Toshiba e-studio550 Service Manual page 216

14. 4. 6. Detection of open-circuited thermistors
A thermistor is a device which resistance decreases when it detects a high temperature. If the fuser roller
thermistor is open-circuited, the control circuit finds that the fuser roller temperature is extremely low and
keeps turning ON the IH coil. As a result, the fuser roller temperature rises, possibly activating the thermostat,
which is a safety protection device. To prevent this, CPU works to detect if the thermistor is open-circuited
or not.
* The following is a circuit diagram of the thermistor open-circuit detection circuit common with the center
and side thermistors.
+5
R1
Thermistor
R2
C
R3
e-STUDIO550/650/810 FUSER UNIT
Main CPU
R4
· The input voltage is obtained by dividing the resistor
1, thermistor, resistor 2 and resistor 3.
· The temperature of the fuser roller surface changes.
The resistance of the thermistor changes.
The input voltage changes.
· The main CPU detects this change of the input
voltage to decide if the thermistor is abnormal or not.
14 - 10 JUNE 2002 © TOSHIBA TEC