92 Theory of operation EN
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.
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