Oki OKIFAX4550 Maintenance Manual page 346

2) Fuser unit temperature control
The heater in the fuser unit is controlled by the thermister, IC2(IOGA6) keeps the heater
roller surface within a prescribed temperature range. The CPU1 supervises the status of
THCHK (A/D converter input section) periodically, turning HEATON signal on and off
according to CPU1 of THCHK status to exercise temperature control.
At power on time, the CPU1 switches the output signal THON from pin 65 (between 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 A.28 (3/4) shows the fuser unit temperature control.
Note: Heater control
2)-1
F1
ACIN
TRA1
THCHK
5V
41309401TH Rev.4
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 CPU1.
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.
MPW1446/MPW1546
+5V
ACON
PC4
AOUT
Thermostat
Heater
Thermistor
Fuser unit
Figure A.28 (3/4) Fuser Unit Temperature Control
R46
HEAT-N
Heater
on/off circuit
CN7
0V
10
CN6
13
P2H Board
IC2
HEATON
98
IOGA6
CPU1
THON 65
THON
SH7034
THCHK
87
THCHK
0V
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