Oki OKIOFFICE 84 Service Manual page 405

Figure A3.1.15 Sensor and Switch Control
2) Fuser unit temperature control
The heater in the fuser unit is controlled by the thermistor, IOGA and CPU to keep the heater roller surface within a
prescribed temperature range. The CPU supervises the status of PC0 (A/D converter input section) periodically,
turning HEATON- N signal on and off according to CPU of PC0 status to exercise temperature control.
At power on time, the CPU switches the output signal THON from pin 65 (between high and low states) to check for
a blown or shorted thermistor according to the status of the THCHK signal.
A built-in thermostat in the fuser unit prevents the heater from being overheated in event of failures in the thermistor,
or temperature control circuit, etc.
Figure A3.1.16 shows the fuser unit temperature control.
Note:
Heater control
Temperature of the heater at the time of printing is 150 °C to 180 °C. This tempera-ture is maintained by controlling
the on and off operation of heater according to the input of the thermistor converted into analogue-digital (A/D)
values by the CPU.
2)-1 Heater control
The ACON control signal on the secondary side is transmitted to the primary side via PC3 to turn TRC1ON for AC
voltage supply to the heater.