1. LDRs & Thermistors
Noadswood Science, 2012

2. LDRs & Thermistors To understand LDRs and thermistors
Wednesday, April 19, 2017
LDRs & Thermistors
To understand LDRs and thermistors

3. Resistance
Look at the VI graph for a LDR and thermistor – what does this tell you about the resistance of the components?
LDR
Thermistor
Bright light
High temperature
Current
Current
Dim light
Low temperature
Potential Difference
Potential Difference

4. Resistance = Voltage ÷ Current
LDR
Resistance = Voltage ÷ Current
At constant light intensity levels the line is straight, so the LDR resistance is constant
If the light intensity increases the resistance decreases
If the light intensity decreases the resistance increases
LDR
Bright light
Current
Dim light
Potential Difference

5. Resistance = Voltage ÷ Current
Thermistor
Resistance = Voltage ÷ Current
At constant temperature the line is straight, so the thermistor resistance is constant
If the temperature increases the resistance decreases
If the temperature decreases the resistance increases
Thermistor
High temperature
Current
Low temperature
Potential Difference

6. Experiment - LDR
Set up a circuit with a LDR in series with a cell and ammeter (place a voltmeter across the LDR)
Record the current and potential difference with 5x different light intensities (place a lamp closer and closer to the LDR (use a fixed distance, i.e. 10cm closer each time))
Calculate the resistance change with increasing light intensities (R = V/I) + - A V

7. Experiment - thermistor
Set up a circuit with a thermistor in series with a cell and ammeter (place a voltmeter across the thermistor)
Record the current and potential difference with different temperatures (sensitive thermistors can have a significant change in resistance from just placing them between finger and thumb, warming them up)
Calculate the resistance change with increasing temperature (R = V/I) + - A V

8. LDRs
Light-dependent resistors (LDRs) are used to detect light levels, e.g. in automatic security lights, burglar detectors etc…
As light levels increase the resistance decreases
As light levels decrease the resistance increases (resistance is highest in darkness)
Resistance (Ω)
Dark
Light
Light Intensity

9. Thermistors
Thermistors are used as temperature sensors, e.g. car engine sensors, fire alarm sensors, fridges etc…
As temperature increases the resistance decreases
As temperature decreases the resistance increases
Resistance (Ω)
Cold
Hot
Temperature

10. Summary Questions
Complete the following using the key words: diode; filament lamp; resistor; and thermistor
The resistance of a... decreases as its temperature increases
The resistance of a… depends on which way round it is connected in a circuit
The resistance of a… increases as the current through it increases
The resistance of a… does not depend on the current through it

11. Summary Questions
A thermistor connected in series with an ammeter and a 3V battery is shown
At 15oC the current through the thermistor is 0.2A and the potential difference across it is 3V – calculate its resistance at this temperature
State and explain what happens to the ammeter reading if the thermistor’s temperature is increased 3V + - A

12. Summary Answers
The resistance of a thermistor decreases as its temperature increases
The resistance of a diode depends on which way round it is connected in a circuit
The resistance of a filament lamp increases as the current through it increases
The resistance of a resistor does not depend on the current through it

13. Summary Questions Resistance = 15Ω (V/I)
The ammeter reading increases because the resistance of the thermistor decreases 3V + - A