Experiments with a thermistor

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What is a thermistor?

A thermistor is a type of temperature-sensitive resistor, meaning that its resistance is affected by a change in temperature. Thermistors can be categorised by their temperature coefficient of resistance, which is ratio of increase in resistance to its resistance temperature rise. Therefore, there are 2 main types, namely positive temperature coefficient (PTC) and negative temperature coefficient (NTC) thermistors. In a PTC thermistor, resistance increases with increasing temperature, whereas in a NTC thermistor, its resistance decreases instead. The graph below illustrates this fact:

Operating over a range of -200°C to + 1000°C, they are supplied in glass bead, disc, chips and probe formats. NTC thermistors should be chosen over PTC thermistors when a continuous change of resistance is required over a wide temperature range. They offer mechanical, thermal and electrical stability, together with a high degree of sensitivity.

The next question is, how does an NTC thermistor work? Why does its resistance decrease when temperature is increased? When the thermistor is subjected to an increase in temperature, the number of electrons able to move about and carry charge increases - it promotes them into the conducting band. Therefore it will be able to carry more current and as a result, its resistance falls. Since semiconductors generally behave this way, they are used to manufacture thermistors. They can also be manufactured from the oxides of transition metals such as manganese, cobalt, copper and nickel.

Thermistors are widely used in potential dividers and in Wheatstone bridge circuits to supply varying potential differences which is done by altering the resistance of the thermistor.


For this sensor project, I have chosen to acquire a series of measurements using a temperature sensor called a thermistor, and design a circuit for an application of a temperature sensor.

Due to availability of various types and capabilities of temperature sensors, I had to perform small experiments to decide on which sensor to use. I have initially chosen to exclude thermocouples because it can only generate a small electromotive force (e.m.f) and therefore needs amplification. The options remaining are the thermistor probe and bead thermistor.

To find out the best one, I decided to monitor their response time, or in other words, compare how quickly the two sensors can respond to sudden changes in temperature. I prepared two beakers, one containing tap water at a temperature of around 20oc (Beaker 1) and the other one containing boiling water at 100oc (Beaker 2). I then set up a circuit as shown below:-

Potential Divider Circuit

The circuit is basically made up of a 5 V power supply with a thermistor and a resistance substitution box connected in series. A digital multi-meter is connected in parallel to the thermistor, to measure the potential difference across the thermistor.

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First of all, I immersed each thermistor into Beaker 1 and after sometime, I transferred the thermistor into Beaker 2, noting the time taken for the digital multi-meter to reach a steady voltage value. The values taken are shown below:

From the values taken, it can be deduced that the bead thermistor was better of the two in terms of response time because it took the shortest time to show a steady value when a sudden change in temperature was applied.


  • Bead thermistor
  • Power supply, 5V dc (When ...

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