Sharp R-930AK-F Service Manual page 30

R-930AK-F
R-930AW-F
Pin No. Signal
48 P10
49-53 P07-P03
54-56 P02-P00
57-59 P37-P35
60-64 P34-P30
(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 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
I/O
OUT Segment data signal.
Signal similar to P23.
Key strobe signal.
Signal applied to touch-key section. A pulse signal is input to P24-P27 terminal while
one of G-5 line keys on key matrix is touched.
OUT Segment data signal.
Signal similar to P23.
OUT Digit selection signal.
Signal similar to P55.
OUT Digit selection signal.
Signal similar to P55.
OUT Used for initial balancing of the bridge circuit (absolute humidity sensor).
ABSOLUTE HUMIDITY SENSOR CIRCUIT
Sensing part Sensing part
(Open vessel) (Closed vessel)
Thermistor
Thermistor
element
element
Cross section view
Absolute humidity vs,
output voltage characteristic
Absolute humidity (g/m )
Description
cooking mode, 16 seconds clearing cycle occurs 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
Sensor
cooking proceeds, the food is heated to generate moisture
case
by which the resistance balance 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
C. Thermistor in
closed vesssl
S. Thermistor in
open vessel
11
F-1
C
F-3
3.57k
2
S
3.32k
F-2
10
VA : -15V
28
IC2(IZA495DR)
4 64
620k
5
63
300k
6 62
150k
7 61
75k
60
8
37.4k
5
3
AN7
47k
+
10k
6
2
AN6
- 47k
1.8k
360k
9
12 1
VA : -15V
VC : -5V
SW1
P30
SW2
P31
SW3
P32
SW4
P33
SW5
P34
LSI
(IC1)