Sharp R-1880LS Service Manual page 37

Pin No. Signal
56 P00 OUT
57-59 P37-P35 OUT
60-64 P34-P30 OUT
(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 glass tube filled with dry air while another in the open
glass tube. Each sensor is provided with the protective cover
made of metal with holes to be protected from the external
airflow.
Sensing part
(Closed glass tube)
(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 R).
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
ο ο
at about 150 C (30 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
+
-
S
R2
S : Thermistor
open vessel
C : Thermistor
R3
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, a 16 seconds clearing cycle occurs. Then 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
I/O
while one of G line keys on key matrix is touched.
Segment data signal.
Key strobe signal.
Signal applied to touch-key section. A pulse signal is input to AN1 and P7 - P4 terminal
while one of G1 line keys on key matrix is touched.
Segment data signal.
Used for initial balancing of the bridge circuit (absolute humidity sensor).
ABSOLUTE HUMIDITY SENSOR CIRCUIT
Sensing part
(Open glass tube)
Absolute humidity vs,
output voltage characterist
Absolute humidity (g/m )
Description
Signal similar to P55.
Signal similar to P55.
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 -.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.
After 16 seconds of operation in the sensor cook mode, the
LSI detects the initial voltage at AN6. If the absolute humidity
sensor is open or shorted, the circuit can not be balanced for
initial voltage comparisons. This will result in the word "error"
on the display and the cooking operation is stopped.
1) Absolute humidity sensor circuit
IC2(IZA495DR)
C. Thermistor in
closed vesssl
S. Thermistor in
open vessel
2
11
F-1
C
F-3
3.57k
S
3.32k
1.8k
F-2
10
9
VA : -15V
37
SW1
64
4
P30
620k
SW2
63
5
P31
300k
SW3
6 62
P32
150k
SW4
7 61
P33
75k
SW5
60
8
P34
37.4k
5
3
AN7
47k
LSI
(IC1)
+
6
10k
2
AN6
- 47k
360k
1
12
VA : -15V
VC : -5V
R-1880LS/LST