Circuit Explanation; Power Supply Circuit - Casio 200CR Service Manual

5. Circuit explanation

5-1. Power supply circuit

PWD
F1
A
2
CN1
TRANS
1
5277-02A
Q1 : Transistor for controlling voltage (VP)
Q1 : Transistor for voltage detection
Q3 : Transistor for controlling power down (PWD) signal
ZD2 : Zenner diode for controlling base current of Q2
ZD1 : Zenner diode for detecting power down (PWD) signal
After plug the AC cord into the outlet, the 24 volts of AC voltage appears between T1 and T2.
Then this voltage is rectified by the diode bridge and it become constant voltage through the
capacitor C1.
[Voltage VP]
A constant voltage (17.9V) appears on the C point. This voltage made from the constant voltage
circuit consist of transistor Q1 and Q2, zenner diode ZD2, resistor R2, R3, R4 and R5.
[Voltage VLED and VCC]
The voltage goes to 1 pin of regurator IC1 from B point and out from 2 pin of IC1 as stable voltage
5.5 volts. Then this voltage down to 5 volts through the diode.
It is used for VCC (for logic) and VLED (for display drive).
Also, it goes to 4 pin of IC1 and using to control the output voltage of 2 pin.
Capacitor C5 is used to reduce the ripple of this voltage.
Inductor L1 is used to stabilize the VCC voltage when the drawer opend.
B
D5
1SR35-100A
D
E
1SR35-100A
D2
C
R9
10K
WITH HEAT-SINK
D1276A
Q1
H
R7 Q3
4.7K
C945
D8
Q2
C945
1S2473
F G
— 10 —
VDD
I
LM2575T-5.0
5 ON/OFF
VIN1
VIN VOUT
2 OUTPUT
IC1
4 FEED BACK
GND
3
L1 L331K
GND
VP
J
VLED
VCC
D10 1S2473
K