Physics Investigatory project

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investigatory class 12 project on thermistor

Text of Physics Investigatory project



Name Sudarshan Thakur Class XII C Roll no. d


This is to certify that Sudarshan Thakur studying in class XII- C of Mount St Marys School has satisfactorily completed his physics investigatory project under the guidance of Mr. Dhiraj Dhall (physics teacher) during the year 2015-2016.

d Signature of External Examiner Signature of physics teacher


I would take this opportunity to express my sincere thanks and gratitude to my physics teacher Mr. Dhiraj Dhall for his invaluable guidance, constant encouragement, constructive comments, sympathetic attitude and immense motivation, which has sustained my efforts at all stages of this project work. His valuable advice and suggestions for the corrections, modifications and improvement did enhance the perfection in performing my job well. I take special pleasure in acknowledging our lab assistant Mr. Pradeep Kumar for his willingness in providing us with necessary lab equipments and constant support without which this effort would have been worthless.



1. Introduction2. ExperimentI. TheoryII. Apparatus requiredIII. ProcedureIV. ObservationV. ConclusionVI. Precautions3. Reference





THERMISTORAthermistoris a type ofresistorwhoseresistanceis dependent ontemperature, more so than in standard resistors. The word is the mixofthermalandresistor. Thermistors are widely used asinrush current limiter, temperaturesensors (NTC type typically),self-resetting overcurrent protectors, and self-regulatingheating elements.

An NTC Thermistor Themistor symbol for circiuit diagram. d


The first NTC thermistor was discovered in 1833 byMichael Faraday, who reported on the semiconducting behavior ofsilver sulfide. Faraday noticed that the resistance ofsilversulfide decreased dramatically as temperature increased. (This was also the first documented observation of a semiconducting material.) Because early thermistors were difficult to produce and applications for the technology were limited, commercial production of thermistors did not begin until the 1930s.A commercially viable thermistor was invented bySamuel Rubenin 1930.


THEORYBASIC CLASSIFICATION AND WORKING OF A THERMISTORAssuming, as a first-order approximation, that the relationship between resistance and temperature islinear, then:

where, change in resistance, change in temperature, first-ordertemperature coefficient of resistanceThermistors can be classified into two types, depending on the classification of. Ifispositive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient(PTC) thermistor, orposistor. Ifis negative, the resistance decreases with increasing temperature, and the device is called anegative temperature coefficient(NTC) thermistor.

ADVANTAGES OF USING A THERMISTORThermistors have some benefits over other kinds of temperature sensors such as analog output chips (LM35/TMP36) or digital temperature sensor chips (DS18B20) orthermocouples. First off, they are much much cheaper than all the above! A bare 5% thermistor is only 10 cents in bulk. They are also much easier to waterproof since its just a resistor. They work at any voltage (digital sensors require 3 or 5V logic). Compared to a thermocouple, they don't require an amplifier to read the minute voltages - you can use any microcontroller to read a thermistor. They can also be incredibly accurate for the price. For example, the 10K 1% thermistor in the shop is good for measuring with 0.25C accuracy! (Assuming you have an accurate enough analog converter) They are difficult to break or damage - they are much simpler and more reliable


PTC Thermistor As heater in automotive industry to provide additional heat inside cabin with diesel engine or to heat diesel in cold climatic conditions before engine injection. In temperature compensatedsynthesizervoltage controlled oscillators.[7] Inlithium batteryprotection circuits.[8] In an electrically actuatedWax motorto provide the heat necessary to expand the wax.

A C831 PTC thermistor

NTC Thermistor Asresistance thermometersin low-temperature measurements of the order of 10K. As sensors in automotive applications to monitor things like coolant or oil temperature inside the engine, and provide data to theECUand to the dashboard. To monitor the temperature of an incubator. Thermistors are also commonly used in moderndigital thermostatsand to monitor the temperature of battery packs while charging. Thermistors are often used in the hot ends of3D printers; they monitor the heat produced and allow the printer's control circuitry to keep a constant temperature for melting the plastic filament. In the Food Handling and Processing industry, especially for food storage systems and food preparation. Maintaining the correct temperature is critical to prevent food borne illness.




Many NTC thermistors are made from a pressed disc, rod, plate, bead orcastchip ofsemiconductingmaterial such assinteredmetaloxides. They work because raising the temperature of a semiconductor increases the number of activecharge carriers- it promotes them into theconduction band. The more charge carriers that are available, the morecurrenta material can conduct. In certain materials like ferric oxide (Fe2O3) with titanium (Ti) doping ann-typesemiconductor is formed and the charge carriers are electrons. In materials such as nickel oxide (NiO) with lithium (Li) doping ap-typesemiconductor is created whereholesare the charge carriers.[4]


Most PTC thermistors are made from doped polycrystallineceramic(containingbarium titanate(BaTiO3) and other compounds) which have the property that that their resistance rises suddenly at a certain critical temperature. Barium titanate isferroelectricand itsdielectric constantvaries with temperature. Below theCurie pointtemperature, the highdielectric constantprevents the formation of potential barriers between the crystal grains, leading to a low resistance. In this region the device has a small negative temperature coefficient. At the Curie point temperature, the dielectric constant drops sufficiently to allow the formation of potential barriers at the grain boundaries, and the resistance increases sharply with temperature.


NTC ThermistorInNTC thermistor, when the temperature increases, resistance decreases. Conversely, when temperature decreases, resistance increases. Thus this type of thermistor is used when we want to increase the current in the circuit as the temperature is increased. example: fire alarm.PTC Themistor In PTC thermistor when the temperature increases above some threshold value, resistance increases drastically.This type of thermistor is required when we want to stop the flow current when temperature is high.Example: Automatic cutoff system in electric geysers.


1. A breadboard 2. A thermistor(NTC) 3. Two resistors 4. A transistor 5. AnLED 6. Few connecting wires





The connectionsaremadesuch thatone legofthe thermistor isconnectedtothebase ofthetransistor anditsotherlegisconnectedtoVcc(positive/high voltage). Theemitterofthetransistor andoneendofthebase resistoraregrounded (negative/low voltage).

Whenthesurfaceofthe thermistorisexposedto heat,theLEDstartsglowing. The LED turns OFF the moment the temperatureofthethermistorfalls belowthethresholdvalue.

Thecircuitisdesigned suchthattheresistance of the thermistor is inversely proportional to the base voltage. d


The thermistor used is a negative temperature coefficient (NTC) resistor.

The reason LEDglowsis that whenthethermistorisheated, its resistancedecreasesduetowhich thebase voltage increasesand pointcomeswhenthe base-emitter junctionis forward biased.

As aresultthetransistor turnsonandanoutputcurrentowsfromitscollector to the emitter, making the LED glow.


1. The current flowing in the circuit is directly proportional to the temperature of the surrounding because we have used an NTC thermistor in the circuit where the resistance of thermistor is inversely proportional to the surrounding temperature in which the thermistor is kept.

2. Increase in base voltage results in the glow of LED. To increase the base voltage we need to use a variable resistor which can forward bias the base-emitter junction of the transistor to initiate the flow of collector current which makes LED glow. Thus we use NTC thermistor to provide variable resistance.

PRECAUTIONS1. All the circuitry connections should be tight.2. While heating the thermistor with a matchstick or a lighter make sure that it is not too close to the sensor.3. Heating of the sensor should not take place for a long time.


1. Google images3.