Circuit Operation - Epson FilmScan 200 Service Manual

2.2.2 Circuit Operation

Figure 2-13 shows the power supply circuitry in block diagram form. AC power feeds into the scanner from
the external power source. A filter circuit removes the noise. The AC voltage then undergoes full-wave
rectification and is smoothed to produce the direct current supply voltage.
This voltage is fed through a switching circuit and secondary smoothing circuit to produce a +24 VDC, +12
VDC, and -12 VDC supply. A +24 V line voltage detector circuit is connected to the switching circuit. This
feedback control arrangement ensures that the +24 VDC supply is kept regulated.
.
The +12 VDC and -12 VDC are regulated by local regulator ICs.
The +5 VDC supply is created by tt2eding the +24 VDC line through the +5 VDC power supply circuit. This
circuit further steps down the +24 VDC voltage and outputs a stabilized +5 VDC supply.
There are two main features of the power supply circuit. First, the power supply switch is in the secondary
circuitry. When this switch is tuned off, the switching circuit is de-energized and output of the +24 VDC,
+12 VDC, and -12 VDC supply stops. However, since the switch is in the secondary circuitry, while the
scanner remains plugged into the external AC supply, current continues to now in the primary circuitry,
whether the power supply switch is tuned oft or on. For this reason, before you perform any maintenance
work, you must unplug the scanner from the external AC power outlet.
Second, there are three circuits to protect the supply circuitry and avoid danger. The +5 VDC line contains
a voltage overload protection circuit. The +5 V voltage overload protection circuit cuts the supply if the
voltage reaches or exceeds +7 VDC. It stops the switching circuit operation, which stops the output of the
+24 VDC line.
The +24 VDC line has a voltage overload protection circuit and a voltage drop protection circuit.
The +24 V voltage overload protection circuit cuts the supply it the voltage reaches or exceeds +30 VDC.
It stops the switching circuit operation, which stops the output of the +24 m line. The voltage drop
protection circuit protects the scanner from such damage as might occur from short circuiting in the
secondary circuitry of the +24 VDC line. If a voltage drop is detected, it stops the switching circuit
operation, which stops the output of the +24 VDC line.
Full wave
rectification
circuit
Filter
circuit
AC IN
Rev. A
Smoothing Switching
circuit circuit
Figure 2-13. Power Supply Circuit Block Diagram
Smoothing +12 VDC
circuit regulator
Smoothing −12 VDC
circuit regulator
Smoothing
circuit
+5 VDC
regulator
+ 5 VDC line
overload
voltage
protection
+24 VDC
line
control
+24 VDC line
overload
voltage
protection
+24 VDC line
voltage
drop
protection
OPERATING PRINCIPLES
+12 VDC
− 12 VDC
+24 VDC
+5 VDC
Operate
Switch
2-9