Humidity Sensor Circuit - Electrolux EI30BM55HB Technical & Service Manual

Operation
6) Power Source Circuit
This circuit generates voltages necessary in the
control unit from the AC line voltage. In addition,
the synchronizing signal is available in order to
compose a basic standard time in the clock
circuit.
Symbol Voltage
VC +5V
7) Relay Circuit
A circuit to drive the magnetron, fan motor, stirrer
motor, hood motor and light the oven lamp and
cook top lamp.
8) Buzzer Circuit
The buzzer is responsive to signals from the LSI
to emit audible sounds (key touch sound and
completion sound).
9) Synchronizing Signal Circuit
The power source synchronizing signal is
available in order to compose a basic standard
time in the clock circuit. It accompanies a
very small error because it works on commercial
frequency.
10)Door Sensing Switch
A switch to "tell" the LSI if the door is open or
closed.

11)Humidity Sensor Circuit

This circuit detects moisture of the cooking food
to allow its automatic cooking.
Humidity Sensor Circuit
(1) Structure of Humidity Sensor
Application
LSI(IC1)
(2) Operational Principle of Humidity Sensor
2-8
The 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.
Ventilation Opening
for Sensing
The figure below shows the basic structure of a
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 150C (302F), 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
Thermistors
Sensing part Sensing part
(Open vessel) (Closed vessel)
Absolute humidity vs,
output voltage characteristic
Absolute humidity (g/m ) 2