Epson Stylus COLOR 1520 Service Manual page 59

Figure 2-18 illustrates the electrical circuit diagram.
‰+5 VDC line over voltage protection circuit
The output voltage level of the 5 V line is monitored by a Zener diode (ZD53). If the voltage level
exceeds 9 V, the status is fed back to the primary switching circuit through the transistor (Q82) and
photocoupler (PC1) to cut the +42 V line to the regulator (OC51).
‰+5 VDC line constant voltage control circuit
Voltage at +5 VDC line is controlled by the regulator IC (IC51). When the abnormal voltage at +5 VDC
line is detected, the status is input to the internal comparator of the regulator to control the voltage.
‰+42 VDC line over voltage protection circuit
The output level of the +42 VDC line is monitored by the 2 Zener diodes ZD52 and ZD87. When the
output level of the +42 VDC line exceeds +48 V, the switching FET operation on the primary side is
stopped via the thyristor (CY52), transistor (Q81) and photocoupler (PC21).
‰+42 VDC line constant voltage control circuit
Voltage at the +42 VDC line is monitored by the Zener diodes (ZD51, ZD81 ZD86). This circuit feeds
back the output voltage level status through photocoupler (PC1) to the primary switching circuit to
control the on/off time of the switching FET for constant output voltage.
‰+42 VDC line overcurrent protection circuit
The output current is monitored by the transistors Q81 and Q82. When the output current is
abnormally low, the status is assumed to be a short circuit and the information is fed back to the
primary circuit to stop the switching FET operation
Rev. A
Figure 2-18. Power Supply Circuit Diagram
Operating Principles
2-17