Thermistors, also known as Resistance Thermometers are basically semiconductor devices which are employed for measurement of temperature. The word thermistor is made up of two words, i.e. thermal and resistor. Thermistors are also referred to as thermally sensitive resistors. These are temperature sensors having their electrical resistance proportional to temperature. Hence, their ability to conduct electric current varies according to temperature. The change in electrical resistance with change in temperature is extremely nonlinear. It is shown in the figure below in comparison to other temperature sensors.
This resistance-temperature relationship of a thermistor can be established by equation given below:
In the above equation: T denotes the temperature in kelvins, TRef is the reference temperature, R represents the resistance of the thermistor in ohms, RRef is the resistance at reference temperature i.e. TRef and Beta.JPGdenotes the calibration constant. The value of calibration constant normally depends upon the thermistor material. It generally varies between 3,000 and 5,000 K.
Key features of a thermistor are mentioned below:
- Usually, thermistors have a temperature range which varies between -40°C to 150°C.
- In actual, the electrical resistance of a thermistor varies between several mega ohms to several ohms whereas its operational resistance is in the range of kilo ohms.
- Thermistors are largely employed in science and engineering applications.
- They are also applied in medical field as temperature sensors or probes for clinical and surgical purposes.
- They are also used as safety devices in electrical circuits.
- Thermistor probes are available in variety of shapes and sizes. They exist in the form of a bead, washer, disk, or rod as shown in the figure below.
Types of Thermistors
Two types of thermistors which are commonly used are detailed below:
- PTC i.e. Positive Temperature Coefficient of Resistance
- NTC i.e. Negative Temperature Coefficient of Resistance
In case of PTC thermistors, the electrical resistance is directly proportional to the temperature. Consequently, the resistance increases with the rise in temperature and decreases with the fall in temperature. This property is known as positive temperature coefficient of resistance. PTC thermistors are formed by using semiconductor materials along with polycrystalline ceramic. PTC thermistors generally behave like an on-off switch. “This on-off behavior of PTC thermistors is useful in situations where equipment can be damaged by easily definable events”.1 The temperature at which this switching phenomenon takes place usually depends upon the thermistor composition. PTC thermistors are mostly employed in areas where a drastic change in resistance is needed at a particular temperature or current level. PTC thermistors are mainly used in following applications:
- For overload protection in electrical circuits
- As thermal switches
- As ordinary thermometers for sensing temperature
- For protection of windings in transformers and electrical motors against excessive heat
- Liquid level detection
NTC thermistors are the most common type of thermistors used for measurement of temperature. The temperature of NTC thermistors is inversely proportional to the resistance which means that with the rise in temperature, their electrical resistance decreases whereas with the fall in temperature, their resistance increases. This property is known as negative temperature coefficient of resistance. NTC thermistors are generally made up from oxides of transition metals like nickel, manganese, copper, iron, and cobalt. Sometimes, semiconductor materials like silicon and germanium can also be used for their construction. The operating temperature range of NTC thermistors varies between -200°C to + 1000°C. They provide good mechanical, thermal and electrical stability. These are extremely sensitive sensors mainly employed in situations where a continuous change of resistance is needed over a wide temperature range. Owing to their low price and excellent performance, NTC thermistors are extensively used in following applications:
- Control and detection of temperature
- Temperature compensation circuits
- Surge and overcurrent protection
- Liquid flow detection
Following are the main benefits offered by thermistors:
- Thermistors offer better accuracy in comparison to RTDs and thermocouples.
- Unlike RTDs and thermocouples, they are highly sensitive. Consequently, lead wire and self-heating errors are insignificant in case of temperature measurements involving thermistors.
- They are smaller in size as compared to thermocouples.
- Thermistors provide faster response than RTDs.
- They offer high stability and brilliant repeatability.
- They are very reliable and convenient to use.
- Unlike thermocouples which provide millivolt outputs, use of thermistors result in reasonable output voltages.
- Thermistors are particularly low cost and easily adaptable temperature sensors. Hence, they are widely employed for simple temperature measurements.
Major two drawbacks associated with the use of thermistors are listed below:
- Since thermistors are semiconductor devices, their operation is highly non linear. This effect of nonlinearity needs to be compensated before applying them in measurement circuits.
- Another disadvantage of thermistors is their limited temperature range due to which they are rendered unsuitable for use at higher temperatures.