Fuser Unit Control Circuit - Toshiba e-STUDIO 5540C Service Manual

3.15.3 Electric Circuit Description

[ 1 ] Fuser unit control circuit

[ 1-1 ] Configuration
This equipment employs an external IH coil unit for heating the fuser belt and two or three (center, side
and sub) heater lamps with a different light emitting (heating) point in the pressure roller. The sub
heater lamp is included only for the triple type. IH coils in the IH coil unit generate a magnetic field to
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heat the fuser unit with a high-frequency current carried inside of them. The heater lamps heat the
pressure roller by turning themselves ON with a command from the ASIC of the LGC board.
The surface temperature of the fuser belt is detected with the fuser belt edge thermistor (THM6),
together with the fuser belt thermopile (THMP1: non-contact sensor). The surface temperature of the
pressure roller is detected with the pressure roller center thermistor (THM3), the pressure roller side
thermistor (THM4), the pressure roller edge thermistor (THM5). The detected temperature data are
sent to the ASIC through an A/D converter. Based on the sent temperature data, the ASIC then turns
the IH coils and the heater lamps ON or OFF to control the surface temperatures.
When the surface temperature of the fuser belt or the pressure roller exceeds the preset temperature
as a result of overheating detection by each thermistor or thermopile, the forcible power OFF circuit
sends a power supply relay OFF signal as well as an overheating signal to the ASIC and the heater
lamp control circuit, and then shuts off the power supply over all the parts, except for the control panel.
If the heater lamp control circuit does not operate due to problems such as thermistor malfunction and
therefore the fuser belt or the pressure roller is abnormally heated, the pressure roller center thermostat
(THMO2), pressure roller side thermostat (THMO3) and fuser belt center thermostat (THMO4) shut off
the power supply to the IH coils and the heater lamps to protect the equipment.
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