9.8 The thermistor

Question 1

What’s a negative temperature coefficient?

Answer 1

For a material, their resistance drops as temperature increases.

In some semiconductors, as the temperature increases, the number density of the charge carriers also increases.

Question 2

What is a thermistor?
Why is it useful in temperature-sensing circuits?

Answer 2

An electrical component made from a semiconductor with a negative temperature coefficient. As the temperature of the thermistor increases, its resistance drops.

It is useful in temperature-sensing circuits because the change in resistance is dramatic. A small change in temperature can be detected by monitoring the resistance of the thermistor.

Question 3

Where are thermistors used?

Answer 3

• Simple thermometers
• Thermostats to control heating and air-conditioning units.
• To monitor the temperature of components inside electrical devices like computers and smartphones so that they can power down before overheating damages them.
• To measure temperature in a wide variety of electrical devices like toasters, kettles, fridges, freezers and hair dryers.
• To monitor engine temperatures to ensure the engine does not overheat.

Question 4

How can you investigate how the resistance of a thermistor changes with temperature change?

Answer 4

• Using an ohmmeter and a water bath
• The ohmmeter in parallel with the thermistor
• Use ohmmeter to get readings of resistance and thermometer to get readings of temperature.

Question 5

Describe the I-V characteristic of thermistor.

Answer 5

An increase in temperature leads to an increase in the number density of the free electrons. This means that the resistance of the thermistor decreases as the temperature increases.

• As p.d increases, current increases which leads to an increase of number density of electrons, leading to less resistance.