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Fusing Section Main Unit Theory of Operation
2-116
II Composition/Operation
16.4.9 Protection against abnormally high temperature
• The machine provides protection at three different stages to prevent abnormally high
temperature of the Fusing Unit.
1. Soft protection
2. Hard protection
3. Thermostat protection
A. 1st stage: soft protection
• If the Heating Roller Thermistor (TH1) and the Fusing Pressure Roller Thermistor (TH2)
detect a temperature exceeding a predetermined value, the malfunction code represent-
ing abnormally high temperature is displayed. At this time, the Heater Lamps are turned
OFF forcibly and the initiation of any new print cycle is prohibited (abnormally high tem-
perature detection control).
B. 2nd stage: hard protection
• The following hard protection control is provided if the CPU overruns and becomes
unable to detect an abnormally high temperature.
1. The Heating Roller Thermistor (TH1) and Fusing Pressure Roller Thermistor (TH2)
detect a temperature exceeding a predetermined value.
2. The Remote signal for the corresponding Heater Lamp of the Low Voltage Unit (LV) is
forcibly turned OFF through the Mechanical Control Board (PWB-A).
3. The triac on the Lower Voltage Unit (LV) is turned OFF to shut down the power supply
to the corresponding Heater Lamp.
4. The Heater Lamp is forcibly turned OFF.
5. The temperature detected by the Heating Roller Thermistor (TH1) and Fusing Pressure
Roller Thermistor (TH2) is decreased to a level below the predetermined value.
6. The Remote signal forcible OFF of the corresponding Heater Lamp is reset so that
power supply to the Heater Lamp is resumed. This operation is repeated until the CPU
recovers to be able to detect the abnormally high temperature condition. This control
allows the power supply to the corresponding Heater Lamp to be shut down before the
thermostat operates, inhibiting the Fusing Unit from being damaged.
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