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Drawer Microwave

(Models MWD24-2 / MWD30-2)

Theory of Operation

3-16

#813632 - Revision B - December, 2009

HUMIDITY SENSOR CIRCUIT

A. Structure of Humidity Sensor (See Figure 3-31):

The humidity sensor includes two thermistors. One
thermistor is housed in the closed vessel filled with
dry air while another in the open vessel. Each sen-
sor is provided with the protective cover made of
metal mesh to be protected from the external airflow.

B. Operational Principle of Humidity Sensor (See

Figure 3-32):

This illustration shows the basic

structure of an absolute humidity sensor. A bridge
circuit is formed by two thermistors and two resistors
(R and R2). The output of the bridge circuit is to be
amplified by the operational amplifier. Each thermis-
tor 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 dif-
ferent 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 varied every minute,
it is amplified by the operational amplifier.

C. Detector Circuit of Humidity Sensor Circuit (See

Figure 3-33):

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, 6
seconds clearing cycle occurs than the detector cir-
cuit starts to function and the LSI observes the initial
voltage available at its AN6 terminal.

With this voltage given, the switches SW to SW5 in
the LSI are turned on in such a way as to change
the resistance values in parallel with R45 ~ R49.
Changing the resistance values results in that there
is the same potential at both F-3 terminal of the
absolute humidity sensor and AN6 terminal of the
LSI. The voltage of AN7 terminal will indicate about
+2.5V. This initial balancing is set up about 6 sec-
onds 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 AN7 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 AN7 terminal 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 disconnection of the absolute
humidity sensor, ERROR will appear on the display and the cooking is stopped.

Ventilation opening

for sensing

Sensing part

(Open vessel)

Sensing part

(Closed vessel)

Thermistors

Figure 3-31. Humidity Sensor

Operational

Amplifier

Output

Voltage