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DMO2420B

DMO2420R

DMO2420S

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.

(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 150½C (302½F), 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.

(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

Pin No. Signal I/O Description

143 COM7 OUT Common data signal : COM9. Connected to LCD signal COM9.

144 COM6 OUT Common data signal : COM10. Connected to LCD signal COM10.

ventilation opening for sensing

Sensing part

(Open vessel)

Sensing part

(Closed vessel)

Thermistors

Operational

amplifier

Output

voltage

S : Thermistor

open vessel

C : Thermistor

closed vessel

Absolute humidity (g/m )

Output voltage

Absolute humidity vs,

output voltage characteristic

SW1

SW2

SW3

SW4

SW5

P20

P21

P22

P23

P24

LSI

(IC1)

AIN4

AIN5

620k

300k

150k

75k

37.4k

47k

10k

1234

8765

0.01uF

0.015uF

0.01uF

VA : -15V

R90

C90

C91

C93

C92

F-2

1.8k

IC2

F-1

F-3

3.57k

3.32k

VC : -5V

0.1

C. Thermistor in

closed vessel

S. Thermistor in

open vessel

R98

R99

R96

R91

360k

R93

R92

R94

R95

D91

D90

R100

R101

R102

R97