Humidity Sensor Circuit - Electrolux FGMO205KFA Service Manual

Section 2 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.
Figure 2-4. Humidity Sensor Diagram
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.
Figure 2-5. Humidity Sensor Operation
2-6
(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 sensor cooking mode, 16 seconds
clearing cycle occurs, then the detector circuit starts
to function and the LSI observes the initial voltage
available at its AN1 terminal.
With this voltage given, 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 AN0 terminal of the
LSI. The voltage of AN1 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 AN1 terminal of the LSI.
Then the LSI observes that voltage at AN1 terminal
and compares it with its initial value, and when the
comparison rate reaches the preset value (fi xed 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
AN1 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.
Humidity sensor circuit: