Thermistors | Italiandirectory

Thermistor, a champion of sensitivity

Also in this series:
  1. Electric Sensors
  2. Temperature sensors: thermocouples
  3. Temperature sensors: RTD, an accurate alternative

If the precision is your priority, you should consider the thermistor as temperature sensor. The Thermistors are available in two types, NTC and PTC. The thermistor NTC (negative temperature coefficient) is made of ceramic and oxides of transition elements (manganese, cobalt, copper and nickel). Given a certain excitation current the NTC has a negative temperature coefficient quite linear and very repeatable.

These temperature-dependent resistors semiconductor, operate in a range from -100 °C to 450 °C. If assembled with the proper packaging, they have a continuous change of resistance in dependence of temperature. This temperature dependence is greater than the RTD (see Figure 1), therefore the thermistor is systematically more sensitive.

The temperature characteristics of a typical NTC thermistor, compared to a 100Ω RTD, are illustrated in Figure 1.

NTC over RTD
Fig. 1: The temperature response versus resistance of the NTC thermistor and the RTD. Credits: Microchip

As we can see, the difference between the temperature coefficients of these two sensors is evident. The thermistor has a negative temperature coefficient, as expected, and the absolute value of the sensor has a scale factor 10,000 over its real usable temperature range.

On the other hand, the RTD has a positive temperature coefficient and its values only change by a 4X factor over the usable temperature range. This increased sensitivity of the thermistor makes it interesting in terms of precision in measurements.

Linearity

The thermistor is less linear than RTD as it requires a 3rd order polynomial for precise temperature corrections. Linearity equations for the thermistor are:

    \[\ln R_{T}=B_{0}+ \frac{B_{1}}{t}+\frac{B_{2}}{t^{2}}+\frac{B_{3}}{t^{3}}\]

over the entire temperature range, where Bx are the material constants of the thermistor. This linearization formula can lead to a total measurement uncertainty of ± 0.005°C.

However, implementing this formula in the microcontroller is quite tedious. Alternatively, look-up tables can be generated for the same purpose, with slightly less precision.

Error analysis

Although the NTC thermistor has a greater precision than the RTD, the two sensors have many things in common. Both are temperature sensitive resistors.

When you use the thermistor, easily an error occurs due to overheating. Indeed, greater attention is needed when designing the excitation of the thermistor because its resistance values are usually highee than the RTD.

For example, consider a packaging with a thermal resistance of 10 °C/W and a nominal thermistor resistance of 10kω at 25 °C with an excitation current of 5mA. The artificial increase of temperature (Δ °C) resulting from self heating is:

    \[\Delta\: ^{\circ}C=I^{2}R_{THERMISTOR}\times \Theta _{PACKAGE}\]

    \[\Delta\: ^{\circ}C=(5mA)^{2}\times 10k\Omega \times 10 \: ^{\circ}C/Watt\]

    \[\Delta\: ^{\circ}C=2.5 \: \: ^{\circ}C\]

With temperature changes of this entity, the measurement is obviously not accurate, but, in addition to that, the thermal coefficient of the thermistor material is delaying the effect of the problem for some seconds until the packaging material stabilizes.

To complicate matters further, this thermal effect, heating the thermistor decreases its resistance (instead of increasing it as seen for the RTD). Because the thermistor has a negative resistive coefficient, the overheating effect reverses because the thermistor resistance becomes less than the excitation voltage/current ratio.

This phenomenon is not easily overcome with calibration software and should be avoided. The PTC has a positive temperature coefficient and is built from barium titanate. PTC sensitivity is considerably higher than the sensitivity of NTC thermistor and should be used when a specific temperature range ( -25 to 150° C) concerns.

In the bottom-right of the resistance/temperature curve, the resistance of the thermistor becomes nearly constant. At higher temperatures, the material shows a threshold temperature (between 80° C and 140° C, dependent on the chemical composition of ceramic) beyond which the resistance/temperature characteristic changes abruptly (Figure 2).

PTC resistance
Fig. 2: PTC thermistor and silistor resistance versus temperature response. Credits: Microchip

At this point, increases in temperature cause an increase in the resistance of the PTC and temperature resistance characteristics become very steep.

A second type of PTC is known as the Silistor. This device is constructed with a silicon-based thermally sensitive material and also has a positive temperature coefficient (from -60 °C to 150 °C) which is linear over the entire operating range.

The thermal characteristics of both types of PTC are shown in Figure 2.