Edexcel A Level (IAL) Physics-5.2 Core Practical 12: Calibrating a Thermistor- Study Notes- New Syllabus

CORE PRACTICAL 12: Calibrate a Thermistor in a Potential Divider Circuit as a Thermostat

This practical investigates how the resistance of a thermistor varies with temperature and how it can be calibrated and used in a potential divider circuit to act as a thermostat.

Aim

To calibrate a thermistor by measuring how the output voltage of a potential divider varies with temperature, and to understand how this circuit can be used as a thermostat.

Apparatus

• Thermistor (NTC type)
• Fixed resistor
• DC power supply
• Voltmeter
• Thermometer or temperature probe
• Water bath
• Beaker
• Connecting leads

Theory

A thermistor is a temperature-dependent resistor. For an NTC thermistor:

• Temperature increases → resistance decreases
• Temperature decreases → resistance increases

In a potential divider circuit, the output voltage depends on resistance:

\( V_{\text{out}} = V_{\text{in}} \dfrac{R_{\text{thermistor}}}{R_{\text{thermistor}} + R_{\text{fixed}}} \)

As temperature changes, \( R_{\text{thermistor}} \) changes, causing \( V_{\text{out}} \) to vary.

Circuit Description 

• The thermistor is connected in series with a fixed resistor.
• A constant supply voltage \( V_{\text{in}} \) is applied.
• The output voltage is measured across the thermistor.

This changing voltage can be used to trigger a switch or relay at a set temperature — acting as a thermostat.

Procedure

1. Set up the potential divider circuit with the thermistor and fixed resistor in series.
2. Place the thermistor in a water bath.
3. Connect the voltmeter across the thermistor.
4. Measure and record the temperature of the water.
5. Record the corresponding output voltage.
6. Heat the water gradually and repeat measurements over a range of temperatures.
7. Allow the water to cool and take repeat readings.
8. Take averages for each temperature value.

Calibration

• Plot a graph of output voltage \( V_{\text{out}} \) against temperature.
• The graph will be non-linear.
• This graph allows temperature to be determined from voltage readings.

This graph is the calibration curve of the thermistor.

Using the Circuit as a Thermostat

• A reference voltage can be set using a comparator.
• When \( V_{\text{out}} \) crosses this reference voltage:
  
  • A heater can be switched off
  • A fan can be switched on
• This allows automatic temperature control.

Sources of Error

• Thermistor not fully immersed in water.
• Temperature gradients in the water bath.
• Inaccurate thermometer readings.
• Electrical noise in voltage measurements.

Improvements

• Stir the water bath continuously.
• Use a digital temperature probe.
• Take multiple readings and average.
• Use insulation to reduce heat loss.