FS-6020
2-3-5
(2) Fixing heater lamp lighting runaway prevention - fixing thermistor disconnection detection circuit
+
R34
R33
+5V1
+5V1
+5V1
7
5
6
U5-2
393
+
-
+
R38
R37
+5V1
1
3
2
U5-1
393
+
-
CR1
PC4
Q5
3
2
1
Fixing
heater lamp
Fixing
thermistor
Engine/high voltage PWB
Fixing heater lamp ON circuit
Fixing heater lamp ON -
Runaway prevention circuit
Fixing thermistor
disconnection detection circuit
Power
source
PWB
CPU
(U1)
AC power source
THDEAD
HTEMPN
THERM
HEATON
62
76
63
64
Figure 2-3-5 Fixing heater lamp lighting runaway prevention -
fixing thermistor disconnection detection circuit
The fixing thermistor detection voltage (THERM) is input to the CPU (U1) and branched to input to the No.6 pin of he
comparator (U5-1). The comparator compares the THERM signal voltage with the voltage set at the No.5 pin (= the voltage
3.2V that the temperature 230 °C is assumed in the case of unusually high temperature) by the divided voltage of resistors
(R33 and R34). If the voltage at the No.6 pin is higher than that of No.5 pin, the output level of No.7 pin is L. For the HEATON
signal, the circuit that forcibly turns off the fixing heater lamp by the hardware-based method and forcibly makes the
HEATON signal to L level regardless of controls by CPU (U1) is provided because the No.7 pin is connected with the output
line of the fixing heater lamp ON signal (HEATON). This circuit is provided for the purpose of fail safe, it usually monitors the
unusual high temperature in the software system by the input voltage of the THERMA signal that is input to the No.76 pin of
CPU (U1), therefore, the circuit operates the control to turn off the fixing heater lamp before the mentioned circuit is
activated and stops the machine, and then indicates the error code 6020 that shows the fixing high temperature failure
according to the self diagnosis function.
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