Epson Stylus COLOR 3000 Service Manual - Page 55

Operating Principles Figure 2-14 illustrates the electrical circuit diagram. Figure 2-14. Power Supply 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 off the +42 V line to the regulator (IC51). +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 (PC1). +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 to constant output voltage level. +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. 2-14 Rev. A