Sharp R-9570 Service Manual page 20

R-9570
ABSOLUTE HUMIDITY SENSOR CIRCUIT
(I) 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.
Sensinq
Part
Internal Structure of
Sensor
OP
"CI
Vent hole
Figure T-10.
(2)
Operational Principle of Absolute Humidity Sensor
The figure below shows the basic strcture of an absolute hu-
midity sensor. A bridge circuit is formed by two thermistors
and two resistors (Rl and R2).
The output of the bridge circuit is to be amplified by the oper-
ational amplifier.
Each thermistor is supplied with a current to keep itself heat-
ed at about 1 50°C (302OF) and the resultant heat is dissipat-
ed in the air and if the two thermistors are placed in different
humidity conditions they show different degrees of heat con-
ductivity leading to potential difference between them caus-
ing 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 will be amplified by the oper-
ational amplifier.
C: Thermistor in
closed vessel
Figure T.
Absolute humtdlty vs.
output voltage character
IstIc
b
11. Absolute humldlty (g/m' 1
(3) Detector Circuit of Absolute Humidity Sensor
This detector circuit is used to detect the output voltage of
the absolute humidity circuit to allow the LSI to control the
sensor cooking of the unit.
When the unit is set in the Sensor Cooking mode, 16 seconds
later the detector circuit starts to function and the LSI observes
the initial voltage available at its AN2 terminal. With this vol-
tage 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 C-3 terminal of the absolute hu-
midity sensor and AN3 terminal of the LSI. The voltage of AN2
terminal will indicate about - 2.5V. This initial balancing is set
up
about 16 seconds after the unit is put in the Sensor Cook-
ing mode.
As the sensor cooking proceeds, the food is heated to gener-
ate moisture by which the resistance balance of the bridge cir-
cuit is deviated to increase the voltage available at AN2
terminal of the LSI. Then the LSI observes that voltage at AN2
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 the sensor cook-
ing; thereafter, the unit gets in the next necessary opeation
automatically.
When the LSI starts to detect the initial voltage at AN2 termi-
nal 16 seconds after the unit has been put in the Sensor Cook-
ing mode, if it is impossible to take a balance of the bridge
circuit due to disconection of the absolute humidity sensor,
ERROR will reappear on the display and the cooking is stopped.
1) Absolute humidity sensor circuit
050
2)
0
RSO-517 23
E22d
it
sw4-i
0
C
Therm,.for ,n I
I,
MO-719 21
closed vessel
I
s
Thermastor tn
open *es,c,
LSI
( IC 1)
AN2
RS3
Figure T- 12.
internal equivalent circuit of 12
CH1
CH2
_-_--------- ---- -
Top
View
VOLrTl
VIN(-)I
VIN(+)l
VF.E
Figure T-l 3.
VW
volrr2
VIN(-12
VIN(+)2
--f+4IN(-)2
.--+VIN(+)2
18