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Thermistor System – Potential Dividers
KS5
Potential Dividers
• Draw a simple potential divider circuit.
• Explain without calculation what a potential
divider is used for.
Potential Dividers
R1
Vin
R2
Vout
• A potential divider is a useful circuit which
allows the user to control the output voltage
from 0V up to a maximum, determined by R1
and the input voltage.
Example
• Using the potential divider equation or otherwise, determine
the output voltage of the following circuit when:
a) R2 has a value of 50Ω
b) R2 has a value of 10Ω
200 Ω
12V
R2
Vout
Solution
• Using the potential divider equation:
𝑉𝑜𝑢𝑡
𝑅2
=
× 𝑉𝑖𝑛
𝑅1 + 𝑅2
a) 𝑉𝑜𝑢𝑡
50
=
× 12 = 2.4𝑉
250
b) 𝑉𝑜𝑢𝑡
10
=
× 12 = 0.6𝑉
210
Output voltage
• It should be clear form the solutions to the
previous example that decreasing the
resistance of R2 leads to a decrease in the
output voltage.
• Connecting a variable resistor in place of R2
would allow the output voltage to be varied
between 0V and a maximum, determined by
R1 and the input voltage.
Sensors
• Connecting any component whose resistance
varies in response to a stimulus in the place of
R2 allows us to build a circuit where the
output voltage varies in response to an
external event.
• Such components are called sensors.
Thermistors
• A thermistor is a circuit component whose
resistance changes with temperature.
• What do you think is going to happen to the
resistance of the thermistor as the
temperature increases?
Investigating thermistors
• For this investigation a
thermistor is mounted inside a
sealed glass tube.
• Resistance measurements can
be taken directly using a
multimeter or through current
and voltage measurement and
calculation.
Taking measurements
• You should measure the resistance of the thermistor for a
range of temperatures.
• Bring a beaker of water to the boil and take readings as the
water cools down.
• Readings should be taken at roughly 5oC intervals. (monitor
the temperature of the water using a thermometer)
• To reduce risk of damage it is advisable that the thermistor
system be supported by a clamp.
Sample Data
16000
• From the data opposite you
should be able to see that
resistance decreases as the
temperature increases.
14000
Resistance Ω
12000
10000
• We say that the thermistor
has a Negative Temperature
Coefficient
8000
6000
4000
• It is an NTC Thermistor
2000
0
0
20
40
60
Temperature oC
80
100
Add the title ‘Resistance against temperature for an
NTC thermistor’ to your graph.
120
Thermistors
• Suggest what you think the relationship
between resistance and temperature would
be for a PTC Thermistor (Positive temperature
coefficient)
Thermistors
• Suggest what you think the relationship
between resistance and temperature would
be for a PTC Thermistor (Positive temperature
coefficient thermistor)
• For a PTC thermistor, as temperature
increases, the resistance of the thermistor
increases.
Using Thermistors
• Look at the potential divider below.
• The thermistor is an NTC type.
• State what will happen to the output voltage as the
temperature increases. Explain your answer as fully as you
can.
R1
Vin
Vout