Canon Color iR C3480, Service Manual

Page: 351 / 774

Chapter 9
9-12
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
F-9-8
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.
RDRV
HTDRV1
HTDRV2
H1
TP
H2
SUB THRM2
SUB THRM1
MAIN THRM
TH1
TH2 TH3
ZEROX
THERR
SOFTRON
Triac
Zero Cross
Detector PCB
AC Driver PCB
Relay
DC Driver PCB
DC Controller
PCB
Safety Circuit