Five most common types of temperature sensors

Temperature sensors are one of the most commonly used sensors. They can be found in devices such as computers, automobiles, kitchen appliances, air conditioners, and home thermostats.

1. Thermistor
   A thermistor (short for “thermal resistor”) is a temperature-sensing device whose resistance is a function of its temperature. There are two types of thermistors: PTC (Positive Temperature Coefficient) and NTC (Negative Temperature Coefficient). The resistance of a PTC thermistor increases with rising temperature, while the resistance of an NTC thermistor decreases with rising temperature. NTC thermistors appear to be the most commonly used type. See Figure 1 below.

NTC thermistorsIt is worth noting that the relationship between the resistance of a thermistor and its temperature is highly nonlinear. Please refer to Figure 2 below.

Figure 2. Relationship between resistance and temperature of an NTC thermistor

The standard equation for the resistance of an NTC thermistor as a function of temperature is:

R_25C is the nominal resistance of the thermistor at room temperature (25°C). This value is typically provided in the datasheet.

β (Beta) is the material constant of the thermistor in Kelvin. This value is typically provided in the datasheet.

T is the actual temperature of the thermistor in degrees Celsius.

However, there are two simple techniques that can be used to linearize the behavior of the thermistor: resistance-mode and voltage-mode.

► Resistance-mode Linearization

A fixed resistor is connected in parallel with the thermistor. If the value of the resistor is the same as that of the thermistor at room temperature, the linearized region will be symmetrical around room temperature. See Figure 3 below.

R_25C is the nominal resistance of the thermistor at room temperature (25°C). This value is typically provided in the datasheet.

β (Beta) is the material constant of the thermistor in Kelvin. This value is typically provided in the datasheet.

T is the actual temperature of the thermistor in degrees Celsius.

However, there are two simple techniques that can be used to linearize the behavior of the thermistor: resistance-mode and voltage-mode.

► Resistance-mode Linearization

A fixed resistor is connected in parallel with the thermistor. If the value of the resistor is the same as that of the thermistor at room temperature, the linearized region will be symmetrical around room temperature. See Figure 3 below.

Thermocouples are commonly used to measure higher temperatures and wider temperature ranges. They operate based on the principle that any conductor subjected to a thermal gradient will generate a small voltage, a phenomenon known as the Seebeck effect. The magnitude of the voltage produced depends on the type of metal involved.

A practical application of the Seebeck effect involves two dissimilar metals connected at one end and separated at the other. The temperature at the junction can be determined by measuring the voltage between the non-junction ends of the wires. Depending on the metal materials used, there are various types of thermocouples. Among these, certain alloy combinations have become popular, driven by factors such as cost, availability, chemical properties, and stability.

Different metal combinations are suitable for different applications, and users typically select them based on the required temperature range and sensitivity. For a chart illustrating the characteristics of thermocouples, refer to Figure 5.

1. Resistance Temperature Detectors (RTDs)

   Resistance Temperature Detectors, also known as resistance thermometers. RTDs are similar to thermistors in that their resistance changes with temperature. However, unlike thermistors, which require special materials sensitive to temperature variations, RTDs utilize a coil wound around a core made of ceramic or glass.

RTD wires are made of pure materials, typically platinum, nickel, or copper. These materials have a precise resistance-temperature relationship, which is used to determine the measured temperature.

1. Analog Temperature ICs

   An alternative to using a thermistor and a fixed-value resistor in a voltage divider circuit is an analog low-voltage temperature sensor, such as the TMP36 from Analog Devices. Unlike thermistors, this analog IC provides an output voltage that is almost linear. Over a temperature range of -40°C to +125°C, it has a slope of 10 mV/°C and an accuracy of ±2°C.

Although these analog temperature sensors are easy to use, their cost is significantly higher compared to the combination of a thermistor and a resistor.

1. Digital Temperature ICs

Digital temperature devices are more complex, yet they can be highly accurate. Additionally, they can simplify your overall design because the analog-to-digital conversion takes place inside the temperature IC itself, rather than in a separate device such as a microcontroller. For example, the DS18B20 from Maxim Integrated offers an accuracy of ±0.5°C over a temperature range of -55°C to +125°C.

1. Summary

   The following is a brief comparison of various types of temperature sensors: