R-362M (serv.man2) - Sharp Home Appliance Service Manual (repair manual). Page 14

R362M - 14

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 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

Sensing part
(Open vessel)

Sensing part
(Closed vessel)

Thermistors

ventilation opening for sensing

SW1

SW2

SW3

SW4

SW5

P54

P53

P52

P51

P50

 LSI
(IC1)

AN0

AN1

620k

300k

150k

75k

37.4k

14

9

17

18

10

16

15

47k

47k

IC2

10k

0.01uF

0.015uF

0.01uF

+

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

360k

R102

R101

R103

R104

D101

R109

R110

R111

R106

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 characteristic

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 AN1 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.