Temperature Sensor Types
Temperature sensors are among the most commonly used sensors. All types of equipment use temperature sensors, including computers, cars, kitchen appliances, air conditioners, and (of course) home thermostats. The five most common types of temperature sensors include:

• Thermistor
• Thermocouple
• RTDs
• Analog thermometer ICs
• Digital thermometer ICs
• This article will give you a succinct intro to each of the sensor types listed.

Thermistor Basics—NTC vs PTC Thermistor
As the name implies, the thermistor (i.e., thermal resistor) is a temperature-sensing device whose resistance is a function of its temperature.

Thermistors are available in two types: PTC (positive temperature coefficient) and NTC (negative temperature coefficient). The resistance of a PTC thermistor increases as the temperature increases. In contrast, the resistance of an NTC thermistor decreases as temperature increases, and this type seems to be the most commonly used thermistor. See Figure 1 below.

It’s important to realize that the relationship between a thermistor’s resistance and its temperature is very non-linear, as seen in Figure 2.

NTC Thermistor Resistance Equation
The standard equation for an NTC thermistor’s resistance as a function of temperature is given by:

Where:

• R25C is the thermistor’s nominal resistance at room temperature (25 °C). This value is normally provided in the datasheet.
• β (beta) is the thermistor’s material constant in Kelvin. This value is normally provided in the datasheet.
• T is the thermistor’s actual temperature in Celsius.

However, there are two easy techniques used to linearize a thermistor’s behavior, namely, resistive mode and voltage mode.

Resistive Mode Linearization
Resistive mode linearization places a normal resistor in parallel with the thermistor. If the value of the resistor is the same as that of the thermistor at room temperature, the linearization region will be symmetrical around room temperature. See Figure 3 below.