2. Protective function
The printer has three protection functions to prevent abnormal temperature rise of the fixing
roller.
• The CPU monitors the ther mistor voltage. If the thermistor voltage becomes lower than
about 1.0V (230°C), the CPU identifies the fixing unit failure, and interrupts power to the
fixing unit and reports it to the video controller.
• If the sur face temperature of the upper fixing roller rises abnormally and the thermistor volt-
age falls below about 0.8V (235°C), the safety circuit in the power supply interrupts power
to the fixing roller heater regardless of outputs from the CPU.
• If the sur face temperature of the upper fixing roller rises abnormally and the thermoswitch
temperature exceeds 200°C, the thermoswitch turns off and cuts power to the fixing roller
heater.
3. Fixing unit failure detection
The CPU assesses a fixing unit failure in the following cases and reports it to the video controller.
a. Thermistor broken wire detection
The detected voltage of the thermistor is 4.6V or more when the thermistor voltage detection
mode (note) is in the error detection mode after the fixing heater is activated.
b. Maximum temperature detection
The fixing temperature rises abnormally and the temperature detected by the thermistor exceeds
about 230°C, during standby or printing.
c. Warmup error detection
The fixing unit does not reach the target temperature (see Table 2-2-1) within 180 seconds after
the start of warm-up.
d. Broken wire detection after the fixing unit is ready
The temperature detected by the thermistor is 150°C, or below during standby or printing.
e. Heater current detection
• The CPU detects a fixing heater broken wir e if the FIXING HEATER CURRENT SENSE signal
(FSRCT) level is same as the all heaters OFF level when the fixing main heater or/and fixing
sub heater is/are driven.
• The CPU detects a fixing heater excursion if the FIXING HEATER CURRENT SENSE signal
(FSRCT) level is higher than the fixing main heater and fixing sub heater ON level when the
fixing main heater or/and fixing sub heater is/are OFF.
• The CPU detects a fixing heater excursion if the FIXING HEATER CURRENT SENSE signal
(FSRCT) level is higher than the specified value when both heaters are OFF.
Note: There are two thermistor voltage detection modes: error detection mode and normal
mode. In the error detection mode, sensitivity in the initial range (low temperature
range) is increased to improve detection accuracy. When the thermistor temperature
exceeds 60°C , it changes to the normal mode.
If the CPU detects a fixing unit failure, it does the following:
1) Stops the fixing roller heater.
2) Sets the RELAY DRIVE signal (RLD) to “L”, and turns the relay in the power supply OFF to
interrupt the power to the fixing roller heater.
3) Reports the fixing unit failure to the video controller.
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CHAPTER 2