Heater Control Circuit - Toshiba 4560 Service Manual

12.4 Heater Control Circuit

12.4.1 Temperature detection unit
To maintain the heat roller temperature, this unit detects the heat-roller temperature by means of ther-
mistor-1, and then performs heater lamp ON/OFF control.
The thermistor is an electrical element whose resistance decreases as its detected temperature in-
creases. If the thermistor is open-circuited, the control circuit erroneously determines that the fuser tem-
perature is extremely low and continues to energize the heater lamp. As a result, the fuser temperature
rises abnormally high, possibly activating the thermostat provided for safety purposes. The CPU works
to detect the breakage of the thermistor and prevents the condition mentioned above.
The following abnormalities are detected:
(1) Abnormal detection during warming up
Maximum 25 sec. after the power has been turned ON, if the thermistor-1 output voltage does not
exceed 0.078 V, CALL SERVICE (C41) is displayed.
(2) Abnormal detection during standby/copying (Thermistor-1)
If the thermistor-1 output voltage decreases to and remains at 0.078 V or lower for 260 msec. or
more, CALL SERVICE (C43) is displayed.
(3) Abnormal detection during standby/copying (Thermistor-2)
If the thermistor-2 output voltage decreases to and remains at 0.192 V or lower for 260 msec. or
more, CALL SERVICE (C45) is displayed.
(4) C40 error counter control
• For improved fuser-unit safety, the CPU works as follows: After the C41 error occurs two times in
succession, even when the power is turned ON, the heater will not come ON and the C41 error will be
immediately displayed.
• However, with "1" or less stored in the C40 counter, if the copier becomes ready correctly, the C40
counter clears to "0".
• After CALL SERVICE (C41 – C45) has occurred and the thermistor, heater lamp, etc. have been
repaired (or replaced), if turning ON the power switch does not cause the heater to energize resulting
in a C40 error, check the C40 counter (08-code 89) and reset it to "0".
4560/4570 FUSER 12 - 4 Nov. 1997 © TOSHIBA