Thermistors

A thermistor is a temperature-sensing element composed of semiconductor material which exhibits a large change in resistance proportional to a small change in temperature or simply Thermistors are temperature sensitive resistors.
Although RTDs and thermistors are both resistive devices, they differ substantially in operation and usage, as thermistors are passive semiconductor devices.

What are thermistors?

Thermistors are semiconductor devices that are used to measure temperature. The name comes from a combination of the words "resistor" and "thermal". Thermistors have an electrical resistance that is proportional to temperature. From a general physics course on electricity and magnetism, you may have learned that this is a property typical for all conductors. For example, devices such toasters, heaters, and light bulbs operate on this principle. Thermistors are different in that they are created to deliberately exploit this effect, and hence are more temperature sensitive than usual.

Two types of thermistors are available:

1. Negative temperature coefficient (NTC), which decreases its resistance as its temperature increases, and
2. Positive temperature coefficient (PTC), which increases its resistance as its temperature decreases.

From the point of view of temperature measurement applications, NTC types are used far more than PTC ones. Due to its characteristics, PTC types are more frequently used as thermostats to sense and regulate temperatures (inside ovens, for instance).
Main advantages of (NTC) thermistors are:

• Large change in resistance versus temperature
• Fast time response
• High resistance eliminates the need for four wire measurement
• Small size
• Inexpensive
• High stability

Main disadvantages of thermistors are:

• Non-linear
• Operating temperature limited to approximately -60 to +300 ºCelsius
• Current source required

Operating principle of thermistors

Thermistors can be encapsulated in glass or epoxy considering a big variety of mechanical models. Most (NTC) thermistors have high resistivities and high negative coefficients, allowing the NTC thermistor to detect changes in temperature that could not be observable with RTDs or thermocouples.
For example, it is common to have an NTC thermistor exhibiting a negative temperature coefficient with a change in resistance of about 4.5%/ºC at 30ºC, and about 1.6%/ºC at 155ºC. Common base values can be in the range of a few ohms to mega-ohms. Normally, high-R thermistors are used for “high” temperatures (lower than 300ºC), and low-R thermistors for “low” temperatures (higher than -60ºC).
Considering the range of some kilo-ohms to mega-ohms, we can conclude that the resistance of the wires connecting the instrumentation to the thermistor is insignificant (in this sense, the three- or four-wire measurement configuration referred for RTDs are not necessary for NTC thermistors with high-R base values).
Figure 2.6 and Figure 2.7 present typical configurations for two- and four-wire thermistor circuits (RL stands for the lead resistances); in cases where the series resistance of the lead configuration is significant, the four-wire circuit can be used. As far as one current source is used, the calculation of the thermistor’s resistance is a straightforward task according to Ohm’s Law.

Figure 2.6 Two-wire thermistor configuration (most common).

Figure 2.7 Four-wire thermistor configuration.

As usually, for each benefit, we should be ready to pay a price; in this case, the price for increasing sensitivity is loss of linearity.
In this sense, the resistance versus temperature characteristic of NTC thermistors is non-linear. The following expression describes the resistance versus temperature characteristic of a thermistor
                                             (2.3)
where:

1. RT is the zero-power R at T(K),
2. R0 is the zero-power R at a known temperature T0,
3. ß is the material constant for the thermistor.

Note: zero-power resistance is the resistance of a thermistor at a temperature measured when there is negligible self-heating (due to Joule’s effect).
Alternatively, the following Steinhart-Hart equation can be used for computation of temperature, giving relatively accurate thermistor curves:

                                          (2.4)
where:

1. T is the temperature in K;
2. RT is the resistance of the thermistor,
3. A, B, and C are constants specific for a given thermistor.

If not provided, constants A, B, and C can be found by solving three equations with known R’ and T’s, and considering:
-40ºC < T1, T2, T3 < 150ºC, and
|T2 – T1| < 50ºC
|T3 – T2| < 50ºC
CONSTRUCTION
They are made up of a mixture of sulphides or oxides or sometimes metals such as copper, iron or cobalt. They tend to be formed into a disc or a bead sealed with plastic or glass. All resistors vary with temperature, but thermistors are constructed of semiconductor material with a resistivity that is especially sensitive to temperature.
Thermistors usually have negative temperature coefficients which means the resistance of the thermistor decreases as the temperature increases.

Construction

The device is manufactured from materials like sintered mixtures of oxides of metals such as manganese, nickel, cobalt, and iron. Their resistances range from 0.4 ohms to 75 mega-ohms and they may be fabricated in wide variety of shapes and sizes. Smaller thermistors are in the form of beads of diameter from 0.15 millimeters to 1.5 millimeters.  Such a bead may be sealed in the tip of solid glass rod to form probe which is easier to mount than bead. Alternatively thermistor may be in the form of disks and washers made by pressing thermistor material under high pressure into flat cylindrical shapes with diameter from 3 millimeters to 25 millimeters. Washers may be stacked and placed in series or parallel to increase power disciplining capability.

USAGE:
Thermistors are most commonly used in bridge circuits.The thermistor can be placed anywhere in the bridge with three constant resistors, but different placements can produce different behavior in the bridge. For example, different placements might cause the output voltage to go in different directions as the temperature changes.

References

Physics for Scientists and Engineers by Douglas Giancoli
Physics For Scientists and Engineers by Raymond Serway