Electrolux EI27MO45GSA Technical & Service Manual Download

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Operation

2-6

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 2-6. 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.

Ventilation

Openings

View of Sensor Case Removed

Sensing Part

(Open vessel)

Sensing Part

(Closed vessel)

Sensing Part

(Open vessel)

Sensing Part

(Closed vessel)

Cross Section View

Sensor

Case

Thermistor

Element

Thermistor

Element

Operational
amplifier

Output
voltage

S : Thermistor

open vessel

C : Thermistor

closed vessel

Absolute humidity (g/m )

atl

Absolute humidity vs,
output voltage characterist

Figure 2-7. Humidity Sensor Operation

(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:

SW2

SW1

SW3

SW4

SW5

P30

P31

P32

P33

P34

LSI