Q1860-90918 Chapter 5 Functional information 73
If a short circuit or other problems on the load side cause an excessive current flow from the
+24 Vdc or +3.4 Vdc power supplies or generate abnormal voltage, the excess-current
protection system and excess-voltage protection system automatically shut off the output voltage
to protect the power supplies.
If an excess-current or excess-voltage protection system is activated and the power supply
circuit does not output dc voltage, it is necessary to turn the power off, correct the problem in the
faulty load, then turn the printer on again.
The circuit has two fuses that break and cut off the output voltage if excess-current flows through
the ac line.
Overcurrent overvoltage protection
This circuit, located on the dc controller PCA, always monitors an abnormal rise in the fusing
roller temperature. If an error occurs, this circuit puts out a signal to turn the relay (RL1) off and
interrupt the power to the fusing roller heater. The circuit operates as follows.
When the fusing roller heater temperature rises and the output voltage of the thermistor exceeds
about 0.5 V (230° C), pin 7 of the comparator (IC304) goes “L.” IC304 pin 1 goes “L,” Q4 turns
off, and then the relay (RL1) turns off.
The printer has the following three protection functions to prevent incorrect activation of the
fusing-roller heater.
The central processing unit (CPU) monitors the thermistor voltage. If it is abnormal, the CPU
identifies the fusing-roller heater error, turns the relay (RL1) off, and reports it to the
formatter.
If the fusing-roller heater temperature rises abnormally and the thermistor voltage falls below
about 0.6 V (220° C), the safety circuit in the fusing-roller heater interrupts the power to the
fusing heater regardless of the CPU output.
If the fusing-roller heater temperature rises abnormally and the thermal fuse temperature
exceeds 240° C, the thermal switch turns off and interrupts the power to the fusing-roller
heater.
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