Humidity Sensor Circuit - Electrolux EI27MO45GSA Technical & Service Manual

Operation

Humidity Sensor Circuit

(1) Structure of Absolute Humidity Sensor:
The absolute humidity sensor includes two
thermistors as shown in the illustration below. One
thermistor is housed in the closed vessel filled with
dry air while another in is 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
Sensing Part
Openings
(Closed vessel)
View of Sensor Case Removed
Figure 2-6. Humidity Sensor
(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
Figure 2-7. Humidity Sensor Operation
Sensing Part
Sensing Part
(Open vessel)
(Closed vessel)
Thermistor
Thermistor
Element
Element
Sensor
Case
Cross Section View
Absolute humidity vs,
output voltage characterist
Absolute humidity (g/m ) 2
(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 first, then 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 R50-1. 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 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 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) Humidity sensor circuit:
IC2(IZA495DR)
C. Thermistor in
closed vesssl
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
2-6
64
4
P30
620k
5
63
P31
300k
6
62
P32
150k
61
7
P33
75k
60
8
P34
37.4k
5
3
AN7
10k
6
2
AN6
47k
360k
1
12
VC : -5V
SW1
SW2
SW3
SW4
SW5
LSI
(IC1)