Absolute Humidity Sensor Circuit - Sharp R-990KS Service Manual

Pin No. Signal I/O
47 P11 OUT
48 P10 OUT
49-53 P07-P03 OUT
54-56 P02-P00 OUT
57-59 P37-P35 OUT
60-64 P34-P30 OUT

[3] ABSOLUTE HUMIDITY SENSOR CIRCUIT

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 Sen-
sor
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 differ-
ence 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 opera-
tional amplifier.
Operational
C
R 1
amplifier
Output
+ voltage
-
R 2
S
S : T hermis tor
open ves s el
R 3
C : T hermis tor
clos ed ves s el
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-6 line keys on
key matrix is touched.
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.
Segment data signal.
Signal similar to P23.
Digit selection signal.
Signal similar to P55.
Digit selection signal.
Signal similar to P55.
Used for initial balancing of the bridge circuit (absolute humidity sensor).
Sensing part Sensing part
(Closed vessel)
(Open vessel)
Thermistor
Thermistor
element
element
Sensor
case
Cross section view
Abs olute humidity vs ,
output voltage characteris tic
Abs olute humidity (g/m ) 2
Description
3. Detector Circuit of Absolute Humidity Sensor Cir-
cuit
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 AN6terminal. With this volt-
age 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. Chang-
ing 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 cooking proceeds, the food is heated to
generate moisture by which the resistance balance the bridge circuit is
deviated to increase the voltage available atAN6 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 pre-
set 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
AN6terminal 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
IC 2(IZA495DR )
C . T hermis tor in
clos ed ves s s l
S . T hermis tor in
open ves s el
11
F -1
C
F -3
3.57k
S
3.32k
1.8k
F -2
10
9
V A : -15V
9 – 4
64
4
P 30
620k
5 63
P 31
300k
6
62
P 32
150k
7 61
P 33
75k
8 60
P 34
37.4k
5
3
AN7
47k
+
10k
6
2
AN6
- 47k
360k
12 1
V A : -15V
V C : -5V
R990K(W)
S W1
S W2
S W3
S W4
S W5
LS I
(IC 1)