Electrolux EW27MO55HSA, Technical & Service Manual

Page: 17 / 48

Operation
2-5
MAGNETRON THERMAL CUT-OUT
The thermal cut-out, located on the waveguide, is
designed to prevent damage to the magnetron if an over
heated condition develops in the magnetron due to
cooling fan failure, obstructed air guide, dirty or blocked
air intake, etc.
Under normal operation, the thermal cut-out remains
closed. However, when abnormally high temperatures
are reached within the magnetron, the thermal cut-out will
open at 257°F (125°C) causing the oven to shut down.
The magnetron thermal cut-out is not reset at room
temperature.
OVEN THERMAL CUT-OUT
The thermal cut-out, located on the side of the steam
duct, is designed to prevent damage to the unit if foods
in the oven catch fire due to over heating produced by
improper setting of cooking time or failure of control unit.
Under normal operation, the thermal cut-out remains
closed. However, when abnormally high temperatures
are reached within the oven cavity, the thermal cut-out
will open at 302°F (150°C) causing the oven to shut
down. When the thermal cut-out has cooled, the thermal
cut-out closes at 266°F (130°C)
Humidity Sensor Circuit
(1) Structure of Absolute Humidity Sensor:
The absolute 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.
(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
C
S
R3
R1
R2
+
-
Operational
amplifier
Output
voltage
S : Thermistor
open vessel
C : Thermistor
closed vessel
2 Absolute humidity (g/m )
e
g
atl
o
v
t
u
pt
u
O
Absolute humidity vs,
output voltage characterist