R-480J

ABSOLUTE HUMIDITY SENSOR CIRCUIT

(1)Structure of Absolute Humidity Sensor

The absolute humidity sensor includes two thermistors as shown in the illustration. One thermistor is housed in the closed vessel filled with dry air while another in the open vessel. Each sensor is provided with the protective cover made of metal mesh to be protected from the external airflow.

 

Thermistors

 

ventilation opening for sensing

Sensing part

Sensing part

(Open vessel)

(Closed vessel)

 

(2)Operational Principle of Absolute Humidity Sensor The figure below shows the basic structure of an absolute humidity sensor. A bridge circuit is formed by two thermistors and two resistors (R1 and R2).

The output of the bridge circuit is to be amplified by the operational amplifier.

Each thermistor is supplied with a current to keep it heated at about 150˚C (302˚F), the resultant heat is dissipated in the air and if the two thermistors are placed in different humidity conditions they show different degrees of heat conductivity leading to a potential difference between them causing an output voltage from the bridge circuit, the intensity of which is increased as the absolute humidity of the air increases. Since the output is very minute, it is amplified by the operational amplifier.

 

 

 

 

Absolute humidity vs,

C

 

Operational

output voltage characteristic

R1

amplifier

voltageOutput

S

R2

 

S : Thermistor

 

 

 

Output

 

 

 

+

voltage

 

R3

 

 

open vessel

 

 

 

C : Thermistor

 

 

 

 

closed vessel

 

 

 

 

 

Absolute humidity (g/m 2 )

(3)Detector Circuit of Absolute Humidity Sensor Circuit This detector circuit is used to detect the output voltage of the absolute humidity circuit to allow the LSI to control sensor cooking of the unit. When the unit is set in the sensor cooking mode, 16 seconds clearing cycle occurs than the detector circuit starts to function and the LSI observes the initial voltage available at its AIN5 terminal.

With this voltage given, the switches SW1 to SW5 in the LSI are turned on in such a way as to change the resistance values in parallel with R98 ~ R102. Changing the resistance values results in that there is the same potential at both F-3 terminal of the absolute humidity sensor and AIN4 terminal of the LSI. The voltage of AIN5 terminal will indicate about -2.5V. This initial balancing is set up about 16 seconds after the unit is put in the Sensor Cooking mode. As the sensor cooking proceeds, the food is heated to generate moisture by which the resistance balance of the bridge circuit is deviated to increase the voltage available at AIN5 terminal of the LSI.

Then the LSI observes that voltage at AIN5 terminal and compares it with its initial value, and when the comparison rate reaches the preset value (fixed for each menu to be cooked), the LSI causes the unit to stop sensor cooking; thereafter, the unit goes in the next operation automatically.

When the LSI starts to detect the initial voltage at AIN5 terminal 16 seconds after the unit has been put in the Sensor Cooking mode, if it is not possible to balance the bridge circuit due to disconnection of the absolute humidity sensor, ERROR will appear on the display and the cooking is stopped.

1) Absolute humidity sensor circuit

 

 

 

R98

 

620k

 

 

26

SW1

 

 

 

 

 

 

P20

 

 

 

 

 

 

 

 

 

 

 

 

R99

 

300k

 

 

25

SW2

 

 

 

 

 

 

 

 

 

P21

 

 

 

R100

 

150k

 

 

24

SW3

 

 

 

 

 

 

 

 

 

P22

 

 

 

R101

 

75k

 

 

23

SW4

 

 

 

 

 

 

 

 

 

P23

C. Thermistor in

R102

 

37.4k

 

 

22

SW5

 

 

 

 

 

 

P24

closed vessel

 

 

 

 

 

 

 

S. Thermistor in

 

 

 

 

 

 

 

 

open vessel

 

R97

 

 

 

50

AIN4

 

 

 

15k

0.01uF

 

 

 

 

 

 

 

0.01uF

 

 

F-1

 

 

 

C93

0.1uF

 

LSI

 

 

 

 

 

C

 

R96

 

 

C91

 

C92

 

(IC1)

 

 

 

 

 

 

F-3

3.57k

 

 

 

 

 

 

 

 

 

 

R94

 

 

 

 

 

 

 

R95

49

 

 

 

C90

0.1 uF

8 7 6 5

AIN5

S

 

1 2 3 4

4.7k

15k

 

 

R91

 

 

 

 

R92

 

 

 

 

F-2

3.32k

IC2

R93

 

 

 

 

 

 

1.8k

360k

 

 

 

 

 

R90

330

 

D90

 

 

 

 

 

VA : -15V

 

VA : -15V

VC : -5V

 

 

20

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Image 22
Sharp R-480J service manual Absolute Humidity Sensor Circuit, Structure of Absolute Humidity Sensor

R-480J specifications

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