Viking RDMOS201SS Service Manual page 26

RDMOS201SS
(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.
Thermistors
Sensing part Sensing part
(Open vessel) (Closed vessel)
(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
o
heated at about 150 C (302
dissipated in the air and if the two thermistors are placed
in 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 inceases.
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
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 then the detector circuit starts to function and
the LSI observes the initial voltage available at its AN1
terminal.
ABSOLUTE HUMIDITY SENSOR CIRCUIT
ventilation opening for sensing
o
F), the resultant heat is
Absolute humidity vs,
output voltage characteristic
Absolute humidity (g/m )
2
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 R107 ~ R111 of IC2
. Changing the resistance value 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 AN1 terminal will indicat about 16 seconds about
-2.50 V. 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 AN1 terminal of the LSI.
Then the LSI observes that the voltage at AN1 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 AN1 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
620k
R98
R99
R100 150k
R101
R102
C. Thermistor in
closed vessel
S. Thermistor in
open vessel R97
47k
F-1
C
R96
F-3
3.57k
8 7 6 5
1 2 3 4
S
R91
R92
3.32k
F-2 IC2
1.8k
D90
VA : -15V VA : -15V
26
SW1
P20
SW2
300k
P21
SW3
P22
SW4
75k
P23
SW5
37.4k
P24
AIN4
LSI
(IC1)
R94 R95
AIN5
10k
47k
R93
D91
360k
VC : -5V