What is a Thermistor?

This is a basic guide to concisely summarize the fundamentals of Thermistors.

The name Thermistor is a combination of 2 words “Resistor” and “Thermal”. It is a Temperature sensing element that changes resistance with temperature.

Thermistors are one of the most cost-effective ways to sense changes in temperature.

Thermistors resistance relationship with temperature is exponential therefore not linear, this subtle but major characteristic is a huge consideration when designing circuits to correctly interpret temperature.

Note Thermistors have NO polarity

What do Thermistor values mean?

Thermistors are referenced to their base resistance this is to 25°C , meaning a 10K Thermistor will read 10K at 25°C , of course there will be errors but the value indicted is the ideal scenario.

For example, if a Thermistor value is described as 10K Ω 1% then at 25°C The Thermistor will measure between 9.9 Ω and 10.1KΩ.

For example, base resistances can be 1K 2K or 47K etc, the value is selected for what best suits the circuitry. 10K is normally the most popular

What are Thermistor Beta Values or ‘B’ Values?

Thermistor Beta values determine the characteristic change in resistance as the temperature changes, they characterise the Resistance v Temperature curve. Beta values is used in the Thermistor equation to calculate the resistance at a particular temperature, If a thermistor is being replaced then it is crucial to know its Value and Beta Value.

The B value is characterized over 2 temperatures, and is obtained from the manufacturer's datasheet.

NTC or PTC?

There are 2 types of Thermistors NTC an PTC

• NTC

NTC stands for Negative Temperature Coefficient , this means the resistance will decrease when the temperature increases.

• PTC

PTC Stands for Positive Temperature Coefficient, this means the resistance will increase when the temperature increases.

NTC thermistors are the more widely used between the 2 therefore the example below will focus on a NTC Thermistor.

Sensing Temperature with a Thermistor

One solution to sensing temperature with a thermistor is to implement a potential divider, then decode the voltage output with the ADC in embedded code.

There are 2 conventional methods when decoding the ADC value.

1. Stein-Hart Hart Equation

This method is requires more processing in the MCU and will use more RAM.

2. Look Up Table

If the MCU is lacking in CPU our ALU then then the lookup table is used although this requires the vales to be calculated outside of the c code and then copied in as an array an uses more ROM

Circuit example

The circuit below uses the common potential divider, but instead of using 2 resistors, one resistor position is a Thermistor. The voltage output is calculated with the potential divider equation.

Conventionally the fixed resistor is selected to match the Thermistor resistance at 25°C e.g. 10K however this depends on the application.

All of our thermistors are available at competitive prices , full circuit and firmware support service is provided to streamline your thermistor circuit design.

Feel free to contact and discuss your requirements and 

Thermistor equations and real world examples will be covered in future articles !

Information our thermistor probes here