Wolf MW24-1100 Service Manual Download Page 15

Page: 15 / 54

Page 15

MW24-1100 MICROWAVE OVEN

Theory of Operation

3-10

#814888 - Revision A - May, 2009

ABSOLUTE HUMIDITY SENSOR CIRCUIT

Structure of AH Sensor

The absolute humidity sensor includes two thermis-
tors

(See Figure 3-12).

One thermistor is housed

in the closed vessel filled with dry air, while the
other in the open vessel. Each sensor is provided
with a protective metal mesh cover to protect it from
external airflow.

ventilation opening for sensing

Sensing part
(Open vessel)

Sensing part
(Closed vessel)

Thermistors

Figure 3-12. AH (Absolute Humidity) Sensor

AH Sensor Operational Principle

Figure 3-13

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 ampli-
fied by the operational amplifier.

Each thermistor is supplied with a current to keep it
heated at about 302°F (150°C). The resulting heat
is dissipated in the air. If the two thermistors are
placed in differing degrees of heat conductivity,
leading to a potential difference between them, an
output voltage from the bridge rectifier is caused,
the intensity of which will increase as the absolute
humidity of the air increases. Since the output is
very minute, it is amplified by the operational ampli-
fier.

Operational
amplifier

Output
voltage

S : Thermistor
open vessel

C : Thermistor
closed vessel

Absolute humidity (g/m )

Output

voltage

Absolute humidity vs,
output voltage characteristic

Figure 3-13. AH Sensor Basic Structure

Detector Circuit of AH Sensor Circuit

The detector circuit detects the output voltage of
the absolute humidity circuit, which allows the LSI
to control sensor cooking. When the unit is set to
sensor cooking mode, a 16 second clearing cycle
occurs, then the detector circuit begins to function
and the LSI observes the initial voltage available at
its AN1 terminal.

With this voltage given, switches SW1 to SW5 in
the LSI are turned on in such a way that it changes
the resistance values in parallel with R107 ~ R111
of IC2. Changing the resistance value creates the
same potential at both the F-3 terminal of the
absolute humidity sensor and the AN0 terminal of
the LSI. The voltage of the AN1 terminal will indi-
cate about -2.50 V for about 16 seconds. This ini-
tial balancing is set up about 16 seconds after the
unit is put into sensor cooking mode. As the sensor
cooking proceeds, the food is heated, generating
moisture causing the resistance balance of the
bridge circuit to deviate and increase the voltage
available at the AN1 terminal of the LSI.

The LSI then observes the voltage at the AN1 ter-
minal and compares it with its initial value. 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
into the next operation automatically. If the LSI
begins detecting the initial voltage at the AN1 termi-
nal, 16 seconds after sensor cooking mode is initi-
ated, but it is not possible to balance the bridge cir-
cuit due to disconnection of the absolute humidity
sensor, ERROR will appear on the display and the
cooking is stopped.

SW1

SW2

SW3

SW4

SW5

P20

P21

P22

P23

P24

LSI
(IC1)

AIN4