Viking DMOC205SS Service Manual page 29

(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.
Sensing part
(Open vessel)
Ventilation
openings
Sensing part
(Closed vessel)
View of sensor case removed
(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
ο
heated at about 150 C (302
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
+
-
R2
S
S : Thermistor
open vessel
R3
C : Thermistor
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, 16 seconds clearing cycle occurs
ABSOLUTE HUMIDITY SENSOR CIRCUIT
Sensing part Sensing part
(Closed vessel)
(Open vessel)
Thermistor
Thermistor
element
element
Sensor
case
Cross section view
ο
F), the resultant heat is
Absolute humidity vs,
output voltage characterist
Absolute humidity (g/m ) 2
than 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.
When the LSI starts to detect the initial voltage at AN6
terminal 16 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.
1) Absolute humidity sensor circuit
IC2(IZA495DR)
C. Thermistor in
closed vesssl
S. Thermistor in
open vessel
11
F-1
C
F-3
3.57k
S
3.32k
1.8k
F-2
10
9
VA : -15V
27
SW1
4 64
P30
620k
SW2
5 63
P31
300k
SW3
6 62
P32
150k
SW4
7 61
P33
75k
SW5
60
8
P34
37.4k
5
3
AN7
47k
LSI
(IC1)
+
10k
6
2
AN6
- 47k
360k
1
12
VA : -15V
VC : -5V
DMOC205SS
VMOC205SS