Sharp R-320EK Service Manual page 27

(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.
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 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
+
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
than the detector circuit starts to function and the LSI
observes the initial voltage available at its ANI5 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 R98 ~ R102. Changing
the resistance values results in that there is the same
potential at both F-3 terminal of the absolute humidity
sensor and ANI4 terminal of the LSI. The voltage of ANI5
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
ABSOLUTE HUMIDITY SENSOR CIRCUIT
Thermistors
Sensing part Sensing part
(Open vessel) (Closed vessel)
Absolute humidity vs,
output voltage characteristic
Absolute humidity (g/m ) 2
resistance balance of the bridge circuit is deviated to
increase the voltage available at ANI5 terminal of the
LSI.
Then the LSI observes that voltage at ANI5 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 ANI5
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) Absolute humidity sensor circuit
R98 620k
300k
R99
R100 150k
R101 75k
R102 37.4k
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
VA : -15V VA : -15V
25
SW1
44
P31
SW2
45
P32
SW3
46
P33
SW4
47
P34
SW5
48
P35
32
ANI4
LSI
(IC1)
R94 R95
31
ANI5
10k
47k
R93
360k
D90
VC : -5V
R-320EK
R-320EQ
R-320EW