Heater Control Circuit; Temperature Detection Section - Toshiba e-STUDIO 160 Service Manual

14.6 Heater Control Circuit

14.6.1 Temperature detection section

This section detects the surface temperature of the heat roller in the fuser unit by means of two ther-
mistors (heater thermistor 1 and 2) and turns ON and OFF the heater lamp to keep the fusing temperature
fixed.
• The thermistor has the property of reducing its resistive value at high temperature. Therefore, the
heater control circuit detects that the higher the thermistor resistive value, the lower the heat roller
temperature, and that the lower the resistive value, the higher the temperature. The CPU controls
turning ON and OFF the heat roller so that the detected temperature is within the set range of fusing
temperatures. Finding the detected temperature abnormal, the CPU performs error processing to stop
the supply of power to the heat roller.
• If the heat roller temperature abnormally rises due to short-circuiting of the triac or temperature control
disabled by breaking of the thermistor, etc., the thermostat cuts off the power supply line to the heater
lamp to prevent the fuser unit from being damaged.
The following abnormal conditions are detected:
(1) Thermistor abnormal or heater lamp broken at warm-up time
SERVICE CALL (C41) is displayed if the heat roller temperature is below 40°C (the output voltage of
heater thermistor 1 is 0.1599V) continuously for 500 milliseconds, 25 seconds immediately after the
start of warming-up immediately following the power ON or the start of warming-up at the time of
restoration from the Super Power Save mode.
(2) Thermistor abnormal after error diagnosis during warming-up
SERVICE CALL (C43) is displayed if the heat roller temperature is below 40°C (the output voltage of
heater thermistor 1 is 0.1599V) continuously for 500 milliseconds after the error diagnosis performed
during warming-up ends.
(3) Heater lamp abnormal after abnormal diagnosis performed during warming-up or after the Ready
state
SERVICE CALL (C44) is displayed if the heat roller temperature does not reach the ready set tem-
perature even 205 seconds after the start of warming-up, or if the temperature at the center of the heat
roller is above 230°C (the output voltage of heater thermistor 1 is 3.09V) continuously for 500 millisec-
onds or below 100°C (the output voltage of heater thermistor 1 is 0.6407V) continuously for 500
milliseconds, after the Ready state.
(4) Thermistor abnormal at the heat roller ends
SERVICE CALL (C45) is displayed if the heat roller end temperature is above 270°C (the output
voltage of heater thermistor 2 is 2.72V) continuously for 500 milliseconds or below 40°C (the output
voltage of heater thermistor 2 is 0.0702V) continuously for 500 milliseconds, after the Ready state.
e-STUDIO160/200/250 FUSER UNIT/EXIT UNIT 14 - 6 December 2002 TOSHIBA TEC