27
R-520LK
R-520LW
(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.
(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 heated
at about 150½C (302½F), 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 very
minute, it is amplified by the operational amplifier.
(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. 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,
ABSOLUTE HUMIDITY SENSOR CIRCUIT
the food is heated to generate moisture by which the
resistance balance of the bridge circuit is deviated to
increase the voltage available at AN1 ter minal of the
LSI.
Then the LSI obser ves 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
the bridge circuit due to disconnection of the absolute
humidity sensor, ERROR will appear on the display and
the cooking is stopped.
ventilation opening for sensing
Sensingpart
(Openvessel)
Sensingpart
(Closedvessel)
Thermistors
C
S
R3
R1
R2
+
Operational
amplifier
Output
voltage
S: Thermistor
openvessel
C: Thermistor
closedvessel
2
Absolutehumidity (g/m )
Outputvoltage
Absolutehumidity vs,
outputvoltage characteristic
SW1
SW2
SW3
SW4
SW5
P57
P56
P55
P51
P53
LSI
(IC1)
AN0
AN1
620k
300k
150k
75k
37.4k
11
17
15
10
9
13
12
47k
47k
10k
12 3 4
87 6 5
0.01uF
0.015uF
0.01uF
VA: -15V VA:-15V
R90
C90
C91
C93
C92
S
F-2 1.8k IC2
F-1
F-3
C
3.57k
3.32k
VC: -5V
0.1
uF
C.Thermistor in
closedvessel
S.Thermistor in
openvessel
R98
R99
R96
R91
360k
R93
R92
R94 R95
D90
R100
R101
R102
R97