25
DMT2420B
DMT2420R
DMT2420S
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 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 AIN5 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 AIN4 terminal of the LSI. The voltage of AIN5
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 AIN5 terminal of the
LSI.
Then the LSI observes that voltage at AIN5 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 AIN5
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) Absolute humidity sensor circuit
ventilation opening for sensing
Sensing part
(Open vessel)
Sensing part
(Closed vessel)
Thermistors
C
S
R3
R1
R2
+
Operational
amplifier
Output
voltage
S : Thermistor
open vessel
C : Thermistor
closed vessel
2
Absolute humidity (g/m )
Output voltage
Absolute humidity vs,
output voltage characteristic
SW1
SW2
SW3
SW4
SW5
P20
P21
P22
P23
P24
LSI
(IC1)
AIN4
AIN5
620k
300k
150k
75k
37.4k
26
23
22
50
49
24
25
47k
47k
10k
1 2 3 4
8 7 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
closed vessel
S. Thermistor in
open vessel
R98
R99
R96
R91
360k
R93
R92
R94
R95
D91
D90
R100
R101
R102
R97
