Sharp FO-90A, Service Manual

Page: 42 / 74

FO-90AAR
[4] Circuit description of power supply PWB
1. Block diagram
2-1. Noise filter circuit
The input noise filter section is composed of L1, C1, C15 and C21 which
reduces normal mode noise from the AC line and common mode noise
to the AC line.
2-2. Rectifying/smoothing circuit
The AC input voltage is rectified by diode D1, 2, 3, 4 and smoothed by
capacitor C2 to supply DC voltage to the switching circuit section.
Power thermistor TH1 suppresses inrush current at power switch-on.
2-3. Switching circuit
This circuit includes MOS FET Q1 and the gate drive circuit, and com-
ponents around Q1.
In this circuit, the DC voltage supplied from the rectifying/smoothing sec-
tion is converted into high frequency pulses by ON/OFF repetition of
Q1.
[5] Circuit description of CIS unit
1. CIS (Contact Image Sensor)
Cis is an image sensor which puts the original paper in close contact
with the full-size sensor for scanning, being a monochromatic type
with the pixel number of 1,728 dots and the main scanning density of
8 dots/mm.
It is composed of sensor, rod lens, LED light source, light-conductive
plate, control circuit and so on, and the reading line and focus are
previously adjusted as the unit.
Due to the full-size sensor, the focus distance is so short that the set
is changed from the light weight type to the compact type.
Fig. 9
Fig. 8
5ms
1.984
µ
s
0V
2V(TYP)
(White original paper)
Approx.5V
øT
CISCLK
VO
5 – 12
2. Waveforms
The following clock is supplied from FC200 of the control board, and
VO is output.
2-4. Control circuit
This circuit controls output voltage of 24V by adjusting ON period of Q1,
looking at signal from photo-coupler PC1.
In this operation IC1 takes charge of important part.
The over current protection is performed by bringing Q1 to OFF state
through detection of voltage of T1 subwiding.
The over voltage protection is performed by operating the over current
protection circuit through detection of zener diode ZD4 and short-circuiting
of load.
2-5. +5V circuit
DC voltage supplied by rectifying the output of transformer T1 with diode
D8 is stabilized by 3-terminal regulator IC1.
Noise
Filter
Circuit
Rectifying
Smoothing
Circuit
Switching
Circuit
(RCC system)
Regulator
Circuit
+5V
+24V
AC IN
Photo Coupler
F1
1.6A/250V
F2
1.6A/250V
Control
Circuit