Sharp R-980J Service Manual page 26

R-980J
Pin No. Signal
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 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.
I/O
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
Sensor
case
Cross section view
Absolute humidity vs,
output voltage characterist
Absolute humidity (g/m )
Description
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
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
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
IC2(IZA495DR)
C. Thermistor in
closed vessel
S. Thermistor in
open vessel
11
F-1
2
C
F-3
3.57k
S
3.32k
1.8k
F-2
10
9
VA : -15V
24
SW1
64
4
P30
620k
SW2
5 63
P31
300k
6 62
P32
150k
7 61
P33
75k
60
8
P34
37.4k
5
3
AN7
47k
(IC1)
+
10k
6
2
AN6
- 47k
360k
1
12
VA : -15V
VC : -5V
SW3
SW4
SW5
LSI