The five most common types of temperature sensors

Temperature Sensors One of the most commonly used sensors, we see temperature sensors in devices such as computers, automobiles, kitchen appliances, air conditioners and home thermostats.

The five most common types of temperature sensors available today include: Thermistors , thermocouples, RTDs (resistance temperature detectors), digital thermometer ICs, and analog thermometer ICs.

A thermistor 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 increasing temperature. Conversely, the resistance of an NTC thermistor decreases with increasing temperature, and this type of thermistor appears to be the most commonly used thermistor. See Figure 1 below.

Figure 1. Electrical symbols for PTC and NTC thermistors

It is important to note that the relationship between the resistance of a thermistor and its temperature is very non-linear. See Figure 2 below.

Figure 2. Resistance versus temperature for NTC thermistors

The standard equation for the variation of NTC thermistor resistance with temperature is:

R25C is the nominal resistance of the thermistor at room temperature (25°C). This value is usually provided in the data sheet. β is the material constant of the thermistor in Kelvin. This value is usually provided in the data sheet. 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 a thermistor, resistance mode and voltage mode.

Resistive mode linearization

Resistance mode linearization connects a normal resistor in parallel with a thermistor. If the value of the resistor at room temperature is the same as the value of the thermistor, the linearized region will be symmetrical around room temperature. See Figure 3 below.

Resistive mode linearization

Resistance mode linearization connects a normal resistor in parallel with a thermistor. If the value of the resistor at room temperature is the same as the value of the thermistor, the linearized region will be symmetrical around room temperature. See Figure 3 below.

Figure 3. Resistive mode linearization

Voltage Mode Linearization

Voltage mode linearization puts the thermistor in series with a common resistor forming a voltage divider circuit that must be connected to a known, fixed and stable voltage reference, V REF. The effect of this configuration is to produce an output voltage that is linear over the entire temperature range. And, similar to resistive mode linearization, if the value of the resistor is equal to the resistance of the thermistor at room temperature, the linearization region will be symmetrical around room temperature. See Figure 4 below.

Figure 4. voltage pattern linearization

Thermocouples are typically used to measure higher temperatures and larger temperature ranges.

Thermocouples work on the principle that any conductor subjected to a thermal gradient produces a small voltage, a phenomenon known as the Seebeck effect. The magnitude of the voltage produced depends on the type of metal.A practical application of the Seebeck effect involves two dissimilar metals that are joined at one end and separated at the other. The temperature of the junction can be determined from the voltage between the wires at the non-junction end. There are several types of thermocouples, depending on the metal material used. Among these, alloy combinations have become popular and the desired combination is driven by factors including cost, availability, chemistry and stability. Different types of metal combinations are used for different applications, and users typically select them based on the desired temperature range and sensitivity. For a graphical representation of thermocouple characteristics, see Figure 5.

Figure 5. Thermocouple Characteristics

3. Resistance temperature detector (RTD)

Resistance Temperature Detectors, also known as resistance thermometers.RTDs are similar to thermistors in that their resistance changes with temperature. However, instead of using special materials that are sensitive to temperature changes, as is the case with thermistors, RTDs use coils wound around a core wire made of ceramic or glass.The RTD wire is a pure material, usually platinum, nickel, or copper, and the material has a precise resistance-temperature relationship that is used to determine the measured temperature.

4. Analog Thermometer IC

An alternative to the use of thermistors and fixed-value resistors in voltage divider circuits is an analog low-voltage temperature sensor, such as Analog Devices' TMP36.In contrast to thermistors, this analog IC provides an output voltage that is nearly linear. The slope is 10mV/°C over a temperature range of -40 to +125°C and is accurate to ±2°C. Although these devices are very easy to use, they are much more expensive than a thermistor plus resistor combination.

5. Digital thermometer IC

Digital temperature devices are more complex, but they can be very accurate. Again, they can simplify your overall design because the analog-to-digital conversion occurs inside the thermometer IC, rather than a separate device such as a microcontroller. For example, Maxim Integrated's DS18B20 has an accuracy of ±0.5°C and a temperature range of -55°C to +125°C. Also, some digital ICs can be configured to harvest energy from their data lines, allowing connections to be made using only two wires (i.e., data/power and ground).