R-761M - Sharp Home Appliance Service Manual (repair manual). Page 13

13

761M -

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.

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.

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 AN7 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 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 AN6 terminal of the LSI. The
voltage of AN7 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 AN7 terminal
of the LSI.
Then the LSI observes that voltage at AN7 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 AN7
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

Sensing part
(Open vessel)

Sensing part
(Closed vessel)

Thermistors

ventilation opening for sensing

C

S

R3

R1

R2

+

-

Operational
amplifier

Output
voltage

S : Thermistor
     open vessel

C : Thermistor
      closed vessel

2

Absolute humidity (g/m  )

Output voltage

Absolute humidity vs,
output voltage characterist

SW1

SW2

SW3

SW4

SW5

D0

D1

D2

D3

D4

 LSI
(IC1)

AN6

AN7

620k

300k

150k

75k

37.4k

11

1

2

14

15

13

12

47k

47k

IC2

10k

0.01uF

0.015uF

0.01uF

360k

+

-

VA : -15V

VA : -15V

R90

C101

C102

C103

C104

S

F-2

1.8k

F-1

F-3

C

3.57k

3.32k

VC : -5V

0.1 uF

C. Thermistor in 
    closed vessel
S. Thermistor in
    open vessel

R107

R108

R112

R105

R101

R103

R104

D101

R109

R110

R111

R106