Sharp R-4H56 Service Manual Download Page 24

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R-4H56

(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 is in the
open vessel. Each sensor is provided with the protective
cover made of metal mesh to be protected from the
external airflow.

Metal
mesh

Sensing part
(Open vessel)

Sensing part
(Closed vessel)

(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 itself heated at about 150˚C (302˚F) and 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 will be amplified by the
operational amplifier.

Operational
amplifier

Output
voltage

S : Thermistor
open vessel

C : Thermistor
closed vessel

Absolute humidity (g/m )

Output voltage

Absolute humidity vs,
output voltage characterist

(3) Detector Circuit of Absolute Humidity Sensor

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 later the detector circuit starts to function and
the LSI observes the initial voltage available at its A6
terminal. With this voltage given, the switches SW1 to
SW5 in the LSI are turned on in such a way as to change

ABSOLUTE HUMIDITY SENSOR CIRCUIT

the resistance values in parallel with R-1. Changing the
resistance values results in that there is the same
potential at both F-3 terminal of the absolute humidity
sensor and A7 terminal of the LSI. The voltage of A6
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 A6 terminal of the LSI.
Then the LSI observes that voltage at A6 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 gets in the next necessary operation
automatically.
When the LSI starts to detect the initial voltage at A6
terminal 16 seconds after the unit has been put in the
Sensor Cooking mode, if it is impossible to take a
balance of the bridge circuit due to disconnection of the
absolute humidity sensor, ERROR will reappear on the
display and the cooking is stopped.