21
KB-6015KS
KB-6015KK
KB-6015KW
(1) Structure of Humidity Sensor
The 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.
(2) Operational Principle of 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
heated at about 150
ο
C (302
ο
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 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 varied
every minute, it is amplified by the operational amplifier.
(3) 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 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 AN6 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 R45 ~ R49. Changing
the resistance values results in that there is the same
HUMIDITY SENSOR CIRCUIT
potential 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 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 AN6 terminal of the LSI.
Then the LSI observes that voltage at AN7 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 AN7
terminal 16 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 humidity
sensor, ERROR will appear on the display and the
cooking is stopped.
1) Humidity sensor circuit
ventilation opening for sensing
Sensing part
(Open vessel)
Sensing part
(Closed vessel)
Thermistors
S
C
R3
R1
R2
+
Operational
amplifier
Output
voltage
S : Thermistor
open vessel
C : Thermistor
closed vessel
2
Absolute humidity (g/m )
O
utput voltage
Absolute humidity vs,
output voltage characteristic
R43 1.8K
VA : +15V
VC : +5V
R4
2 3.57K
R4
1 3.32K
S
C
R51 47K
R45 620K
R46 300K
R47 150K
R48 75K
R49 37.4K
R44 360K
R50 10K
3
F2
8
4
1
2
IC5
+
-
R40
430
C42 0.1μ
F
D40
D41
R52 47K
86
63
P04
P03
P02
P01
P00
AN6
SW1
SW2
SW3
SW4
SW5
AN7
LSI
(IC1)
64
52
51
50
49
48
C. Thermistor in
closed vessel
S. Thermistor in
open vessel
F3
F1
