Oki OF5600, Service Manual

Page: 322 / 448

Board Descriptions A - 38 OF 5300/5600 Serv Man, P/N 59278301
2) Fuser unit temperature control
The heater in the fuser unit is controlled by the thermister, IOGA3 and CPU to keep
the heater roller surface within a prescribed temperature range. The CPU super-
vises the status of the port PC0 periodically, turning HEATON signal on and off
according to CPU of PC0 (A/D converter input section) status to exercise tempera-
ture 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 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 A3.2.18 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 thermister converted into analogue-digital (A/D) values by the CPU.
Filter ACIN
SW1
F1
L101, R1, etc.
IC2
HEAT-N
THERM
5 V
5 V
AOUT
C4
L2
PC1
D1
1VP/2VP (Power supply unit) M17
Thermostat
Thermister
Heater
Fuser unit
CPU
IOGA
0 V
PA11
PC0
Heater
control
HEATON
CN1
B7
HEAT-N
TR506
THON
65
87
THCHK
A12
IC18, TR502
Fuser Unit Temperature Control (Figure A3.2.18)