All subjects · Physics · WPH15 (Unit 5)

Core practical 12: calibrating a thermistor as a thermostat

Core practical 12 calibrates a thermistor in a potential divider, so that the output p.d. can be used to read or control temperature, as in a thermostat.

Method

Connect an NTC thermistor in series with a fixed resistor (or a variable resistor) across a d.c. supply, with a voltmeter across the fixed resistor (the output).
Put the thermistor in a beaker of water with a thermometer close to it. Start with iced water and heat gradually, stirring all the time.
At regular temperature intervals, record the temperature and the output p.d.
Repeat while cooling, and average.

The calibration curve

Plot output p.d. against temperature. For an NTC thermistor, its resistance falls as temperature rises, so the fixed resistor takes a larger share of the supply p.d.: the output across the fixed resistor rises. Use the curve to read off the temperature for any measured p.d.

As a thermostat

A thermostat switches a heater off when the temperature reaches a set value. The output p.d. is fed to a switching circuit that operates at a fixed threshold p.d. Adjusting the variable resistor changes the temperature at which the threshold is reached. The calibration curve shows what output p.d. matches the chosen temperature.

Worked example

A 6.0 V supply is connected across a thermistor and a 2.2 kΩ resistor. At 20 °C the thermistor's resistance is 3.3 kΩ. Find the output p.d. across the fixed resistor.

V_out = V_in x R / (R + R_th) = 6.0 x 2.2 / (2.2 + 3.3)

Answer: V_out = 2.4 V

Key idea: Thermistor plus fixed resistor as a potential divider; stir and heat slowly; plot V_out against temperature; read temperatures off the curve.

Check yourself

A 6.0 V supply is connected across a 2.2 kΩ fixed resistor and a thermistor of resistance 3.3 kΩ. Find the p.d. across the fixed resistor in V.
Show answer
2.4 V. V = 6.0 x 2.2 / 5.5 = 2.4 V.
When the water is heated, the thermistor's resistance falls to 1.8 kΩ. Find the new p.d. across the 2.2 kΩ resistor (supply 6.0 V), in V.
Show answer
3.3 V. V = 6.0 x 2.2 / 4.0 = 3.3 V.
With a 6.0 V supply, the output across a fixed resistor must be 4.0 V when the thermistor's resistance is 1.0 kΩ. Find the fixed resistance in kΩ.
Show answer
2 kΩ. The thermistor has 2.0 V across it, so the fixed resistor has twice its resistance: 2.0 kΩ.
Why is the water stirred throughout the experiment?
- To cool the thermistor
- So the water, thermometer and thermistor are all at the same temperature
- To stop the water boiling
- To increase the current
Show answer
So the water, thermometer and thermistor are all at the same temperature. Without stirring, the thermometer and thermistor could be at different temperatures.
As the temperature rises, what happens to the output p.d. across the fixed resistor in series with an NTC thermistor?
- It decreases
- It stays the same
- It falls to zero
- It increases
Show answer
It increases. The thermistor's resistance falls, so the fixed resistor takes a larger share.
How is the calibration curve used?
- To find the supply p.d.
- To find the temperature from a measured output p.d.
- To find the resistance of the voltmeter
- To find the current in the heater
Show answer
To find the temperature from a measured output p.d.. That is what calibration means: linking the reading to the quantity being measured.
In a thermostat circuit, why is a variable resistor used instead of a fixed one?
- To make the thermistor more sensitive to light
- To protect the supply
- To keep the output p.d. constant
- To set the temperature at which the circuit switches
Show answer
To set the temperature at which the circuit switches. Changing R changes the temperature at which the output reaches the switching threshold.

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Last updated 7 November 2026. Spotted a mistake? Tell us.