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a.
Structure of Humidity Senso
The humidity sensor includes two thermistors as shown
in the illustration. One thermistor is housed in the closed
vessel ¿ lled 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 airÀ ow.
Ventilation opening for sensing
b.
Operational Principle of Humidity Sensor
The ¿ gure below shows the basic structure of an abso-
lute humidity sensor. A bridge circuit is formed by two
thermistors and two resistors (R3-9 and R2).
The output of the bridge circuit is to be ampli¿ ed by the
operational ampli¿ er.
Each thermistor is supplied with a current to keep it
heated at about 3-950 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 differ-
ence 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 varied every minute, it is ampli¿ ed by the operational
ampli¿ er.
R3
Operational
S
R1
amplifier
Output
voltage
+
C
R2
S : Thermistor
open vessel
C : Thermistor
closed vessel
c.
Detector Circuit of Humidity Sensor Circuit
This detector circuit is used to detect the output voltage
of the absolute humidity circuit to allow the LSI to con-
trol sensor cooking of the unit. When the unit is set in
the sensor cooking mode, 3-96 seconds clearing cycle
occurs than the detector circuit starts to function and
the LSI observes the initial voltage available at its AN6
terminal.
Drawer Microwave
HUMIDITY SENSOR CIRCUIT
r
Thermistors
Sensing part
Sensing part
(Open vessel)
(Closed vessel)
Absolute humidity vs,
output voltage characteristic
Absolute humidity (g/m )
With this voltage given, the switches SW3-9 to SW5 in
the LSI are turned on in such a way as to change the re-
sistance values in parallel with R45 ~ R49. Changing the
resistance values results in that there is the same poten-
tial at both F-3 terminal of the absolute humidity sensor
and AN6 terminal of the LSI. The voltage of AN7 terminal
will indicate about +2.5V. This initial balancing is set up
about 3-96 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 resis-
tance 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 AN7 terminal and
compares it with its initial value, and when the compari-
son rate reaches the preset value (¿ xed 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 AN7
terminal 3-96 seconds after the unit has been put in the
Sensor Cooking mode, if it is not possible to balance the
bridge circuit due to disconnection of the absolute humid-
ity sensor, ERROR will appear on the display and the
cooking is stopped.
VA : +15V
C. Thermistor in
closed vessel
S. Thermistor in
F2
open vessel
F3
F1
R43 1.8K
R51 47K
R45 620K
R46 300K
R47 150K
R48 75K
R49 37.4K
2
3-9
Theory of Operation
VC : +5V
D40
D41
86
3
8
+
63
1
IC5
-
R50 10K
R52 47K
4
2
R44 360K
64
52
51
50
49
48
AN7
LSI
(IC1)
AN6
SW1
P00
SW2
P01
SW3
P02
SW4
P03
SW5
P04
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