Canon Color iR C3480 Service Manual page 351

Chapter 9
The machine is equipped with the following mechanisms that detect overheating of the fixing assembly to cut the power to the heater:
- cutting off the power using the DC controller (CPU)
- cutting off the power using a safety circuit
- cutting off the power using a thermal switch
TH2
TH1
H1
MAIN THRM
SUB THRM2
SUB THRM1
Cutting Off the Power (thermistor) Using the DC Controller (CPU)
The CPU monitors the output voltage (MAIN_THRM,SUB_THRM1,SUB_THRM2) of the main thermistor/sub thermistor 1/sub thermistor 2; when the voltage
of MAIN_THRM is about 0.41 V or lower (equivalent of 260 deg C or more), or that of SUB_THRM1/SUB_THRM2 is about 0.77 V or less (equivalent of 260
deg C or more), the machine assumes a fault in the fixing assembly and executes the following:
- Go low the fixing heater drive signal (HEAT_ON1, HEAT_ON2), thus turning off the heater.
As a result, the triac (of the AC driver PCB) goes off to cut the power to the heater.
- Go low the relay drive signal (SOFTRON), thus the safety circuit on the DC driver PCB turns to low the relay drive command (RDRV).
As a result, the relay turns off to cut off the power to the heater.
Cutting Off the Power Using the Safety Circuit
The machine monitors the output voltage (MAIN_THRM, SUB_THRM1, SUB_THRM2) of the main thermistor/sub thermistor 1/sub thermistor 2. If the output
of MAIN_THRM is about 0.38 V or less (equivalent of 258 deg C or more), or that of SUB_THRM1/SUB_THRM2 is about 0.7 V or less (equivalent of 255 deg
C or more), the safety circuit on the DC driver PCB outputs the themistor error signal (THERR) to the DC controller PCB.
- The DC controller PCB makes the heater drive signal (HTDRV1,HTDRV2) "L" and turns off the fixing heaters.
Triac (in the AC driver PCB) is turned off, energizing to the fixing heater is intercepted.
- The safety circuit in the DC driver PCB makes the relay drive signal (RDRV) "L". The relay is turned off, and energizing to the fixing heater is intercepted.
Cutting Off the Power Using a Thermal Switch
If the fixing heater overheats and the reading of the thermal switch exceeds about 270 deg C, the thermal switch (TP1) goes off to cut the power to the fixing heater.
9-12
TH3
Zero Cross
TP
Detector PCB
H2
Relay
Triac
AC Driver PCB
DC Driver PCB
ZEROX
RDRV
Safety Circuit
HTDRV1
HTDRV2
F-9-8
THERR
SOFTRON
DC Controller
PCB