ELECTRICITY 4: Circuit devices and how are they used e.g. thermistor, LDR, LED and
diode
Doc Brown's Physics Revision
Notes
Suitable for GCSE/IGCSE Physics/Science courses or
their equivalent
INDEX:
Introduction to testing a circuit device
What is an LDR?
Uses?
LDR
Describe and explain a practical use of an LDR
What is a thermistor?
Uses?
THERMISTOR
Describe and explain a practical use of a thermistor.
What is the function of a diode?
Uses?
DIODE
* What is an LED?
Uses?
LED
Introduction to circuit devices and how to
investigate their characteristics
Some components are designed to change
resistance in response to changes in the environment e.g.
the resistance of an
LDR varies with light intensity,
the resistance of a thermistor varies with
temperature,
and these properties used in
sensing systems to monitor changes in the
environment.
These kind of circuit components
can be used to turn systems on and off, increase or decrease power
to control the output depending on the ambient (surrounding)
conditions.
Circuit 31 (left) is
the sort of circuit you can use to test a device in terms of its current -
voltage behaviour.
By varying the voltage from the power supply using the variable
resistor you can readily get lots of pairs of readings of p.d. (V) and current
(A).
Then use Ohm's Law equation (R = V/I) to calculate the value of
the resistance of the device for any pair of p.d. and current readings.
To test some devices you also need to be
able to vary the temperature (thermistor) and light intensity (LDR)
Thermistor - temperature dependent
resistor
Circuit 42 shows how you can
investigate the resistance of a thermistor.
The voltmeter is wired in parallel with
the thermistor, the p.d. V is measured in volts (V).
The variable resistor allows you to vary
the p.d. and current flow.
The ammeter, wired in series, gives you
the current I reading in amps (A).
You must decide on the initial p.d. and
see how the current varies.
You calculate the resistance of the
thermistor from Ohm's Law equation: V = IR, so R = V/I
Somehow you need to vary the temperature
of the thermistor resistor e.g. dipping it into a beaker of water of varying
temperature, making sure the circuit is insulated from the water.
You can make measurements from 0 to
60oC by using ice and then warm-hot water and try to get
measurements for every 5 or 10oC incremental rise in
temperature.
You
should find that the resistance falls with increase in temperature because
a thermistor is a temperature dependent resistor.
The higher its temperature, the lower
a thermistor's resistance (e.g. tens of ohms) and much higher at low
temperatures (e.g. thousands of ohms).
High resistance in a cool environment and
low resistance in a warm environment.
You can see this trend clearly in the resistance -
temperature graph for a thermistor.
Thermistors can therefore respond to changes in temperature.
Thermistors can act as temperature
detectors and are used in thermostats, temperature sensors - cooling systems
in car engines etc.
Circuit
32 shows in principle how to control a cooling fan in a room.
(real thermistor circuits are more
complicated)
The fixed resistor and cooling fan are
wired in parallel. This means they always have the same potential difference
across them.
However, the thermistor is a variable
resistor.
The p.d. of the power supply is shared
out between the thermistor and the 'loop' consisting of the fixed resistor
and fan wired in parallel.
The output component (fan) and the
thermistor are wired in series.
(I've indicated this with blue arcs -
not meant to be wires!)
The greater the component's resistance,
the greater proportion of the p.d. it takes.
If the room gets hotter, the resistance
of the thermistor decreases, so it takes a smaller shared of the p.d.
Therefore the p.d. across the fixed
resistor and fan rises (V1 increases, V2 decreases).
The fixed resistor and cooling fan
motor are wired in parallel, so have the same p.d. V1 across
them.
The greater the p.d. across the fan, the
faster it goes as the power output can increase (P = IV).
If the room cools, the thermistor's
resistance increases and the process reverses and the fan slows down or
stops.
Thermistors are used as
temperature detectors e.g. electronic thermostats in heating and cooling
systems in the home or electric kettles (relatively low temperatures), or in
high temperature situations like a car engine.
