Electrolux E30MO65GSSA Service Manual page 29

(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.
Sensing part
(Open vessel)
Ventilation
openings
Sensing part
(Closed vessel)
View of sensor case removed
(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 (R 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
ο ο
at about 50 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
-
S R2
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, 6 seconds clearing cycle
occurs than the detector circuit starts to function and
ABSOLUTE HUMIDITY SENSOR CIRCUIT
Sensing part Sensing part
(Closed vessel)
(Open vessel)
Thermistor
Thermistor
element
element
Sensor
case
Cross section view
Absolute humidity vs,
output voltage characterist
Absolute humidity (g/m ) 2
the LSI observes the initial voltage available at its AN6
terminal.
With this voltage given, the switches SW to SW5 in
the LSI are turned on in such a way as to change the
resistance values in parallel with R50-. Changing the
resistance values results in that there is the same potential
at both F-3 terminal of the absolute humidity sensor and
AN7 terminal of the LSI. The voltage of AN6 terminal
will indicate about -2.5V. This initial balancing is set
up about 6 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
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 6 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.
) Absolute humidity sensor circuit
IC2(IZA495DR)
620k
300k
C. Thermistor in
closed vesssl
37.4k
S. Thermistor in
open vessel
47k
11
F-1
C
F-3
3.57k
+
-
S
3.32k
1.8k
F-2
10
9
VA : -15V
VA : -15V
29
E30MO65GSSA
SW1
4 64
P30
SW2
5 63
P31
SW3
6
62
P32
150k
SW4
7 61
P33
75k
SW5
8 60
P34
5
3
AN7
LSI
(IC1)
10k
6
2
AN6
47k
360k
1
12
VC : -5V