Oki OKIPAGE 10ex Troubleshooting Manual page 21

For heater breakdown detection, the heater must first be turned on. When a temperature rise
which corresponds to the switching on of the heater does not occur, then a heater breakdown is
detected. To shorten the breakdown detection time, the following circuit is used. Immediately
after the power is turned on, the thermistor is checked and THERM-CMP signal is turned on to
turn the resistor Q44 on. The reading resolution is increased through the variation of the
thermistor resistance value.
If, for whatever reason, temperature control fails and the temperature rises abnormally, the
abnormal high temperature detection circuit shown below forcibly cuts the power supply to the
fuser.
Thermistor
From CPU
40990901TH Rev.1
5V
Thermistor Breakdown Detection Circuit
R24
1.5k
THERM-CMP
Abnormal High Temperature Detection Circuit
+5V
R26 R28
IC4
1k 1k
+
324
–
R27
1.8k
Converter
R25
100k
Q6
HEAT-N0
+5V
Q7
IC2
A/D
To PC1
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