While selecting a temperature sensor type, one restriction concerns range over which it can stay reasonably accurate. A thermistor is a resistance thermometer sensor whose electrical resistance varies in response to a change in its surrounding temperature.
Change in temperature contribute to change in its electrical properties like resistance, it can also be called a Transducer.
Thermistor symbol as per the IEC standard.
Thermistors are miniature metallic oxide elements which generate the most sensitive and fastest response of any of the thermometers. The word thermistor is derived from the term “thermally sensitive resistor” since the resistance of the thermistor varies as a function of temperature.
Thermistors usually employ polymers or ceramics – usually, semiconductors made by sintering mixtures of metallic oxides such as manganese, cobalt, iron, copper, and more.
Comparison with RTD or Thermocouple:
A standard thermistor is over ten times more sensitive than an RTD. This allows the thermistor circuit to detect minute changes in temperature that could not be observed with an RTD or thermocouple circuit.
As seen in below figure, the thermistors have a much steeper resistance temperature slope compared to RTD and thermocouple, which indicates the higher temperature sensitivity.
A thermistor connected to a bridge circuit can readily indicate a temperature change of as little as 0.0005°C. The cost of this increased sensitivity is the loss of linearity.
The thermistor is an exceptionally nonlinear device that is highly dependent on process parameters. Therefore, the thermistor manufacturers have not standardized thermistor curves to the same extent as they have RTD and thermocouple curves.
Thermistors are generally available in several configurations.
# Hermetically sealed flexible thermistor
# Bolt washer type
# Self-adhesive surface mount type.
Thermistors are encapsulated to protect thermistor element either by a epoxy resin or glass. These coatings protect thermistors and the connecting wires from humidity, corrosion, and mechanical stress.
Thermistors with epoxy coatings are available for use at a lower temperature range -50⁰C to 150⁰C.
Thermistors with glass coatings are available for use at the temperature range of -50⁰C to 300⁰C.
Advantages of thermistors:
1. They have high sensitivity to small temperature change.
2. Thermistors are interchangeable across a broad range of temperatures.
3. They are insensitive to lead wire lengths.
4. They can be read relatively easily with handheld devices.
5. They are relatively inexpensive.
Disadvantages of thermistors:
- Small temperature range.
- Non linear device.
3. Thermistors can drift over time as they age.
4. Exposure to high temperatures accelerates the aging process.
5. Self-heating when exposed above designed temperature applications.
6. They are not encouraged for temperature sensing in the industrial environment because, they do not offer an advantage over RTD, Thermocouple.
Types of Thermistors (based on temperature coefficient):
Based on the temperature coefficient, thermistors are classified into two categories. Depending upon sensor composition thermistors can exhibit either increase in resistance or decrease in resistance as the temperature increases.
1.PTC- Positive Temperature Coefficient.
2.NTC- Negative Temperature Coefficient.
PTC- Positive Temperature Coefficient: PTC thermistors are those whose resistance increases with an increase in temperature.
Thermistors whether PTC or NTC has a body made of metallic oxides. The metallic body of a thermistor can be pressed into different shapes and sizes.
They can be pressed into the bead, disc, rod shapes. The one pressed into a bead shape is known as bead thermistor. Similarly, one that is pressed in a disc is known as a disc thermistor, one that pressed in the shape of a cylinder is known as a cylindrical thermistor.
The resistance verses temperature graph below shows how the PTC reacts to the temperature changes. Tb = the transition temperature. The resistance at this point is much higher than the resistance below this transiton temperature. This property allows PTCs to be used as fuses.
- Ceramic PTC thermistors are replacement of a conventional fuse to protect loads such as motors, transformer, etc.,
- Positive temperature coefficient models are used for specialized applications over the low-temperature range (control and safety applications.)
- PTC thermistors are used as heaters and resettable fuses.
NTC- Negative Temperature Coefficient: NTC thermistors are those whose resistance decreases with an increase in temperature. Only NTC is commonly used to measure temperature.
The material of construction:
Unlike other resistors, these are made of ceramics of polymers, which composed of metal oxides that are dried and sintered to obtain the desired form.
In the case of NTC, Cobolt, Nickel, Iron, Copper oxides
NTC thermistors are used in wheatstone bridge, where it uses four resistors. One is thermistor among four resistors. There wont be any change in current through ammeter. A change in temperature cause change in resistance of thermistor and hence current flow through ammeter.