GB1291855A - Improvements relating to thermographic machines - Google Patents

Abstract

1291855 Photo-electric temperature sensing FRANK R FORD Ltd 19 Feb 1970 [22 Feb 1969] 9644/69 Heading G1A [Also in Division G2-3] In a thermographic copying machine, between rollers 14, 15 Fig. 1 of which passes temperature sensitive paper sheet and an 'original' sheet to be copied, the mark/space ratio of current pulses fed to an infrared lamp heater 24 within roller 14 is controlled to maintain substantially constant. The output temperature of the heater and hence also the heat applied to the paper. Alternatively the machine may be used to stick sheets together. A thermistor Th on a bracket 28 senses the surface temperature of the roller 14 and picks up radiant heat from the lamp transmitted through a hole 25a in a reflector and through the transparent wall of the roller. Alternatively the sensor may be within roller 14. The temperature sensed by the thermistor therefore corresponds to that experienced by the lateral area of the sheets passing under a slot 27 in the reflector. SCR1 Fig. 3 is controlled in dependence upon the impedance of the thermistor whereby to control the average D. C. through heater lamp R1. A feeler member 37 senses the entry of sheets into a passage 11 and closes a switch S1 which energizes a relay coil 4. Relay contact A1 is thereby caused to change over from the position shown in Fig. 3. Thermistor Th controls the conductivity of a transistor T 1 and hence also the charging time of a capacitor C 1 charged via a diode D 1 and the transistor T 1 . A uni-junction transistor UJT 1 is triggered, when the voltage in C 1 reaches the necessary value, and applies a firing pulse to SCR1. The delay angle of this pulse is dependent upon the magnitude of the charging current of C 1 and the time it takes to charge which in turn is dependent on the resistance of Th, The lower the temperature sensed the earlier the firing pulse of SCR 1 will occur and the longer the conduction periods of the SCR and heater, and vice versa. A resistor R 10 is preset to enable the magnitude of compensating effect of Th to be varied to suit operational requirements. Time delay means including C2 ensure that heating will continue after the switch S1 opens on passage of the trailing edge of the sheets. Instead of controlling the lamp current a shutter may be moved across the lamp, the shutter being controlled by a sensor in the form of a bimetal strip and directly or indirectly coupled mechanically thereto.