Sharp SF-1020, SF-1120 service manual Heater lamp control circuit, General

(6) Heater lamp control circuit

				+5V1
R67	R71
1KJ	47KJ
11
13	R7 5
10		Q7			C6
IC20D	75J	2SC945			R80	OP-LATCH
NJM2901	C7
R6 8			R84		24KJ	CPU 17PIN
5.6KJ				D41	0.22µ F
	0.22µ F		10KJ

Vref

R91

R87

R89

8.2KJ

1MJ

4.7KF

8

14

D3 8

9

D6

D7

IC20C

C55

MA700

D16

NJM2901

R93

R9 2

DSM101

680J

22000PF

47KF

8 ~ 20

PR

Q 5

2SA573

R54

R53

+24V

IC9

5.6KJ

47KJ

16

1

O1

I1

15

2

14

O2

I2

3

Heat roller

O3

I3

R32

PR

13

O4

I4

4

2.7KJ

CPU

12

O5

I5

5

HL

AN1

11

O6

I6

6

10

O7

I7

7

9

8

SK

G

ULN2004

R35

PR

+5V2

L2

270J

1/4W

10K

Q3

2.2K

GATE

T1

DTA123Y5

T2

PR

GND

AC PWB

AC PWB

200V AREA

MLSW

ML

AC

MLSW ML

ML : 1KW

N1

1General

The heater lamp control circuit detects the heat roller surface temperature with the thermistor, converts it into a voltage, and out- puts it to the CPu analog input pin.

The CPU converts analog signals into digital signals, compares them with the value set by test commands to turn on/off the heater lamp, maintaining the heat roller surface temperature at a constant level.

The higher the heat roller surface temperature is, the greater the thermistor resistance is, and vice versa.

Therefore, the lower the heat roller surface temperature is, the higher the thermistor pin voltage is. The thermistor pin voltage is inputted to the analog port of CPU, which controls ON/OFF of the heater lamp according to the input voltage level.

[High temperature protect circuit when the CPU is hung up (out of control)]

1)The IC20 8 pin voltage (reference voltage) is divided by R91 and R93. The thermistor pin voltage is inputted to the IC20 9 pin. When the voltage at 9 pin becomes lower than the voltage at 8 pin (the heat roller surface temperature about 240V), the IC20 14 pin is driven to LOW, pulling down the PR signal to the GND level, turning off the AC power of the heater lamp.

2)Since the OP-latch signal is fixed to HIGH or LOW, the collector of Q7 is open. The voltage at IC20 10 pin is higher than the voltage at IC20 11 pin (reference voltage), and IC20 13 pin is driven to LOW.

With the above operations, IC9 13 pin is opened to turn off Q5, turning off the power relay. As a result, the AC power of the heater and the copy lamp is turned off.

*The double protection with 1) and 2) is provided.

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