Frigidaire GLBM188HB, Руководство по обслуживанию

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Page 26
GLBM188HB
9 – 2
[2] HUMIDITY SENSOR CIRCUIT
1. Structure of Humidity Sensor
The humidity sensor includes two thermistors as shown in the illustra-
tion. One thermistor is housed in the closed vessel filled with dry air
while another in the open vessel. Each sensor is provided with the pro-
tective cover made of metal mesh to be protected from the external
airflow.
2. Operational Principle of Humidity Sensor
The figure below shows the basic structure of a 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 opera-
tional 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 differ-
ent degrees of heat conductivity leading to a potential difference
between them causing an output vo ltage from the bridge circuit, the
intensity of which is increased as the humidity of the air increases.
Since the output is very minute, it is amplified by the operational ampli-
fier.
3. Detector Circuit of 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 cook ing mode, 32 seconds clearing cycle
occurs than the detector circuit starts to function and the LSI observes
the initial voltage available at its P 65 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 P58 ~ P62 of IC2. Chang-
ing the resistance values results in that there is the same potential at
both F-3terminalof the humidity sensor and P66 terminal of the LSI.
The voltage of P65 terminal will indicate about -2.5V. This initial bal-
ancing is set up about 32 seconds after the unit is put in the Sensor
Cooking mode. As the sensor cook ing proceeds, the food is heated to
generate moisture by which the resistance balance the bridge circuit is
deviated to increase the voltage available at P65 terminal of the LSI.
Then the LSI observes that voltage at P65 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 P65
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 discon-
nection of the humidity sensor, ERROR will appear on the display and
the cooking is stopped.
1)Humidity sensor circuit
Ventilation opening for sensing
Sensing part
(Open vessel) Sensing part
(Closed vessel)
Thermistors
C
S
R3
R1
R2
+
-
Operational
amplifier
Output
voltage
S : Thermistor
open vessel
C : Thermistor
closed vessel
2 Humidity (g/m )
O
ut
pu
tvolta
ge
humidity vs, output voltage
characteristic
SW1
SW2
SW3
SW4
SW5
P70
P71
P72
P73
P74
LSI
(IC1)
P66
P65
620k
300k
150k
75k
37.4k
28
31
32
26
25
30
29
47k
47k
10k
0.01uF
0.1uF 0.01
uF
VA : -15V VA : -15V
R50
C60
C61
C62
C9
2
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
R58
R59
R50
R51
360k
R53
R52
R54 R55
D61
D60
R60
R61
R62
R57
IC2
+
-