Table of Contents
Advertisement
(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
heated at about 150˚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 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
-
R2
S
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 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 R56 ~ R60 of IC4.
Changing the resistance values results in that there is
the same potential at both F-3 terminal of the absolute
humidity sensor and AN5 terminal of the LSI. The
voltage of AN6 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 AN6 terminal and
ABSOLUTE HUMIDITY SENSOR CIRCUIT
Thermistors
Sensing part
Sensing part
(Closed vessel)
(Open vessel)
Absolute humidity vs,
output voltage characterist
Absolute humidity (g/m )
2
17
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 AN6
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
R56
620k
R57
300k
R58
150k
R59
75k
R60
C. Thermistor in
closed vessel
37.4k
S. Thermistor in
R55
open vessel
47k
F-1
C
R61
F-3
3.57k
+
IC2
-
S
R54
R50
3.32k
R51
F-2
1.8k
360k
VA : -16V
VA : -16V
R-390B
SW1
95
PA0
SW2
96
PA1
SW3
97
PA2
SW4
98
PA3
SW5
99
PA4
85
AN5
LSI
(IC1)
R52
R53
86
AN6
10k
47k
D101
VC : -5V
Hide quick links:
Advertisement
Table of Contents
