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R-430BK
E%F
ABSOLUTE HUMIDITY SENSOR CIRCUIT
Structure of Absolute Humidity Sensor
the sensor cooking of the unit.
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 is in an
open vessel. Each sensor is provided with a protective
cover made of metal to protect it from the external air-
flow. The open vessel cover has two small holes in it to
allow moisture laden air from the oven cavity to surround
it.
Thermistors
Sensing part
ventilation openings for sensing
(Open vessel)
Sensing part
(Closed vessel)
Operational Principle of AH Sensor
The figure below shows the basic structure of an abso-
lute humidity sensor. A bridge circuit is formed by two
thermistors and two resistors (RI and R2).
The output of the bridge circuit is amplified by an opera-
'
tional amplifier.
Each thermistor
is supplied with a current to keep it
heated to about 150" C (302" F) and the resultant heat
is dissipated in the air, if the two thermistors are placed
in different humidity conditions they show different hu-
midity conditions and different degrees of heat conduc-
tivity leading to a potential difference between them
causing an output voltage from the bridge circuit, the in-
tensity 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
+
R3I
-
I
open vessel
I
Absolute humidity vs.
\
output voltage characteristic
[
C: Thermistor
'
closed vessel
Absolute humidity (g/m3)
)
Detector Circuit of AH Sensor Circuit
This detector circuit is used to detect the output voltage
of the absolute humidity circuit to allow the LSI to control
When the unit is set in the sensor cooking mode, 16 sec-
onds later the detector circuit starts to function and the
LSI observes the initial voltage available at its AN1 ter-
minal.
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 RI 07-RI 11 of IC2.
Changing the resistance values results in that there is
the same potential at both the F-3 terminal of the abso-
lute humidity sensor and the AN0 terminal of the LSI.
The voltage of AN1 terminal will indicate about -25V.
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 the AN1 terminal of the LSI. Then the LSI
observes that voltage at AN1 terminal and compares it
with its initial value. When the comparison rate reaches
the preset value (fixed for each menu to be cooked), the
LSI caused the unit to stop sensor cooking; thereafter,
the unit goes into 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 impossible to take a bal-
ance of the bridge circuit due to disconnection of the ab-
solute humidity sensor, ERROR will reappear on the dis-
play and cooking is stopped.
Absolute Humidity Sensor Circuit
r
--------_____
R107 620k Q
80
SW1
R108 300k '
I 81
P1O-o-Q
SW2
R1O 150kQ
Pll -o-=-J
0
L%vL
82
SW3
P12-o-o-J
83
SW4
I
P137
R11137.4k Q
I
SW5
I
"
-
84
P14-o-o-J
Va: -15V
S:
Thermistor open vessel
Va: -15V
vc: -5v
C: Thermistor closed vessel
19
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