2015-2016 PHYSICS INVESTIGATO RY PROJECT Name Sudarshan Thakur

Physics Investigatory project

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investigatory class 12 project on thermistor

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Name Sudarshan Thakur Class XII C

Roll no. d

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


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Signature of External Examiner Signature of physics teacher

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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.


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1. Introduction2. Experiment

I. TheoryII. Apparatus requiredIII.ProcedureIV.ObservationV. ConclusionVI.Precautions

3. Reference


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A thermistor is a type of resistor whose resistance is dependent on temperature, more so than in standard resistors. The word is the mix of thermal and resistor. Thermistors are widely used as inrush current limiter, temperature sensors (NTC type typically), self-resetting overcurrent protectors, and self-regulating heating elements.

An NTC Thermistor Themistor symbol for circiuit diagram. d

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The first NTC thermistor was discovered in 1833 by Michael Faraday, who reported on the semiconducting behavior of silver sulfide. Faraday noticed that the resistance of silversulfide 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 by Samuel Ruben in 1930.



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Assuming, as a first-order approximation, that the relationship between resistance and temperature is linear, then:

where, change in resistance, change in temperature

, first-order temperature coefficient of resistance

Thermistors can be classified into two types, depending on the classification of  . If   is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, or posistor. If   is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor.

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Thermistors have some benefits over other kinds of temperature sensors such as analog output chips (LM35/TMP36 ) or digital temperature sensor chips (DS18B20) or thermocouples.

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 ±0.25°C accuracy! (Assuming you have an accurate enough analog converter)

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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 compensated synthesizer voltage controlled oscillators.[7]

In lithium battery protection circuits.[8]

In an electrically actuated Wax motor to provide the heat necessary to expand the wax.

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A C831 PTC thermistor

NTC Thermistor

As resistance thermometers in low-temperature measurements of the order of 10 K. As sensors in automotive applications to

monitor things like coolant or oil temperature inside the engine, and provide data to the ECU and to the dashboard.

To monitor the temperature of an incubator. Thermistors are also commonly used in

modern digital thermostats and to monitor the temperature of battery packs while charging.

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Thermistors are often used in the hot ends of 3D 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.

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Many NTC thermistors are made from a pressed disc, rod, plate, bead or cast chip of semiconducting material such as sintered metal oxides. They work because raising the temperature of a semiconductor increases the number of

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active charge carriers - it promotes them into the conduction band. The more charge carriers that are available, the more current a material can conduct. In certain materials like ferric oxide (Fe2O3) with titanium (Ti) doping an n-type semiconductor is formed and the charge carriers are electrons. In materials such as nickel oxide (NiO) with lithium (Li) doping a p-type semiconductor is created where holes are the charge carriers.[4]


Most PTC thermistors are made from doped polycrystalline ceramic (containing barium titanate (BaTiO3) and other compounds) which have the property that that their resistance rises suddenly at a certain critical temperature. Barium titanate is ferroelectric and its dielectric constant varies with temperature.

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Below the Curie point temperature, the high dielectric constant prevents 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 ThermistorIn NTC 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  

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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. An LED   6. Few connecting wires

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The connections are made such that one leg of the thermistor is connected to the base of the transistor and its other leg is connected to Vcc(positive/high  voltage).  The emitter of the transistor and one end of the base resistor are grounded (negative/low voltage).

When the surface of the thermistor is exposed to heat, the LED starts glowing. The LED turns OFF the moment the

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temperature of the thermistor falls below the threshold value.

The circuit is designed such that the resistance of the thermistor is inversely   proportional to the base voltage. d


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

The reason LED glows is that when the thermistor is heated, its resistance decreases due to which the base voltage   increases and point comes when the base-‐emitter   junction is forward biased.

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As a result the transistor turns on and an output current flows from its collector 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

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transistor to initiate the flow of collector current which makes LED glow. Thus we use NTC thermistor to provide variable resistance.


1. 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. https://en.wikipedia.org/wiki/Thermistor

2. Google images

3. https://in.answers.yahoo.com/

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