Noadswood Science, 2012

To understand LDRs and thermistors

Wednesday, April 19, 2023

Look at the VI graph for a LDR and thermistor – what does this tell you about the resistance of the components?

Potential Difference

Current

Thermistor

High temperature

Low temperature

Potential Difference

Current

LDR

Bright light

Dim light

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

Potential Difference

Current

LDR

Bright light

Dim light

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

Potential Difference

Current

Thermistor

High temperature

Low temperature

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)

+ -

V

A

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)

+ -

V

A

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)

Light Intensity

Resistance (Ω)

LightDark

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

Temperature

Resistance (Ω)

HotCold

1. Complete the following using the key words: diode; filament lamp; resistor; and thermistor

a) The resistance of a... decreases as its temperature increases

b) The resistance of a… depends on which way round it is connected in a circuit

c) The resistance of a… increases as the current through it increases

d) The resistance of a… does not depend on the current through it

2. A thermistor connected in series with an ammeter and a 3V battery is shown

a) At 15oC the current through the thermistor is 0.2A and the potential difference across it is 3V – calculate its resistance at this temperature

b) State and explain what happens to the ammeter reading if the thermistor’s temperature is increased

+ -

A

3V

1. a) The resistance of a thermistor decreases as its

temperature increasesb) The resistance of a diode depends on which way round

it is connected in a circuitc) The resistance of a filament lamp increases as the

current through it increasesd) The resistance of a resistor does not depend on the

current through it

2.

a) Resistance = 15Ω (V/I)b) The ammeter reading increases because the resistance

of the thermistor decreases

+ -

A

3V