Thermistors

Thermistors are thermally sensitive resistors produced with semiconducting materials.

Although RTDs and thermistors are both resistive devices, they differ substantially in operation and usage, as thermistors are passive semiconductor devices.

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