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R-9Zoo
ABSOLUTE
HUMIDITY
SENSOR
CIRCUIT
(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.
(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
(RI
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.
b
Absolute humidity
(g/m* 1
(3)Detector
Circuit
of Absolute
Humidity
Sen-
sor 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 IN1 terminal.
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 IN0 terminal
of the
LSI. The voltage of IN1 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
bal-
ance of the bridge circuit
is deviated to increase the
voltage
available
at IN1 terminal
of the LSI.
Then
the LSI observes
that voltage
at IN1 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
IN1 terminal
16 seconds after the unit has been put
in the Sensor Cooking
mode, if it is not possible to
I
balance, of the bridge circuit
due to disconnection
of the absolute
humidity
sensor,
ERROR will
ap-
,
I
pear on the display
and the cooking
is stopped.
1 )Absolute
humidity
sensor circuit
IC2( IZA3
1
ODR)
--.-.
IT-z
--I4
SW1
620K
lo
P47d
I5
11
9.
*
300K
'6
12
P46pT
r
.
P45s3,
150K
7
13
"1
I-
P44 s%
C. Thermtstor
in
8
75K
16
14
SW5
"1
closed
vessel
I '-
37.4K
I
P43
4
"1
S. Thermistor
In
open vessel
I
IN0
VA:-1SV
VA:
-15v
VC:
-5v
19
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