Currently, the five most common types of temperature sensors include: Thermistors Thermocouples RTDs (Resistance Temperature Detectors) Digital Thermometer ICs Analog Thermometers I

1. Thermistor A thermistor (i.e., THERM or RES iStor) 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 as temperature increases. In contrast, the resistance of an NTC thermistor decreases as temperature increases, and this type of thermistor appears to be the most commonly used thermistor.

R 25°C is the nominal resistance of the thermistor at room temperature (25°C). This value is usually provided in the datasheet. β(β) is the material constant of the thermistor in Kelvin. This value is usually 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 a thermistor, namely resistance mode and voltage mode. Resistor mode linearization Resistor mode linearization connects an ordinary resistor in parallel with a thermistor. If the resistor has the same value as the thermistor at room temperature, the linearized region will be symmetrical around room temperature. See Figure 3 below.

3. Resistor Mode Linearization Voltage Mode Linearization Voltage mode linearization places the thermistor in series with an ordinary resistor forming a voltage divider circuit that must be connected to a known, fixed and stable voltage reference VREF. The effect of this configuration is to produce an output voltage that is linear over temperature. And, similar to resistive mode linearization, if the value of the resistor is equal to the resistance of the thermistor at room temperature, the linearized region will be symmetrical around room temperature. See Figure 4 below.

2. Thermocouples Thermocouples are usually used to measure higher temperatures and larger temperature ranges. Thermocouples work on the principle that any conductor subjected to a thermal gradient will produce a small voltage, a phenomenon known as the Seebeck effect. The magnitude of the voltage generated depends on the type of metal. A practical application of the Seebeck effect involves two dissimilar metals that are connected at one end and separated at the other. The temperature of the junction can be determined from the voltage between the conductors at the non-junction ends. There are many types of thermocouples depending on the metal materials used. Among these, alloy combinations have become popular, and the combination required is driven by factors including cost, availability, chemistry and stability. Different types of metal combinations are suitable for different applications, and users usually select them based on the required temperature range and sensitivity. See Figure 5 for a graph of thermocouple characteristics.

3. Resistance temperature detector (RTD) Resistance temperature detector, also known as resistance thermometer. RTDs are similar to thermistors in that their resistance changes with temperature. However, RTDs do not require the use of special materials that are sensitive to temperature changes like thermistors, but instead use a coil wrapped around a core made of ceramic or glass. RTD wires are pure material, usually platinum, nickel, or copper, and that material has a precise resistance-temperature relationship that is used to determine the measured temperature.

4. Analog Thermometer IC Alternatives to using thermistors and fixed-value resistors in voltage divider circuits are analog low-voltage temperature sensors, such as the TMP36 from Analog Devices. Contrary to the thermistor, the output voltage provided by this analog IC is almost linear. Slope is 10mV/°C and is accurate to ±2°C over the temperature range of -40 to +125°C. See Figure 6 below.

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. Likewise, 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).