Vi-5. Power Supply Unit - Canon A-200 series Service Manual

VI-5. Power Supply Unit
The 230V AC power sent through the AC inlet on the rear panel goes through fuse FS1, noise
filter L 1 then enters bridge diode D1. This AC power source is rectified and smoothed by 01,
C6 and C7. Then this DC power is sent to the primary coil of oscillation transformer T1 via the
transistor TR3. This transistor is controlled by IC1 on the sub printed board to regulate the
+
5V output voltage.
Triac TRK1 and resistor R2 protect the circuit against excessive current flow. When the AC
power switch is turned on, a surge current is fed through resistor R2 and then TRK1 is turned
on. Even if TRK1 does not operate, the internal thermal fuse of R2 is blown to protect the
circuit.
IC1 (MPC494C) on the sub printed board controls TR3. At the first pin of this IC, it monitors
+
5 VDC output voltage which is obtained by photo-coupler PC2. IC1 then makes TR3 control
the pulse width and frequency.
The
+
12 V power is generated by adding this
+
5 V and
+
7 V generated by 022 and C24.
The
+
5 V power supply has a over voltage protection circuit which is controlled by photo
coupler PC1. When an over voltage is detected, the photo coupler PC1 activates and then
control voltage for IC1 on the sub printed board is cut off.
IC2 acts as an over current protector for the
+
5 V and
+
12 V power supply lines. This IC
detects voltage drops at R53 and R52 which are inserted at the
+
5 V power supply line and
at R51 for
+
12 V power supply line. If an excessive current greater than the specified value,
IC1 -reduces the pulse width of the signal to be sent to TR3 and as a result, the voltage of the
+
5 V line drops.
This power supply also has a protection circuit against short circuiting of the output. Ther­
mostat TH1 is mounted on the heatsinker together with 021, and when the temperature of the
heatsinker reaches to 95 degrees centigrade, photo coupler PC1 activates, and finally, inter­
nal oscillation of the IC1 stops.
The -12 V voltage is regulated by the three-terminal voltage regulator IC1 and another con­
trol voltage for IC1 on the sub printed board is regulated by TR1 and Z01 on the sub printed
board.
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