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EN AC and DC power distribution 139
3
Overcurrent/overvoltage protection
If a short-circuit or other problem on the load side causes an excessive
current flow or generates abnormal voltage, the overcurrent/
overvoltage protection systems automatically cut off the output voltage
to protect the power-supply circuit.
If the overcurrent or overvoltage protection system are activated and
the power-supply circuit does not generate DC voltage, it is necessary
to turn the power off, correct the problem, and then turn the printer on
again.
The circuit has two fuses (FU1, FU2), which break and cut off the output
voltage if overcurrent flows through the AC line.
Fuser over-temperature protection
The fusing-heater safety circuit is located on the engine controller board
and constantly monitors the fusing temperature.
To protect the fuser from excessive temperatures, the printer has the
following three protective functions:
• The CPU monitors the voltage of the FSRTH1 and FSRTH2
thermistor signals. If the fuser temperature reaches 240° C (464° F),
then the CPU turns off the relay (RL101) to interrupt the power to
the fusing heater.
• If the temperature of the fusing heater continues to rise abnormally,
and the temperature of the thermal switch (TP101) exceeds about
250° C (482° F), TP101 opens up to cut off the power supply to the
fusing heater.
• The power supply on the coil side of RL101 and RL102 is connected
to +24 VB. When the top cover is opened, the power supply to
RL101 and RL102 is cut off, and the relay is turned off.