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21
R-4G75(W)M
R-4G75(B)M
R-4G75(B)M
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 the
open vessel. Each sensor is provided with the protective
cover made of metal mesh to be protected from the
external airflow.
as shown in the illustration. One thermistor is housed in
the closed vessel filled with dry air while another is 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 itself
heated at about 150˚C (302˚F) and 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 will be amplified by the operational
amplifier.
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 itself
heated at about 150˚C (302˚F) and 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 will be amplified by the operational
amplifier.
(3) Detector Circuit of Absolute Humidity Sensor
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 later the detector circuit starts to function and
the LSI observes the initial voltage available at its P66
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 R-1.
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 later the detector circuit starts to function and
the LSI observes the initial voltage available at its P66
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 R-1.
Changing the resistance values results in that there is
the same potential at both F-3 terminal of the absolute
humidity sensor and P67 terminal of the LSI. The
voltage of P66 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 P66 terminal of the LSI. Then the LSI
observes that voltage at P66 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 get in the next necessary operation
automatically.
When the LSI starts to detect the initial voltage at P66
terminal 16 seconds after the unit has been put in the
Sensor Cooking mode, if it is impossible to take a
balance of the bridge circuit due to disconnection of the
absolute humidity sensor, ERROR will appear on the
display and the cooking is stopped.
the same potential at both F-3 terminal of the absolute
humidity sensor and P67 terminal of the LSI. The
voltage of P66 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 P66 terminal of the LSI. Then the LSI
observes that voltage at P66 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 get in the next necessary operation
automatically.
When the LSI starts to detect the initial voltage at P66
terminal 16 seconds after the unit has been put in the
Sensor Cooking mode, if it is impossible to take a
balance of the bridge circuit due to disconnection of the
absolute humidity sensor, ERROR will appear on the
display and the cooking is stopped.
Absolute humidity sensor circuit
Sensing part
(Open vessel)
(Open vessel)
Sensing part
(Closed vessel)
(Closed vessel)
Metal
mesh
mesh
C
S
R3
R1
R2
+
-
Operational
amplifier
amplifier
Output
voltage
voltage
S : Thermistor
open vessel
open vessel
C : Thermistor
closed vessel
closed vessel
2
Absolute humidity (g/m )
Output voltage
Absolute humidity vs,
output voltage characterist
output voltage characterist
SW1
SW2
SW4
SW3
SW5
P30
P31
P32
P33
P34
VA : -15V
VA : -15V
VC : -5V
LSI
(IC1)
(IC1)
+
-
D
D
C
C
C
R
R
R
R
F-1
F-2
F-3
R3
R4
R5
R6
R7
R8
P67
P66
R1
R2
64
63
62
61
60
4
5
6
R51
10
11
7
8
1
4
C
S
12
C. Thermistor in
closed vessel
S. Thermistor in
open vessel
closed vessel
S. Thermistor in
open vessel
3
5
2
6
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