A Thermistor is an electronic component which allows us to monitor the temperature.

This ability to monitor the temperature sees the thermistor used in many applications where temperature sensing is required. 

So, where are thermistors actually used?

The thermistor can be found in safety applications like fire alarms, in kitchen appliances such as microwaves, fridges, and toasters, in the Automotive Industry, in comfort applications like Air Conditioning Units and many more.

However, other than temperature sensing, they are used in voltage regulation, volume control and circuit protection.

I will cover more applications where the thermistor is used later in this article.

It is a versatile, cheap and reliable option compared to many other alternatives available. 

While thermistors can be used in multiple ways, their main purpose is to serve as a temperature sensor.

What is a Thermistor?

A thermistor is essentially a resistor, whose resistance changes in relation to the change in ambient temperature. 

Over small increments, the relationship between a thermistor and temperature can be assumed as linear, but in most cases the relationship is non-linear.  

The word Thermistor is a combination of two words; Thermal and Resistor.

There are two main types of Thermistors; Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC).

Negative Temperature Coefficient (NTC) Thermistor

The relationship of a NTC Thermistor’s resistance with temperature is inversely proportional.

This means that an increase in temperature sees a decrease in resistance.

Positive Temperature Coefficient (NTC) Thermistor

PTC thermistors on the other hand have a proportional relationship.

This means that an increase in temperature sees an increase in resistance.

Materials Used

Another factor that distinguishes Thermistor types is the material used.

The two most common materials used are Metallic compounds which include oxides (this is mostly in NTC Thermistors), and single crystal semiconductors.

Both these material types cover different temperature ranges.

Brief History

The earliest known Thermistor was observed by Michael Faraday in 1833. 

The material that he was using was a metallic compound which had a negative temperature coefficient. 

The material he was using and noticed a change in resistance was silver sulphide.

The rise of metallic oxides only came later in the 1940’s.

Due to the research that was focused on semiconductor materials after the Second World War, silicon thermistors were studied more.

Where Thermistors are used?

Due to them being cheap and highly effective, thermistors can be used in many applications.

The applications of where Thermistors are used can be divided into the two categories of NTC and PTC thermistors.

NTC Thermistor Applications

  • When there is a  need for sensing temperatures at very low values, NTC thermistors are the most effective
  • These variety of thermistors are commonly found in digital thermostats
  • Charging batteries can be dangerous if temperatures are not monitored. They can overheat and cause fires. NTC thermistors provide a means of making sure temperature levels are maintained during the charging process
  • When turned on, power supplies have a high in-rush-current. NTC thermistors provide a way of limiting this in rush current with their high resistance. As the temperature increases gradually, the resistance of the thermistor decreases letting the current increase at a steady pace.

PTC Thermistor Applications

  • Similar to the in-rush- current limiter that NTC thermistors are used for, PTC thermistors are used in current limiting applications. Since, the resistance of the PTC thermistor rises with a rise in temperature, a large surge in current will cause a rise in temperature, which in turn will increase the resistance of the thermistor thus limiting the flow of current.

However, thermistors are more versatile than just being used in applications where temperature sensing is required.

They are used in applications of voltage regulation, volume control, time delay and protection circuits (as we saw above).

Common products where thermistors are used?

Microwaves are a common if not essential product in many households. 

They provide a means of heating up your food and beverages with a press of a couple of buttons. 

But, a microwave would not be able to function correctly without proper temperature regulation. The risks of fires due to overheating are very high. 

Thermistors help microwaves maintain their temperature levels.

Surge Protectors are devices that protect your devices which are plugged into the mains outlets. 

A lightning strike to a power line near your home could cause a large spike in current which can potentially damage devices plugged in.

As we saw above PTC thermistors provide a way to limit this current and are the reason they are used in surge protectors.

Automobiles like your car, have an indicator on the dashboard that has an ‘H’ (hot) and ‘C’ (cold).

This indicator lets you know the temperature levels of the oils and coolants in your car thanks to a thermistor.

While the thermistor does not prevent overheating, it’s just a way to indicate temperature levels.

Why use thermistors?

From some of the applications of where thermistors are used you can see their importance.

Thermistors provide feedback that lets us know whether the temperature is too high or low depending on the application.

So, thermistors need to be used when we need to regulate temperature within the ‘safe zone’.

They are a common option because of their affordability, low cost and effectiveness.