Oki OKIFAX 5400 Maintenance Manual page 299

E17
2)
CPU
HEATON
THON
THCHK
40838301TH Rev.1
Fuser unit temperature control
The heater in the fuser unit is controlled by the thermister, CPU to keep the heater roller
surface within a prescribed temperature range. The CPU supervises the status of the
port THCHK periodically, turning HEATON signal on and off according to CPU of
THCHK (A/D converter input section) status to exercise temperature control.
At power on time, the CPU switches the output signal THON from pin 59 (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 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 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 CPU.
E17
TR502
HEATON
Heater
control
27
HEAT-N
TR506
THON
0 V
59
THCHK
47
Figure A3.1.16 Fuser Unit Temperature Control
SW1
CN1
ACIN
B7
5 V
PC1
HEAT-N
A12 THERM
5 V
A3 - 18
1VP/2VP (Power supply unit)
L101, R1, etc.
F1
Filter
C4
L2
D1
AOUT
Thermostat
Heater
Thermister
Fuser unit