therm is tor characteristic determination and temperature control
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THERMISTORCHARACTERISTIC
DETERMINATION ANDTEMPERATURE CONTROL
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INTRODUCTION
Temperature is defined as degree ofhotness or coldness of any body. Nowa days temperature control is very
important in industries.
Temperature can be measured usinga device called Thermometer.
There are many ways to controltemperature, but our main concern isto control it with the help of
THERMISTOR.
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ABSTRACT
Negative temperature coefficient(NTC) Thermistor will be used as thetemperature controller.
We will take a liquid in a container,and a Thermometer and Thermistorwill be immersed in it.
The whole setup is heated, andgradual increase in temperature isrecorded by the Thermometer.
The Thermistor will be connected to
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The value of the current flowing inthe circuit will be recorded by a milli
ammeter. As the temperature of the liquid will
increase there will be a decrease in
the resistance of the Thermistor as ithas negative temperature coefficientof resistance.
Based on the above readings a graphbetween temperature and resistancewill be drawn, and this graph will
give us THERMISTOR
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The enabling point of the Relay willbe connected to the output of thecomparator whereas the input andoutput points of the Relay will beconnected to the two terminals of the
Battery. Comparator has two inputs, one will
be supplied with reference voltage
which is given by the battery, whilethe other input voltage is given bythe Thermistor circuit.
If the Thermistor circuit voltage rises
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ThermistorsTHERMAL RESISTORS
A Thermistor is a type of resistor used tomeasure temperature changes, relying onthe change in its resistance with changingtemperature.Thermistor is a combination
of the words thermal and resistor. TheThermistor was invented by Samuel Rubenin 1930, and has U.S. Patent #2,021,491.
Leads, coated Glass encased Surface mount
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Thermistors are made ofsemiconductor materials (metalliccompounds including oxides such asmanganese, copper, cobalt, andnickel, as well as single-crystalsemiconductors silicon and
germanium).
Contrast Common carbon
resistors, made from carbon powder
ThermistorsTHERMAL
RESISTORS
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Assume a simple linear relationshipbetween resistance and temperature forthe following discussion:
R = k T
where
R = change in resistanceT = change in temperature
k= first-order temperature coefficientof resistance
ThermistorsTHERMAL
RESISTORS
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Thermistors can be classified into twotypes depending on the sign of k.
If k is positive, the resistance increaseswith increasing temperature, and thedevice is called a positive temperaturecoefficient (PTC) Thermistor, Posistor.
If k is negative, the resistance decreaseswith increasing temperature, and thedevice is called a negative temperature
coefficient (NTC) Thermistor.
ThermistorsTHERMALRESISTORS
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ThermistorsTHERMAL RESISTORS
Applications:
1. Temperature measurement.
2.
Time delay (self heating from largecurrent opens the Thermistor so it canbe used as a slow switch). Heating = i2R where R is the resistance and i is the
current.3. Surge suppression when a circuit is first
energized. Current needs to flow throughthe Thermistor for awhile to heat it so
that it opens, and acts again as a
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THERMISTORCHARACTERISTIC GRAPH
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RELAY
Relays areelectrical switches
that open or closeanother circuitunder certain
conditions.
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Relay Purpose
Isolate controlling circuit fromcontrolled circuit.
Control high voltage system withlow voltage. Control high current system with
low current. Logic Functions
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Relay Operation
When a current flows through thecoil, the resulting magnetic fieldattracts an Armature that is
mechanically linked to a movingcontact, the movement either makesor breaks contractor with a fixed
contact.
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There is no fault free system.
It is neither practical nor economical to builda fault free system.
Electrical system shall tolerate certaindegree of faults.
Usually faults are caused by breakdown ofinsulation due to various reasons: system
aging, lighting, etc. Detect system failures when they occur and
isolate the faulted section from theremaining of the system.
Mitigating the effects of failures after theyoccur. Minimize risk of fire, danger to
personal and other high voltage systems.
Need For Relay
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Thermal Overload Relays
Use bimetallic strips toopen/close relay contacts when
temperature exceeds/drops tocertain level.
Require certain reaction time
Inverse time/current relationship
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IC 741
Op Amp 741 is an 8 pin IC.
We will be using it as a voltagecomparator.
A comparator is a circuit thatcompares two input voltages. Onevoltage is called reference voltageand other is called input voltage.
When input voltage rises or fallsbelow reference voltage then outputchan es olarit .
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4/25/12 PIN DIAGRAM OF IC 741
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IC 741
IC 741 has inputs at pins 2 and 3.
Output at pin 6.
Offset null at pin 5. -Vcc at pin 4.
+Vcc at pin 7.
No connection at pin 8.
Pin 3 is non inverting input with sign
+, pin 2 is inverting input with sign-.
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When the voltage at pin 3 goes lowerthan the fixed voltage at pin 2 thenoutput voltage at pin 6 goes high.
The point at which voltage at pin 2 isequal to voltage at pin 3 is called the
Threshold voltage. It is the point at
which the output at pin 6 switchesfrom high to low.
The voltage of the battery falls from
original value of 9 volts until it
IC 741
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Temperature
measurementThe high sensitivity of an NTC thermistormakes it an ideal candidate for temperaturesensing applications.
These low-cost NTC sensors are normallyused for a temperature range of -40 C to+300 C.Selection criteria for NTC thermistors are temperature range resistance range measuring accuracy environment (surrounding medium) response time
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One of the circuits suitable fortemperature measurement is a
Wheatstone bridge with an NTCthermistor used as one bridge leg.
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With the bridge being balanced, any changein temperature will cause a resistance
change in thethermistor and a significant current will flowthrough the ammeter. It is also possible touse a variable
resistor R3 and to derive the temperaturefrom its resistance value (in balancedcondition).
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TEMPERATURE CONTROL-A system can be designed using a thermistorwith a known temperature/ resistancecurve to form one leg of an AC bridge and avariableresistor calibrated in temperature to formanother leg.When the resistor is set to a desired
temperature, bridgeunbalance occurs. This unbalance is fed into anamplifierwhich actuates a relay to provide a source of
heat or
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