Viking Designer DMOR200SS Service Manual Download Page 26

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DMOR200SS

ABSOLUTE 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 (R 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 50ΩC (302ΩF), 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.

(3) Detector Circuit of Absolute Humidity Sensor

Circuit

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  circuit  starts  to  function  and

the LSI observes the initial voltage available at its AN
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  R98  ~  R02  of  IC2.
Changing the resistance values results in that there is
the same potential at both F-3 terminal of the absolute
humidity sensor and AN0 terminal of the LSI. The voltage
of  AN  terminal  will  indicate  about  -2.5V.    This  initial
balancing is set up about 6 seconds 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 the bridge circuit is deviated
to increase the voltage available at AN terminal of the
LSI.

Then the LSI observes that voltage at AN 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 AN
terminal 6 seconds after the unit has been put in the
Sensor Cooking mode, if it is not possible to balance, of
the bridge circuit due to disconnection of the absolute
humidity sensor, ERROR will appear on the display and
the cooking is stopped.

) Absolute humidity sensor circuit

ventilation opening for sensing

Sensing part
(Open vessel)

Sensing part
(Closed vessel)

Thermistors

Operational
amplifier

Output
voltage

S : Thermistor
open vessel

C : Thermistor
closed vessel

Absolute humidity (g/m )

utput voltage

Absolute humidity vs,
output voltage characteristic