Epson Stylus COLOR 800 Service Manual page 54

EPSON Stylus &2/25 800
The operating principles of various protection circuit and controller circuit in the figure above are described
below.
‰+5 VDC line over voltage protection circuit
The output voltage level of +5 V is monitored by a Zener diode (AD53) on the secondary circuit. If the
voltage level exceeds 9 V, the switching FET Q1 goes Off in the following order, and no induced
electromotive is generated and production of +5 VDC and +42 VDC stops as the result.
ƒ Zener diode (ZD53) detects voltage which exceeds +9 V at +5 V line.
ƒ Transistor Q81 goes On.
ƒ Photo coupler PC1 goes On.
ƒ FET Q31 goes On and the gate current for the switching FET Q1 is cut off.
ƒ Switching FET Q1 goes Off.
‰+5 VDC line constant voltage control circuit
Voltage at the +5 VDC line is monitored by the regulator IC51. Abnormal voltage at the +5 VDC line is
detected and the information is fed back to the +5 V conparator in the IC, then +5 VDC is controlled or
cut off.
‰+5 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 Q1 goes Off in the following
sequence.
ƒ Zener diodes (ZD52, ZD87) detect the voltage over 48 V at the +42 V line.
ƒ Transistor Q81 goes On.
ƒ Photo coupler PC1 goes On.
ƒ FET Q31 goes On and the gate current for the switching FET Q1 is cut off.
ƒ Switching FET Q1 goes Off.
‰+42 VDC line constant voltage control circuit
Voltage at the +42 VDC line is monitored by the Zener diodes ZD51 and ZD81 to ZD86. When the
voltage at the +42 VDC line exceeds, the Switching FET Q1 is controlled in the following sequence.
ƒ Zener diodes (ZD52 and ZD81 to ZD86) detect the voltage over 38V at the +42 VDC line.
ƒ Transistor Q81 goes On.
ƒ Photo coupler PC1 goes On.
ƒ The transistors Q3 and Q2 go off and gate current for the switching FET Q1 is cut off.
ƒ Switching FET Q1 goes Off.
ƒ When the voltage level drops under +38 V, the photo coupler PC1 and transistors Q3 and
Q2 go Off and the swathing FET Q1 goes back On.
‰+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 information is fed back to the primary circuit via the photo coupler PC1 to stop the
switching operation.
2-14
Figure 2-15. VDC Circuit Block Diagram
Rev. A