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PLMV169DCC
(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
o
heated at about 150
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
C
R1
amplifier
Output
voltage
+
S
R2
S : Thermistor
open vessel
R3
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 AN1 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 of IC2.
Changing the resistance values results in that there is the
same potential at both F-3 terminal of the absolute humidity
sensor and AN0 terminal of the LSI. The voltage of AN1
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
ABSOLUTE HUMIDITY SENSOR CIRCUIT
Thermistors
Sensing part
Sensing part
(Open vessel)
(Closed vessel)
o
C (302
F), the resultant heat is
Absolute humidity vs,
output voltage characteristic
Absolute humidity (g/m )
balance the bridge circuit is deviated to increase the
voltage available at AN1 terminal of the LSI.
Then the LSI observes that voltage at AN1 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 AN1
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 appear on the display and
the cooking is stopped.
1) Absolute humidity sensor circuit
C. Thermistor in
closed vessel
S. Thermistor in
open vessel
F-1
C
R96
F-3
3.57k
S
R91
R92
3.32k
F-2
1.8k
VA : -15V
2
30
620k
30
R98
P33
300k
31
R99
P34
R100
150k
32
P35
33
R101
75k
P36
R102
37.4k
34
P37
R97
91
AN0
47k
+
R94
R95
92
IC2
AN1
10k
47k
R93
360k
D90
VA : -15V
VC : -5V
SW1
SW2
SW3
SW4
SW5
LSI
(IC1)
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