Sharp R-3H57 Service Manual page 24

R-3H57
(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.
(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, 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 amplifier
R1
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
than the detector circuit starts to function and the LSI
observes the initial voltage available at its AN1 terminal.
With this voltage given, the switches SW1 to SW5 in the
ABSOLUTE HUMIDITY SENSOR CIRCUIT
Sensing part
Sensing part
(Open vessel)
(Closed vessel)
Absolute humidity vs,
output voltage characterist
Absolute humidity (g/m ) 2
LSI are turned on in such a way as to change the
resistance values in
Changing the resistance values 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 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 AN1 terminal
of the LSI.
Then the LSI observes that 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
IC2(IZA495DR)
R3
R4
R5
R6
R7
C. Thermistor in
closed vesssl
S. Thermistor in
open vessel
R8
47k
11
F-1
C
F-3
3.57k
S
3.32k
1.8k
F-2
10
0
VA : -15V
VA : -15V
22
parallel with R3 - R7 of IC2.
SW1
9
4
P97
620k
SW2
5
8
P96
300k
SW3
6
7
P95
150k
SW4
7 6
P94
75k
SW5
8 5
P93
37.4k
11
3
AN0
LSI
(IC1)
+
10k
12
2
AN1
- 47k
360k
1
12
VC : -5V