Sharp R-4H55 Service Manual page 26

R4H5.5
R-4X5.5
R-4X5.W
ABSOLUTE HUMIDITY SENSOR CIRCUIT
(1)Structure of Absolute Humidity Sensor
The absolute humidity sensor includes two thermis-
tors as shown in the illustration. One thermistor 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 ab-
solute humidity sensor. A bridge circuit is formed by
two thermistors and two resistors (Rl 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 resultant 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 between them caus-
ing an output voltage from the bridge circuit, the inten-
sity 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.
~~~~~, l/q;
Absolute humidity (g/m 2)
(3)Detector Circuit of Absolute Humidity Sensor
This detector circuit is used to detect the output volt-
age 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 later the detector circuit starts to function and
the LSI observes the initial voltage available 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 volt-
age of IN1 terminal will indicate about -2.5V. This ini-
tial 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 IN1 terminal of the LSI.
Then the LSI observes that voltage at IN1 terminal and
compares it with its initial value, and when the com-
parison 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
terminal16 seconds after the unit has been put in the
Sensor Cooking mode, if it is impossible to take a bal-
ance of the bridge circuit due to disconnection of the
absolute humidity sensor, ERROR will reappear on
the display and the cooking is stopped.
Absolute humidity sensor circuit
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