Epson Stylus Color 1520 Service Manual page 60

Figure 2-18 illustrates the electrical circuit diagram.
Primary Circuit
Full Wave
Smoothing
Rectifier
Circuit
Circuit
Filter
Fuse
AC Input
The operating principles for the various protection and controller circuits shown above are described below.
Y
+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, information is fed back to the primary switching circuit through the transistor (Q82) and
Photo coupler (PC1) to cut the +42 V line to the regulator (OC51).
Y
+5 VDC line constant voltage control circuit
Voltage in the +5 VDC line is controlled by the regulator IC (IC51). When abnormal voltage in the +5
VDC line is detected, information is input to the internal comparator of the regulator so it controls the
voltage.
Y
+42 VDC line over-voltage protection circuit
The output level of the +42 VDC line is monitored by 2 Zener diodes, ZD52 and ZD87. The circuit
operates as follows:
S
Output level of the +42 VDC line exceeds +48 V
S
Thyristor (CY52) goes ON
S
Transistor (Q81) goes ON
S
Photo coupler (PC21) goes ON
S
Switching FET operation on the primary side goes OFF
Y
+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 Photo coupler (PC1) to the primary switching circuit to
control the on/off time of the switching FET for constant output voltage.
Y
+42 VDC line overcurrent protection circuit
The output current is monitored by the transistors Q81 and Q82. When the output current is abnormally
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
2-17