Table of Contents

What is Thermistor

What is Thermistor :

The thermistor is a word formed by combining thermal with a resistor. Thermistors such as RTDs are temperature – sensitive resistors. Thermistors are non-linear devices. Their resistance will decrease with an increase in temperature but at a much faste/rate than that of RTDs. The resistance can change by more than 1000 times. As a result, thermistors can sense minute changes in temperature which are undetected RTDs and thermocouples. The basic equation is given by

Thermistors are small and inexpensive devices that are most commonly made of metal oxides such as those of chromium, nickel, manganese, and cobalt. The metals are oxidized through a chemical reaction, ground to a fine powder, then compressed and subject to very high heat. Theses oxides are semiconductors.

Basic Principle of Thermistors :

Thermistors are non-metallic resistors that is semiconductors of a ceramic material having a negative coefficient of resistance.
When the thermistor is subjected to a temperature change, the resistance of the thermistor changes. This change in resistance of the thermistor becomes a measure of the change in temperature when calibrated.
The resistance of the thermistor decreases with an increase in temperature and vice-versa.

Parts Of Thermistors :

The main parts of a thermistor are as follows:

A metal tube which houses a thermistor sensing element.
An insulation separates the thermistor sensing element from the metal tube.
Lead wires are drawn out from the thermistor sensing element as shown in the diagram.
The metal tube, sensing element, and leads together become a thermistor used to measure temperature.
The leads of the thermistor are connected to a wheat stone bridge as shown in the diagram.

Thermistor Diagram ;

Working Of thermistor :

The procedure for measuring temperature in an as follows:

A known constant current is passed through the thermistor sensing element and the initial resistance of the thermistor sensing element is measured using the wheat stone bridge.
Now the thermistor is introduced into the medium whose temperature is to be measured. Due to a change in temperature (assume the change is in the positive direction), the sensing element changes (decreases). (It should be noted that the same constant current is passed through the sensing element during measurement).
Now this change in resistance of the sensing element of the thermistor is measured using the wheat stone bridge. This change in resistance becomes a measure of temperature when calibrated.

Types of Thermistors :

There are two types of thermistors based on the lead attachment:

1. Beads and
2. Metallized surface-contact.

Bead types have platinum wires sintered into a ceramic body ( bead ) as shown in figure . (a) Metallized surface-contact thermistors are called chips or flakes. In contrast to bead types, leads are not sintered directly into the ceramic. Instead, the sintered ceramic is coated with a metallic contact shown in Figure 1.43(b). Either the chip manufacturer or user attaches leads to this contact. One advantage of chip thermistors over bead types is that chips are easily trimmed by cutting or grinding. Thus, they are easy to match and therefore, they are interchangeable. While matched bead thermistors are available, they cost more than interchangeable chips.

Thermistors can be classified into two types depending on the temperature coefficient of resistance ( kf ) If k is positive, the resistance will increase with increasing temperature and the device is called positive temperature coefficient (PTC) thermistor or posistor. If k is negative, the resistance will decrease with’ increasing temperature and the device is called negative temperature coefficient (NTC) thermistor. NTC thermistors are mostly used in temperature sensing devices whereas PTC thermistors are mostly used in electric current control devices.

Advantages of thermistors

It produces more accurate output and fast.
It is suitable for the usage in remote location.
It can be manufactured in almost any shape and size.
A high degree of accuracy is obtained.
Good stability and repeatability are ensured.
It has the ability to withstand mechanical and electrical stresses.

Disadvantages of thermistors

It produces highly non-linear behavior over its range of operation.
It has a limited measuring range.
Self-heating may occur.
A power supply is required.
It is fragile in nature.

Applications of thermisters :

As the thermistors have good sensitivity, they are used for measuring varying temperatures.
They are used for temperature compensation in electronic equipment.
They are used in time delay circuits.
They are used to measure thermal conductivity.
They are used to measure the pressure and flow of liquids.
Used in precision temperature measurement (in the range of 100 degrees C to 300 degrees C).