Thermistors
- A thermistor is a non-ohmic conductor and sensory resistor whose resistance varies with temperature
- Most thermistors are negative temperature coefficient (ntc) components
- This means that if the temperature increases, the resistance of the thermistor decreases (and vice versa)
The resistance through a thermistor is dependent on the temperature of it
- The temperature-resistance graph for a thermistor is shown below
Graph of temperature against resistance for a thermistor
- Thermistors are temperature sensors and are used in circuits in ovens, fire alarms and digital thermometers
- As the thermistor gets hotter, its resistance decreases
- As the thermistor gets cooler, its resistance increases
A digital thermometer uses a thermistor
Worked example
A thermistor is connected in series with a resistor R and a battery.
The resistance of the thermistor is equal to the resistance of R at room temperature.
When the temperature of the thermistor decreases, which statement is correct?
A. The p.d across the thermistor increases
B. The current in R increases
C. The current through the thermistor decreases
D. The p.d across R increases
ANSWER: A
- The resistance of the thermistor increases as the temperature decreases
- Since the thermistor and resistor R are connected in series, the current I in both of them is the same
- Ohm’s law states that V = IR
- Since the resistance of the thermistor increases, and I is the same, the potential difference V across it increases
- Therefore, statement A is correct
Examiner Tip
Make sure you remember the shape of the temperature-resistance graph for a thermistor, as it is a common exam question to draw and interpret this.The graph should not touch the x-axis, as this implies 0 resistance which is only possible in superconductors.
