Epson Stylus COLOR 1520 Service Manual page 59

Figure 2-18 illustrates the electrical circuit diagram.
Primary Circuit
Full Wave
Smoothing
Rectifier
Circuit
Circuit
Filter
Fuse
AC Input


+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
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
Switching
Circuit
(Q1)
Photo-coupler
PC1
Figure 2-18. Power Supply Circuit Diagram
Secondary Circuit
Half Wave
Rectifier
Circuit
Chopping
Regulator
(IC51)
+42 V Line Detector
Circuit (ZD51, 81 to 86)
+42 V Line Over Current
Protection Circuit
(Q81, Q82)
+42 V Line Over Voltage
Protection Circuit
(ZD52, ZD53, ZD87, Q82, Q83)
+5 V Over Voltage
Protection Circuit
(ZD53, Q82)
Power Off Delay Circuit
(ZD88, C84, Q84)
Power Off Signal
Operating Principles
+42 V
+5 V

ZD86). This circuit feeds
2-17