Sharp Carousel R-990C Service Manual - Page 26

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R-990C(S)

Pin No.

Signal

57-59

P37-P35

60-64

P34-P30

(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.

Sensing part

(Open vessel)

Ventilation

openings

Sensing part

(Closed vessel)

View of sensor case removed

(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

amplifier

Output

voltage

S : Thermistor

open vessel

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 AN6

terminal.

I/O

OUT

Digit selection signal.

Signal similar to P55.

OUT

Used for initial balancing of the bridge circuit (absolute humidity sensor).

ABSOLUTE HUMIDITY SENSOR CIRCUIT

Sensing part

(Open vessel)

(Closed vessel)

Thermistor

element

Sensor

case

Cross section view

Absolute humidity vs,

output voltage characteristic

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 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 AN7 terminal of the LSI.

The voltage of AN6 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

AN6 terminal of the LSI.

Then the LSI observes that voltage at AN6 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 AN6

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

IC2(IZA495DR)

C. Thermistor in

closed vesssl

S. Thermistor in

open vessel

F-1

F-3

3.57k

3.32k

1.8k

F-2

VA : -15V

SW1

P30

620k

SW2

P31

300k

SW3

P32

150k

SW4

P33

75k

SW5

P34

37.4k

AN7

47k

(IC1)

10k

AN6

47k

360k

VA : -15V

VC : -5V

LSI