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DMOR206SS

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 glass tube filled with dry air while another in
the open glass tube. Each sensor is provided with the
protective cover made of metal with holes 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 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.

(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, a 16 seconds clearing cycle occurs.

Then the detector circuit starts to function and the LSI
observes the initial voltage available at its AN6 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 R50-1. Changing the
resistance values results in that there is the same potential
at both F-3 terminal of the absolute humidity sensor and
AN7 terminal of the LSI.  The voltage of AN6 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 AN6 terminal of the LSI.

Then the LSI observes that voltage at AN6 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.

After 16 seconds of operation in the sensor cook mode,
the LSI detects the initial voltage at AN6. If the absolute
humidity sensor is open or shorted, the circuit can not
be balanced for initial voltage comparisons. This will
result in the word "error" on the display and the cooking
operation is stopped.

1) Absolute humidity sensor circuit

Operational

amplifier

Output

voltage

S : Thermistor

open vessel

C : Thermistor

closed vessel

Absolute humidity (g/m )

Output voltage

Absolute humidity vs,