Sharp KB-5121KS Service Manual page 31

(1) Structure of Humidity Sensor
The 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.
ventilation opening for sensing
(2) Operational Principle of 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 varied
every minute, it is amplified by the operational amplifier.
R3
Operational
S R1
amplifier
Output
voltage
+
C R2
S : Thermistor
open vessel
C : Thermistor
closed vessel
(3) Detector Circuit of 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
than the detector circuit starts to function and the LSI
observes the initial voltage available at its AN5 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 R45 ~ R49. Changing
the resistance values results in that there is the same
HUMIDITY SENSOR CIRCUIT
Thermistors
Sensing part Sensing part
(Open vessel) (Closed vessel)
ο
F), the resultant heat is
Absolute humidity vs,
output voltage characteristic
Absolute humidity (g/m )
potential at both F-3 terminal of the absolute humidity
sensor and AN5 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 AN5 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 the
bridge circuit due to disconnection of the absolute humidity
sensor, ERROR will appear on the display and the
cooking is stopped.
1) Humidity sensor circuit
VA : +15V
C. Thermistor in
closed vessel
S. Thermistor in
F2
open vessel
F3
F1
R43 1.8K
R51 47K
R45 620K
R46 300K
R47 150K
R48 75K
R49 37.4K
2
29
VC : +5V
D40 D41
86
3
8
+
90
1
IC5
AN6
-
R50 10K R52 47K
4
2
LSI
(IC1)
R44 360K
91
AN5
SW1
30
P70
SW2
29
P71
SW3
28
P72
SW4
27
P73
SW5
26
74
KB-5121KS/K/W