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Automatic
temperature controller
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The
circuit presented here main- tains the temperature of
the load (e.g., electric irons, heaters, etc) between
two preset levels, i.e., a higher and a lower level which
may be manually adjusted with the help of potentiometers
VR1 and VR2 respectively. The load supply has been connected
via a relay such that the load is ‘on’ only
when the relay is energised. The circuit makes use of
two negative temperature coefficient (NTC) thermistors
having a resistance of approximately 3 kilo-ohms at room
temperature. The thermistors should be placed in the vicinity
of the load for sensing its temperature. The thermistors
are connected to the op-amps which are operated in open
loop mode, i.e., as comparators. IC2 (555) is used in
monostable mode and is wired in such a way that its output
‘on time’ not only depends on RC time constant
(1.1x R9 x C1) but also on the output status of op-amp
B at pin 7. Initially, when the circuit is switched on
and the temperature is normal, output pins 1 and 7 of
op-amps A and B and pin 3 of timer 555 are at low level.
Transistor T3 is off and transistor T4 is forward biased
via zener D3. The relay is energised and hence the power
supply to the load is connected. When the temperature
of the load increases and crosses the lower level for
the first time, pin 7 of the op-amp B goes high but nothing
happens since collector of transistor T2 is open circuited
as relay RL1 is in energised state. However, when temperature
further rises and crosses the higher level, output pin
1 of op-amp A also goes high, triggering IC 555 via transistor
T1. The output of 555 goes high and results in conduction
of transistor T3, cutting off transistor T4. This in turn
switches off the relay and thus the supply to the load
is disconnected. Timer 555 will continue to remain in
this state and will come out only when the temperature
falls below the low level, at which stage output pin 7
of op-amp B would go low and the load would be again switched
on (connected to the supply). A buzzer with its positive
terminal connected to the collector of the transistor
T4 and the negative terminal grounded can be used to give
audio indication when the temperature is between the desired
limits.
The current rating of the relay contacts should be high
enough to withstand the load current. This circuit, when
incorporated in the electric water heater or any other
electric heating appliance, would effectively maintain
the temperature within the desired limits. |
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