Viking DMOR206SS Service Manual page 33

(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 glass tube filled with dry air while another in
the open glass tube. Each sensor is provided with the
protective cover made of metal with holes to be protected
from the external airflow.
ventilation opening for sensing
(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
ο ο
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.
Operational
C
R1 amplifier
Output
voltage
+
-
S
R2
S : Thermistor
open vessel
C : Thermistor
R3
closed vessel
(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, a 16 seconds clearing cycle occurs.
ABSOLUTE HUMIDITY SENSOR CIRCUIT
Thermistors
Sensing part Sensing part
(Open vessel) (Closed vessel)
Absolute humidity vs,
output voltage characterist
Absolute humidity (g/m ) 2
33
Then the detector circuit starts to function and the LSI
observes the initial voltage available at its AN6 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 R50-1. Changing the
resistance values results in that there is the same potential
at both F-3 terminal of the absolute humidity sensor and
AN7 terminal of the LSI. The voltage of AN6 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 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 AN6 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.
After 16 seconds of operation in the sensor cook mode,
the LSI detects the initial voltage at AN6. If the absolute
humidity sensor is open or shorted, the circuit can not
be balanced for initial voltage comparisons. This will
result in the word "error" on the display and the cooking
operation is stopped.
1) Absolute humidity sensor circuit
IC2(IZA495DR)
620k
300k
150k
75k
C. Thermistor in
closed vesssl
37.4k
S. Thermistor in
open vessel
47k
11
F-1
C
F-3
3.57k
+
S -
3.32k
1.8k
F-2
360k
10
9
12
VA : -15V
VA : -15V
DMOR206SS
SW1
4 64
P30
SW2
5 63
P31
SW3
62
6
P32
SW4
7 61
P33
SW5
60
8
P34
5
3
AN7
LSI
(IC1)
6
10k
2
AN6
47k
1
VC : -5V