Thermistor Working Principle with Applications

Thermistors

A thermistor is a temperature sensor constructed of semiconductor material that exhibits a large modification in resistance in proportion to a tiny low modification in temperature. Thermistors are inexpensive, rugged, reliable and responds quickly. Because of these qualities thermistors are used to measure simple temperature measurements, but not for high temperatures. Thermistor is easy to use, cheap, durable and respond predictably to a change in temperature. Thermistors are mostly used in digital thermometers and home appliances such as refrigerator, ovens, and so on. Stability, sensitivity and time constant are the final properties of thermistor that create these thermistors sturdy, portable, cost-efficient, sensitive and best to measure single-point temperature. Thermistors are available in different shapes like rod, disc, bead, washer, etc. This article gives an overview of thermistor working principle and applications.

What is Thermistor and How it Works?

A thermistor is an inexpensive and easily obtainable temperature sensitive resistor, thermistor working principle is,it’s resistance is depends upon temperature. When temperature changes, the resistance of the thermistor changes in a predictable way. The benefits of using a thermistor is accuracy and stability.

Types of Thermistors

Thermistors are classified into two types such as:

Thermistor Elements

Thermistor Probes

Thermistor Elements

Thermistor elements are the simplest form of thermistor, it is commonly used when space is very limited. OMEGA offers a wide variety of thermistor elements which vary not only in form factor, but also in their resistance Vs temperature characteristics. Since thermistors are non-linear, the device used to read the temperature must linearize the reading.

Thermistor Probes

The standalone thermistor element is comparatively delicate and cannot be located in a ruuged environment. OMEGA offers thermistor probes that are thermistor elements fixed in metal tubes. Thermistor probes are much more suitable for industrial environments than thermistor elements.

Simple Fire Alarm Circuit Diagram Using Thermistor

This simple fire alarm circuit is based on thermistor and fire detection is possible through this circuit.

Simple Fire Alarm Circuit Using Thermistor

This circuit is very useful in home security systems.

This circuit works based on the principle of switching property of the transistor

The thermistor and resistor R1 forms potential divider n/w to drive the transistor.

The semiconductor materials used for thermistors are sensitive to temperature

The transistor is switched ON by the voltage drop through the resistor R1.

Consider the atmosphere’s temperature is around 25°C, and then the resistance of the thermistor changes, then the voltage across the thermistor changes according to the principle of ohm’s law V=IR.

When the voltage across resistor R1 is low, then it is not sufficient to turn ON the transistor.

When the temperature increases, the resistance of thermistor decreases, so that the drop across the resistor R1 increases which turns ON the transistor.

When the transistor is turned ON, the current from Vcc starts to flow via 6V buzzer which generates a beep sound.

The diode is used for enabling unidirectional conduction and the capacitor removes sudden transients from the thermistor.

The temperature of the thermistor can be calculated from the measured resistance by using the Steinhardt-Hart equation.

1/T=A+B*ln(R)+C*(ln(R))3 Where, R in W and T in oK

In the above equation, A, B, and C are the constants, that can be determined from experimental measurements of resistance. For a typical thermistor, here are some data points.

Data Points of a Typical Thermistor

Using these three values we can get three equations in A, B and C.

(1/273) = A + B ln(16330) + C (ln(16330))3

(1/298) = A + B ln(5000) + C (ln(5000))3

(1/323) = A + B ln(1801) + C (ln(1801))3

These equations can be solved and calculated for A, B, C

A = 0.001284

B = 2.364x 10-4

C = 9.304x 10-8

Using these values you can calculate the temperature and the reciprocal and we get a plot of resistance vs Kelvin.

Plot of Resistance Vs Kelvin

Calculating the Temperature from Resistance

If you have resistance value, then you can solve for the temperature by from the above Steinhardt-Hart equation.

1/T=A+B*ln(R)+C*(ln(R))3

From the above equation, we can calculate the temperature

T=1/[A+B*ln(R)+C*(ln(R))3]

Testing of a Thermistor This is just a sample test for a thermistor. The multimeter has to be kept in the mode of resistance. The two terminals of the multimeter are to be connected to the two terminals of the thermistor.

Testing of a Thermistor

We need not to focus on polarity here. Now, heat the thermistor by moving the heated soldering iron tip to it. Now you can observe the changes in the multimeter readings smoothly depending on whether the thermistor under test is PTC or NTC.

For faulty thermistors, the following things should observe.

The change in the readings of thermistor will never be smooth For a short thermistor, the meter reading will be always zero, whereas in an open thermistor the meter reading will be always infinity.

Applications of thermistors

A Thermistor is used to measure the temperature.

The thermistor is used as an electrical circuit component

For temperature compensation Circuit protection

Voltage regulation

Time delay, and Volume control.

Thermistors are used in an automotive applications

Instrumentation and Communication Consumer electronics

Food handling and processing

Industrial electronics

Medical electronics

Military and aerospace

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