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Section 2 Operation

2-8

HUMIDITY SENSOR CIRCUIT

1. Structure of Absolute Humidity Sensor:

The absolute humidity sensor includes two

thermistors as shown in the illustration below. One

thermistor is housed in the closed vessel filled with
dry air while another in is 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 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.

Ventilation opening for sensing

S ensing part
(Open vessel)

S ensing part
(C losed vessel)

Thermistors

R 3

R 1

R 2

Operational
amplifier

Output
voltage

S : Thermistor

open vessel

C : Thermistor

closed vessel

Humidity (g/m )

tpu

lta

humidity vs, out put voltage
characteristic

Figure 2-5. Humidity Sensor Operation

Figure 2-4. Humidity Sensor Diagram

(3) Detector Circuit of Absolute Humidity Sensor
Circuit:

This detector circuit is used to detect the output
voltage of the humidity circuit to allow the LSI to
control sensor cooking of the unit.  When the unit is
set in the sensor cooking mode, 32 seconds clearing
cycle occurs than the detector circuit starts to
function and the LSI observes the initial voltage
available at its AIN4 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 R98 ~ R102 of IC2. Changing
the resistance values results in that there is the
same potential at both F-3 terminals of the humidity
sensor and AIN6 terminal of the LSI. The voltage of
AIN4 terminal will indicate about -2.5V. This initial
balancing is set up about 32 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 AIN4 terminal of the LSI.  Then the LSI observes
that voltage at AIN4 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 AIN4 terminal 32 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 humidity sensor,
ERROR will appear on the display and the cooking is
stopped.