INTRODUCTION
A thermistor is the transducer used for this project. you will
remember that thermistor is really a resistor,
the value of which changes with its temperature.
one useful kind of
thermistor has a resistance which decrease with temperature rise- it is known
as negative temperature coefficient (n.t.c) thermistor
in the circuit to be described
,the temperature change of the thermistor is used to control the switching of a relay .the relay contact are use to
switch on and off the current to an electric heater and some of this heat
provides the essential feedback common to all control circuit .the block
diagram shows the essential requirement of a thermal-control circuit or
thermostat.
CIRCUIT
The circuit is known as
Schmitt trigger and in this application it switches on or energizes the relay
when heat falls on the thermistor .
the circuit works with
the aid a variable resistor and a thermistor wired in
voltage dividers mode and the value of VR determines the level of heat at which
the relay energize.that is to say in the thermostat an increase of temperature
of the thermistor causes Tr1 to switch on and so Tr3 to switch off.this means
that if the source of heat for the thermistor is reduced ,the thermistor
cools,when its resistance has increase sufficiently,Tr1
turns off and tr3 on ,so that heat is again generated by the heater switched on
by the relay contacts.
Types of thermistor
VA1098 has resistance of about 1.5k ohms at 25
degree census
VA1066s has resistance
of about 4.7k ohms at 25 degree census
Whichever types used
one should be able to check the power dissipation by measuring the current and
voltage across under operating condition
to ensure it does not dissipate more than mazimum.
The mazima are listed in the catalogues of the supplies of
thermistor
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