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 is in an open vessel. Each
sensor is provided with the protective cover made
of metal mesh to protect it from the external
airflow.
Sensing part Sensing part
(Open vessel 1 (Closed vessel 1
(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 amplified by the
operational amplifier.
Each thermistor is supplied with a current to keep
*it heated at about 150 “C (302 OF), 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 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 is amplified by the
operational amplifier.
Operational
amplifier C
R,
T
is”i!l
i
-
8 *u
s: : RI
+
%a”,“e’
P-
S: Thermistor
open vessel
R3 u
C: Thermistor
closed vow
t
Absolute humidity vs.
output voltage characteristis
Absolute humidity (g/m’ 1
(3)Detector Circuit of Absolute Humidity Sen-
sor Circuit
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,
a 16 second clearing cycle occurs than 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 re-
sistance values in parallel with R50-1.
Changing the resistance values results in that there
is the same potential at both the 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 goes in
the next 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 not possible to
balance, of the bridge circuit due to disconnection
of the absolute humidity sensor, ERROR will an-
pear on the display and the cooking is stopFed. ’
( D72QOlU)
1 )Absolute humidity sensor circuit
C
Thermamtar In
closed v.sseI
5
Tharmfslor In
op.” P.S..,
R50 Rdatof block
SW4 1
P441
sw59.
P43 ---01
-1
IN0
LSI
( IC 11
IN1
VA:‘-1 5V
2)lnternal equivalent circuit of I2
CH1
CH2
Top View
29
To Next Page
Loading...
