Sharp SF-A57 Service Manual page 39

[f] Paper feed motor (DFM) current limiting circuit
This circuit limits the motor start current to a constant level, and is composed of the resistor for detection of the current value and the voltage
comparator.
The negative voltage side of the paper feed motor is connected to the;pickup resistor composed of R97, R98, and R99, which converts the current
flowing in the driver circuit into a voltage value.
The converted voltage value is compared with the reference voltage by IC9 comparator.
The reference voltage is obtained by dividing the zenor voltage generated from R84 and ZD2 with R39 and R67. It is about 0.3V. When the
converted voltage value exceeds the reference voltage (about 2.8A in the current value), IC9 14 pin turns LOW and the signal is inputted to the G/A
62 pin, interrupting +24V supply to the motor. As a result, the current value is limited.
When the converted value falls below the reference value, +24V is supplied again to resume conduction to the motor.
Since the operating current of this circuit is considerably great,
it operates only when starting the motor and does not operate in the normal state.
[g] Rush current limiting circuit
This circuit limits a rush current which flows into the current generation capacitors (C53, C54) in the transport motor (DTM)/reverse motor (DRM)
drive circuit, and is composed of the limiting resistor and the FET which allows to flow a constant current.
After closing the ADF open/close switch (AUOD) and the reverse guide open/close switch (TGOD) and until the cathode voltage of ZD3 reaches
16V, the base current is not supplied to Q10, which is kept OFF, driving IC4 3 pin HIGH.
At that time, the base current of Q9 is not supplied to turn off Q9, turning off Q3, flowing a current through R92.
On the other hand, when the cathode voltage of ZD3 exceeds 16V, the base current of Q10 flows to conduct the transistor, turning on Q13. As a
result, the current which was flowing through R92 flows through Q13 to cancel current limiting.
R25 is the discharging resistor which discharges electric charge in C53 and C54 when the ADF open/close switch (AUOD) or the reverse guide
open/close switch (TGOD) is opened.
G/A
+5V
(IC7)
R109
TP27
4.7K
62
MICUR
FD+24V
ZD2
12V
PGND
Paper feed motor current control circuit
DC+24V
Power supply
from main body
ZD3
16V
Rush current prevention circuit
TP28
IC9-3
R20
8
14
10K
9
C7
R97
µ PC339G2
1000PF 0.39
R84
R37
2.2K
10K
R39
24K
R67
750
R92
4.7
2SJ265
R83
+5V
1.0K
R68
4.7K
IC4-6
R105
13 12
Q10
20K
C56 TC74HC14AF
0.22µ F
FA1L3N
– 38 –
From paper feed motor
drive circuit
R98 R99
0.39 0.39
PGND
Q13
+24V
R25
4.7K
R106
PGND
2.0K
Q9
FA1L3N