Sharp R-530EK-T Service Manual Download Page 23

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R530EWT

9 – 2

[2] 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 Sen-
sor

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, 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 differ-
ence 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 opera-
tional amplifier.

3. Detector Circuit of Absolute Humidity Sensor Cir-
cuit

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 AIN5 terminal.With this volt-
age 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 R98 ~ R102.
Changing the resistance values results in that there is the same poten-
tial at both F-3 terminal of the absolute humidity sensor and AIN4 ter-
minal of the LSI. The voltage of AIN5 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 AIN5 ter-
minal of theLSI.

Then the LSI observes that voltage at AIN5 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 operationau-
tomatically.When the LSI starts to detect the initial voltage at
AN1terminal 16 seconds after the unit has been put in the Sensor
Cooking mode, if it is not possible to balance 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

Ventilation opening for sensing

Sensing part
(Open vessel)

Sensing part
(Closed vessel)

Thermistors

R 3

R 1

R 2

Operational
amplifier

Output
voltage

S : T hermis tor

open ves s el