Sharp R-880B Service Manual page 27

(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.
ventilation opening for sensing
(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 differ-
ent degrees of heat conductivity leading to a potential
difference between them causing an output voltage
from the bridge circuit, the intensity of which is in-
creased 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 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 AN1
terminal.
ABSOLUTE HUMIDITY SENSOR CIRCUIT
Thermistors
Sensing part
Sensing part
(Open vessel) (Closed vessel)
Absolute humidity vs,
output voltage characterist
Absolute humidity (g/m ) 2
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 R107 ~ R111 of IC2.
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 compari-
son 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 abso-
lute humidity sensor, ERROR will appear on the dis-
play and the cooking is stopped.
1) Absolute humidity sensor circuit
R107
R108
R109
R110
R111
C. Thermistor in
closed vessel
S. Thermistor in
open vessel
R106
47k
F-1
C
R112
F-3
3.57k
S
R105
R101
3.32k
F-2
1.8k
VA : -15V VA : -15V
25
SW1
620k 14
P54
SW2
300k 15
P53
SW3
150k 16
P52
SW4
17
75k
P51
SW5
37.4k
18
P50
10
AN0
LSI
(IC1)
+
R103 R104
9
IC2
AN1
10k
47k
R102
360k
D101
VC : -5V
R-880B