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45

The Gate Array (IC301) works only for writing the ASCII code from the data bus (D4~D7). V0 is supplied for the crystal drive. 
R310, R311 and R312 are density control resistors. 
Consequently, in this unit, the timing (positive clock) is generated by the LCD interface circuitry in the gate array (IC301).

DIsplay mode

Density

Normal

Dark

2 lines (X1.0)

LED1 (IC301-22pin)

H

L

XL14 (IC301-20pin)

Hi-Z

L

XL15 (IC301-21pin)

L

L

1 line (X1.5)

LED1

H

H

XL14

Hi-Z

Hi-Z

XL15

Hi-Z

L

46

This power supply board uses the switching regulator method.

The input current goes into the input rectifier circuit through the filter circuit. The filter circuit decreases the noise voltage and the
noise electric field strength.

The input current is rectified by D101, D102, D103 and D104 and charges C106 to make DC voltage. Then it supplies power to
the converter circuit.

Bias is applied to the Q101 gate via this circuit when the AC power is turned on and Q101 begins operating.

A-B Voltage Wave Form

C-D Voltage Wave Form

E-F

G-H Voltage Wave Form

0

0

0

Input
Circuit

AC
Input

Surge
Absorber
Circuit

G

H

4V

24V

E

IC101

D110

R104

F

9~6V

GND

Control
Circuit

Kick-on
Voltage
Circuit

Converter
Circuit

C

D

4V
Output
Circuit

24V
Output
Circuit

+

-

Rectifier
Circuit

A

B

C106

Error Detecting
Circuit

Surge
Absorber
Circuit

Q101

47

The following is an overview of how the power supply unit is controlled. 
The control method of this power supply unit is pulse width modulation.

When Q

1

 is ON, the energy is charged in the transfer primary coil according to E

1

. When Q

1

 is OFF, the energy is output from

the secondary transfer as follows.
    L 

→ D

1

 

→ Load → L

Then the power is supplied to the Load. When Q

1

 is ON, power is not output from the secondary side. The output voltage is fed

back in the control IC according to the error amp rectifier. Then depending on how T

ON

 is controlled, stabilization occurs. Also,

when the current load becomes too large, in order to decrease the voltage output, the increase in 

 is controlled and the output

voltage is stabilized.
Therefore, basically the timing: Ton/Toff of Q1 controls the output voltage.

48

This circuit is for absorbing surge voltage generated by the transformer.

The control circuit amplifies the output with increased voltage detected in the error detecting circuit. Then it drives the main tran-
sistor.
In this power supply, the duty ratio is defined by changing the ON period of the main transistor.
This is shown as follows.
When the output voltage of the 24V circuit increases, the current of the photo coupler PC101 increases, the pulse width of the
output control IC becomes narrower and the ON period of Q101 becomes shorter.

The highest drain current (Q101) is limited by a limiter circuit (IC101) of 24V. The 24V output is limited by this circuit.

If the 24V output increases because the error detecting circuit or control circuit is broken, IC101 will recognize this signal and
output becomes 0V.

Refer to Power Supply Board Section (P.131).