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(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.

ventilation opening for sensing

(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.

Operational

amplifier

Output

voltage

S : Thermistor

open vessel

C : Thermistor

closed vessel

(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 ANI5 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 ANI4 terminal of the LSI. The voltage of

ANI5 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

ABSOLUTE HUMIDITY SENSOR CIRCUIT

Thermistors

Sensing part

(Open vessel)

(Closed vessel)

Absolute humidity vs,

output voltage characteristic

Absolute humidity (g/m )

which the resistance balance of the bridge circuit is

deviated to increase the voltage available at ANI5

terminal of the LSI.

Then the LSI observes that voltage at ANI5 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 ANI5

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

R99

300k

R100

150k

R101

75k

R102

37.4k

C. Thermistor in

closed vessel

S. Thermistor in

open vessel

R97

47k

F-1

R96

F-3

3.57k

8 7 6 5

1 2 3 4

R91

R92

3.32k

R93

F-2

IC2

1.8k

360k

VA : -15V

R- 326FS

SW1

P31

SW2

P32

SW3

P33

SW4

P34

SW5

P35

ANI4

LSI

(IC1)

R94

R95

ANI5

10k

47k

D90

VC : -5V