resistance value decreases with an increase in temp. is used as a temp. sensing element for detecting the temp. of a heat fixing device 23, and a bridge circuit is constituted of said thermistor 51 and resistances 52- 55. The conveyance passage for image receiving paper 15 is disposed with a pair of sensing electrodes 56A, 56B spacially in a manner as to contact the image receiving paper 15 on the side before the heat fixing device 23 when viewed in its conveyance direction. These electrodes are connected in parallel with the resistance 55 of the bridge circuit. The difference between the potential of the connection point (a) of the thermistor 51 and resistance 52 of the bridge circuit and the potential of the connection point (b) of the resistances 53, 54 is detected in a comparing sensing circuit 57, based on the output whereof a power source 59 is turned On and OFF and its conduction angle is controlled by an amplifying control circuit 58, whereby the power supply to a ribbon heater 26 is controlled.