Humidity Sensor Circuit - Electrolux EI30MH55GBA Technical & Service Manual

Humidity Sensor Circuit

(1) Structure of Humidity Sensor
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
(2) Operational Principle of Humidity Sensor
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 1 50C (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
(3) Detector Circuit of Humidity Sensor Circuit
This detector circuit is used to detect the output
voltage of the 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 than the detector circuit
starts to function and the LSI observes the initial
voltage available at its AN1 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 R98 ~ R102.
Changing the resistance values results in that
there is the same potential at both F-3 terminal of
the 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 heat-
ed 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 (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
AN1 terminal 1 6 seconds after the unit has been
put in the Sensor Cooking mode, if it is not
possible to balance the bridge circuit due to dis-
connection of the humidity sensor, ERROR will
appear on the display and cooking is stopped.
1) Humidity sensor circuit
620k
R98
R99 300k
R100 150k
R101
75k
R102 37.4k
C. Thermistor in
closed vessel
S. Thermistor in
open vessel R97
47k
F-1
C
R96
F-3
3.57k
8 7 6 5
1 2 3 4
S
R91
R92
3.32k
F-2 IC2
1.8k
VA : -15V VA : -15V
2-11
Operation
SW1
18
P50
SW2
17
P51
SW3
16
P52
SW4
15
P53
SW5
14
P54
10
AN0
LSI
(IC1)
R94
R95
9
AN1
15k
47k
R93
360k
D90
VC : -5V