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# How A Thermistor Works

## Introduction:

A thermistor is a component that has a resistance that changes with temperature. There are two types of thermistor, those with a resistance that increase with temperature (Positive Temperature Coefficient – PTC) and those with a resistance that falls with temperature (Negative Temperature Coefficient – NTC).

## Temperature coefficient:

The most common type of thermistors are those in which resistance decreases as the temperature increases (NTC).

The amount by which the resistance decreases as the temperature increases is not constant, it varies in a non linear way. A formula can be used to calculate the resistance of the thermistor at any given temperature. Normally these are calculated for you and the information can be found in the devices datasheet.

## Thermistor Applications:

There are many applications for a thermistor, three of the most popular are listed below.

### Temperature sensing:

The most obvious application for a thermistor is to measure temperature, they are used to do this in a wide range of products such as thermostats.

### In rush current limiting:

In this application the thermistor is used to initially oppose the flow of current (by having a high resistance) into a circuit. Then as the thermistor warms up (due to the flow of electricity through the device) its resistance drops letting current flow more easily.

### Circuit protection:

In this application the thermistor is used to protect a circuit by limiting the amount of current that can flow into it. If too much current starts to flow into a circuit through the thermistor this causes the thermistor to warm up. This in turn increases the resistance of the thermistor reducing the current that can flow into the circuit.

### Example:

The circuit shown below shows a simple way of constructing a circuit that turns on when it goes hot. The decrease in resistance of the thermistor in relation to the other resistor which is fixed as the temperature rises will cause the transistor to turn on. The value of the fixed resistor will depend on the thermistor used, the transistor used and the supply voltage.

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## 9 thoughts on “How A Thermistor Works”

• Rahim Alli Saha October 25, 2016 at 8:03 am

What is thermister

• Mark Donnison October 25, 2016 at 11:33 am

Hi Rahim, A thermistor is a component that has a resistance that changes with temperature.

• Jervis November 23, 2016 at 12:10 am

When the temperature rises in the 3rd Diagram , what will happen to the base voltage? Why?

• Rob Haywood December 8, 2016 at 2:24 pm

The base voltage will increase.
As the temperature rises the resistance of the thermistor decreases. As the other resistor is fixed there comes a point (depending on value of resistors etc) when the transistor receives power and turns on.

Rob

• Farzana December 11, 2016 at 1:32 pm

How can a thermistor be used to measure temperature?

• Mark Donnison December 13, 2016 at 10:33 am

Hi Farzana, as with most things, there is more than one way to go about achieving this. You could, for example, use a microcontroller such as the BBC micro:bit for this purpose. As the micro:bit is not able to read changes in resistance you would have to convert these changes in resistance to changes in voltage, one way of doing this is to use a potential divider circuit where one of the resistors in series is fixed and the other resistor is replaced with the thermistor. If you take a voltage reading between the two, the voltage will change as the resistance changes and you can monitor this using the analogue read pins on the micro:bit, with a bit of maths and some code you can convert these changes of voltage into a more meaningful number.

We have a guide detailing how a potential divider circuit works, which you can find here: https://www.kitronik.co.uk/blog/potential-divider-voltage-divider

At the foot of the guide you will find a list of products that feature a potential divider circuit, one of which is a heat activated switch: https://www.kitronik.co.uk/2113-heat-activated-switch.html

I hope this answered your question.

• Zoya January 3, 2017 at 1:28 pm

Tq it was helpful

• David yap March 1, 2017 at 6:30 am

I have a driver pcb to drive a spindle .I saw a thermistor is installed in the pcb . Now my spindle is not running when I switch ON. How do I know if this thermistor is faulty. ?
Thank you very much for your advice.

• Mark Donnison March 1, 2017 at 11:34 am

Hi David, I would use a multimeter to measure the resistance of the thermistor and then warm up the thermistor with my hand or breath, you should see a change in resistance as you do so.