Absolute Humidity Sensor Circuit; Structure Of Absolute Humidity Sensor; Operational Principle Of Absolute Humidity Sensor - Sharp R-310AK Supplemental Service Manual

R-310AK
R-330AK
R-330AW
Pin No. Signal
40 COM3 OUT
41 COM4 OUT
42 VDD
ABSOLUTE HUMIDITY SENSOR CIRCUIT (R-330AK/AW)

(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 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
+
-
R2
S
S : Thermistor
open vessel
R3
C : Thermistor
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, 16 seconds clearing cycle occurs than the
detector circuit starts to function and the LSI observes the
initial voltage available at its AN1 terminal.
I/O
Common data signal: COM1.
Connected to LCD (Pin No. C3)
Terminal not used.
IN Power source voltage input terminal.
Connected to GND.
Thermistors
Sensing part
Sensing part
(Open vessel) (Closed vessel)
Absolute humidity vs,
output voltage characterist
Absolute humidity (g/m )
Description
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. 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 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 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 absolute humidity
sensor, ERROR will appear on the display and the cooking
is stopped.

1) Absolute humidity sensor circuit

C. Thermistor in
closed vesssl
S. Thermistor in
open vessel
F-1
C
R112
F-3
3.57k
S
R105
2
3.32k
F-2
VA : -15V
8
19
R107
R47
620k
R108 20
R46
300k
R109
21
R45
150k
R110
22
R44
75k
R111 23
R43
37.4k
R106
10
AN0
47k
+
R103 R104
9
IC2
AN1
- 10k
47k
R101
R102
1.8k
360k
D101
VA : -15V VC : -5V
SW1
SW2
SW3
SW4
SW5
LSI
(IC1)