With conventional resistors, we would normally require resistance to remain the same over a wide range of temperatures. On the other hand, there are applications in which we could use the effect of varying resistance to detect a temperature change. The component that allows us to do this is known as a thermistor. The name is a contraction of a thermally sensitive resistor. The resistance of a thermistor changes markedly with temperature and these components are widely used in temperature sensing and temperature compensating applications. Two basic types of thermistors are available, NTC and PTC. Using thermistors have many benefits and drawbacks. These pros and cons are listed below.
Advantages of thermistor
The benefits of the thermistor are:
- Thermistor has a compact size.
- Thermistor has a higher output. Thus, it has a high sensitivity. It has the best sensitivity compared to other temperature sensors.
- Highly sensitivity allows it to work well over a small and narrow temperature range. Therefore, a small temperature change is easier to detect.
- It is easy to use.
- It operates rapidly.
- It has a fast response in the narrow temperature range. It is a good choice when measuring very small temperature changes.
- It is cost-friendly. You can buy it at an affordable price.
- It is very responsive to changes in temperature.
- It has high accuracy.
- It is repeatable
- It is customizable.
- It is polarity insensitive.
- Thermistor has a long history. The operating principle of a thermistor was described by Michael Faraday when during his experiments he noticed that the resistance of silver sulfide decreased as the temperature increased. In the light of this research, Samuel Ruben invented the first commercial thermistor in 1930 and was awarded U.S. Patent No. 2,021,491.
- It can be fabricated in many forms. The more common include discs, beads, and rods.
- It is usually made from specially processed oxides of cobalt, magnesium, manganese, nickel, uranium, or mixtures of such substances.
- There are two kinds of thermistors: NTC (Negative Temperature Coefficient) thermistors and PTC (Positive Temperature Coefficient) thermistors.
- Thermistors can be used in many applications such as measurement and control of temperature (thermometers, oil temperature in automobiles), time delay, temperature compensation in electronic equipment (rechargeable batteries), household electronics (refrigerators, washing machines, and electric cookers), computer electronics (CPU, LCDs and hard disk drives)
Disadvantages of thermistor
The drawbacks of the thermistor are:
- It needs shielding in power lines.
- It is extremely non-linear. They are especially non-linear at high temperatures, so it is best to use them for measurements of less than 100°C.
- It is a passive device.
- The nonlinear characteristics of thermistors often create a problem for temperature measurement. It is not suitable for a large temperature range.
- It has a narrow operating temperature range compared to RTD and thermocouples.
- It is fragile.
- It is susceptible to self-heating errors.
- It is unstable due to drift and recalibration.
- Self-heating errors from applied power.
- It needs an excitation current, more commonly known as a voltage source.