R-4H54
ABSOLUTE HUMIDITY SENSOR CIRCUIT
(1) Structure of Absolute Humidity Sensor
The absolute humidity sensor includes two
thermistors as shown in the illustration. One
thermister is housed in the closed vessel filled with
dry air while another is 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 (RI
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
itself heated at about 150°C (302°F) and the re-
sultant heat is dissipated in the air and if the two
thermistors are placed in different humidity condi-
tions they show different degrees of heat
conductivity leading to a potential difference be-
tween 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 will be amplified by the
operational amplifier.
~~~~~ /iqy;
Absolute humidity (g/m* )
(3)Detector Circuit of Absolute Humidity Sen-
sor
This detector circuit is used to detect the output
voltage of the absolute humidity circuit to allow the
LSI to control the sensor cooking of the unit.
When the unit is set in the sensor cooking mode,
16 seconds later the detector circuit starts to func-
tion and the LSI observes the initial voltage avail-
able at its IN1 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 R-l.
Changing the resistance values results in that there
is the same potential at both F-3 terminal of the
absolute humidity sensor and IN0 terminal of the
LSI. The voltage of IN1 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 bal-
ance of the bridge circuit is deviated to increase the
voltage available at IN1 terminal of the LSI. Then
the LSI observes that voltage at IN1 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 gets in
the next necessary operation automatically.
When the LSI starts to detect the initial voltage at
IN1 terminal 16 seconds after the unit has been put
in the Sensor Cooking mode, if it is impossible to
take a balance of the bridge circuit due to discon-
nection of the absolute humidity sensor, ERROR
will reappear on the display and the cooking is
stopped.
Absolute humidity sensor circuit
ii=F
(RD72QOl U)
19
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