Oki OKIOFFICE84 Maintenance Manual page 287

2) Fuser unit temperature control
The heater in the fuser unit is controlled by the thermister, IOGA4 keeps the heater
roller surface within a prescribed temperature range. The IOGA4 supervises the
status of THCHK (A/D converter input section) periodically, turning HEATON signal
on and off according to IOGA4 of THCHK status to exercise temperature control.
At power on time, the IOGA4 switches the output signal THON from pin 59 (be-
tween high and low states) to check for a blown or shorted thermister 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 thermister, or temperature control circuit, etc.
Figure A3.1.14 shows the fuser unit temperature control.
Note: Heater control
2)-1 Heater control
The ACON control signal on the secondary side is transmitted to the primary side
via PC4 to turn triac TRA1 for AC voltage supply to the heater.
F1
ACIN
TRA1
THCHK
5V
Figure A3.1.14 Fuser Unit Temperature Control
Temperature of the heater at the time of printing is 150 ˚C to 180 ˚C. This
temperature is maintained by controlling the on and off operation of heater
according to the input of the thermister converted into analogue-digital (A/D)
values by the IOGA4.
MPW1446/MPW1546
+5V
ACON
PC4
AOUT
Thermostat
Heater
Thermistor
Fuser unit
A3 - 17
R51
HEAT-N
Heater
on/off circuit
CN7
0V
10
CN6
13
P2H Board
IC2
HEATON
27
IOGA4
IC2
THON
59
THON
IOGA4
THCHK
47
THCHK
0V
40672201TH Rev. 4
R51