LDRs & Thermistors

Report
Noadswood Science, 2012
Friday, July 17, 2015

To understand LDRs and thermistors

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
Current
High temperature
Current
Dim light
Potential Difference
Low temperature
Potential Difference
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
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



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



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

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 Intensity
Light

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
Temperature
Hot
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
A thermistor connected in series with an ammeter and a 3V
battery is shown
2.
+
3V
-
A
a)
b)
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
1.
a)
b)
c)
d)
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
2.
+
3V
-
A
a)
b)
Resistance = 15Ω (V/I)
The ammeter reading increases because the resistance of
the thermistor decreases

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