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R-9370
REHEAT SENSOR CIRCUIT
1. Operational
Principle
of Reheat
Sensor
As shown in Fig. T-8,2
pieces of temperature sensitive resis-
and RH, are used in the detector of the reheat sensor. These
temperature
sensitive resistors are cased for protection.
Fig. T-9 shows the basic circuit. The temperature
sensitive
resistor RH, is receiving approx. 52mA of current at all times,
keeps itself heated to approx. 460°F
(240°C).
A minute current is applied to the temperature sensitive resis-
tor RN to measure the ambient temperature.
VOUT
(output
voltage)
in Fig. T-9
can be expressed
as
follows.
RH
VOUT =
-
RN
lo
l
Rf
Where,
lo is a constant
current and Rf is a fixed
resistance.
Therefore,
VOUT
will be the voltage in proportion to the
resistance ratio between RH and RN.
The temperature sensitive resistors RH and RN have a linear
and positive characteristic against the temperature. Therefore,
when the temperat;;
is the only varying force with humidity
being constant,
R
value at a constant
evel.
r
becomes
constant,
keeping
VOUT
When vapors are produced from the food, the RH resistor side
is cooled
down
reducing
the
temperature,
making
the
resistance lower. Therefore,
a
VOUT
voltage is generated in
proportion to the variation of vapor volume.
Internal Structure al Fft-i Sensor
Figure T-8.
Operational
amplifier
Rf
output
voltage
2. Reheat
Sensor
Circuit
This circuit is composed of a part which transmits a change
of the reheat sensor voltage to PO0 terminal of LSI and of the
which supplies a constant current to the temperature
sensi-
tive resistor RH.
When sensor cooking is started, 16 seconds of cleaning time
will follow to let out residual moisture.
After this cleaning time, the magnetron is oscillated. Forty se-
conds later, SW1 to SW5 in LSI are turned on so that the in-
put voltage to PO0 terminal of LSI will become approx. -4.8V.
Then, in compliance with the changes of vapor volume com-
ing from the food, the resistance ratio between RN and RH
varies to feed PO0 terminal of LSI with the voltage in propor-
tion to this ratio.
When this voltage level reaches the preset detection point, the
cooking is terminated.
if the initial voltage setting cannot be made due to open or
short-circuit the of sensor elements approximately 40 seconds
after the sensor cooking was started, the indicator shows ER-
ROR and sensor cooking is terminated.
ADVOl.
- 24"
f
-6.15V
34
P63
35
36
P9a
P91
-
'9
PM)
1ooP
33b
6
rn4T
49
SW
49
SW2
4%
SW3
49
SW4
47
sws
Figure T-l 0.
Figure T-9.
18
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