Unit 7: Electrical devices
LO2: Understand electrical sensors and actuators
Sensors – temperature – the thermistor
Learner activity sheet
Activity 1
A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in
standard resistors.
Thermistors come in a variety of shapes and sizes, and are usually made from ceramic or
semiconducting polymer. Their resistance changes as temperature increases or decreases. This
causes the current flowing through the thermistor to change as more or less charge carriers become
available.
Begin this activity by investigating different types of thermistor available. You may wish to use the
internet, and will find useful information including datasheets on component supplier’s websites.
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Answer the following questions:
1.
What is a NTC thermistor and how does it work?
2.
What is a PTC thermistor and how does it work?
3.
What applications are thermistors used for?
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Activity 2
In Activity 2 you are going to conduct an experiment to investigate the characteristics of a thermistor.
Equipment you will need:
Thermistor (an NTC type with 100 at 25C is suitable)
Power supply (regulated, fixed voltage, DC)
Ammeter (A) (you could use the ammeter function in a multimeter)
Connecting wires
Timer or stopwatch
Thermometer
Beaker
Heat resistant mat
Supply of hot water (from a kettle)
1.
Set up the experiment as shown below using a heat-resistant beaker. Make sure that the beaker is
on a heat-resistant mat or surface.
A
5 – 9 V DC
Thermometer
Hot water
Thermistor
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2.
Pour boiling water into the beaker (take care to use suitable personal protective equipment).
3.
Take readings of current at regular time intervals as the water cools (you will have to determine
what time intervals you are going to use as you start to see the temperature fall).
Tabulate your results of temperature and current (and also resistance calculated using Ohm’s Law
R = V/I):
Temperature
4.
Current
Resistance
Plot a graph of temperature (x axis) vs. current (y axis). You could plot this using graph paper, or
could use a spreadsheet.
5.
Plot also a graph of temperature (x axis) vs. resistance (y axis).
Questions:
1.
What do you notice about the shape of the graphs?
2.
What is happening to the resistance of the thermistor?
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3.
If you have access to the datasheet for the thermistor being used, do your results agree with the
current vs. temperature and/or resistance vs. temperature graphs in the data?
If not, explain why you think they do not match.
You could repeat the activity using a thermocouple. You would need to begin by investigating how a
thermocouple works, and how you can measure its output signal.
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