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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Therm is Tor Characteristic Determination and Temperature Control

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