Footnote
on the I-V graph for a thermistor
(graph (2)
on the right)
The graph of current versus voltage for a
thermistor is similar to that of a filament bulb.
Its a non-linear graph and the
phrase non-linear component may be used.
When the current (A) is NOT
proportional to the p.d (V) so the thermistor is described as a
non-ohmic conductor
(doesn't obey Ohm's Law!).
The
passage of current heats up the filament and the rise in
temperature causes the resistance to increase.
As the current increases, more heat energy is released and the
filament gets hotter and hotter, so further increase in temperature
further increases the resistance.
This decreases the rate at which the current increases with
increase in potential difference.
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LDRs - light dependent resistors
Circuit 44 shows how you can
investigate the resistance of an LDR in varying light conditions.
The voltmeter is wired in parallel with
the LDR, the p.d. V is measured in volts (V).
The variable resistor allows you to vary
the p.d. and current flow.
The ammeter, wired in series, gives you
the current I reading in amps (A).
You must decide on the initial p.d. and
see how the current varies.
You calculate the resistance of the
thermistor from Ohm's Law equation: V = IR, so R = V/I
Somehow you need to vary the light
intensity shining on the LDR resistor e.g. using a lamp working of variable
resistor and taking a reading with a light meter as well as the p.d. and
current readings.
Since
an LDR is a light dependent resistor and
you should find ....
The higher the light intensity, the
lower an LDR's resistance, the greater the current flow for a fixed p.d..
i.e. high resistance in darkness and low
resistance in bright light.
You can see this trend clearly in the resistance -
temperature graph for an LDR.
Therefore an LDR can respond to changes in light intensity
e.g. daylight/night time.

At constant temperature and
constant light intensity, the current voltage graph for an LDR is linear,
the same as for a fixed resistor (left graph 1), so it is an ohmic
resistor at constant temperature.
Since the circuit system will sense the
presence of light, that is the basis of a thermistor's applications.
LDR resistors are used automatic
control of lights at night - outdoor lighting, burglar alarm circuits, light
intensity meters.
Circuit
33 shows in principle how to control the output of a lamp bulb.
(real LDR circuits are more
complicated)
In this case the 'active' component, the
bulb, is wired in parallel with the LDR response resistor.
In this case the p.d. across the LDR and
the lamp bulb is the same, though the LDR is a variable resistor.
In dim light or darkness, the p.d. across
the LDR and bulb is high because LDR's resistance is high.
The greater the p.d. across the lamp the
greater the power output (P = IV), so the bulb lights up - glows more brightly as the
surroundings get darker.
If the surroundings e.g. a room or a
garden path gets brighter, the LDR's resistance decreases, the p.d.
decreases, so the power output decreases and the lamp glows dimmer or 'goes
out'.
As well as automatic night lights, an LDR
can be used in a burglar alarm circuit.
A small and constant beam of light is
shone on an LDR (it can be from an invisible infrared emitting LED). If
the shadow (of the burglar) crosses the light beam, the intensity of
light falling on the LDR is reduced. Therefore the resistance of the LDR
is reduced and this triggers an alarm.
A simple light meter can be made by
connecting an LDR in series with a battery and an ammeter.
The brighter the light, the lower the
resistance of the LDR.
The lower the resistance, the greater
the current flow, so the ammeter reading is a measure of the light
intensity.
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The diode
See
also
an experimental investigation of I-V
characteristics of a diode
The
current through a diode flows in one direction only
This is because a diode has a very high resistance in
the reverse direction.
A diode is a special device made from a
semiconducting material based on silicon (classed as a semi-conductor).
Since the current only flows one way
through a diode, it can be used to convert an ac current into a dc
current.
With alternating current (ac), the
current changes direction in a cycle, but with direct current (dc) there is
no reversal in current direction, it flows one way with a constant voltage.
Oscilloscope traces comparing ac and dc
current signals - showing the alternating + <=> - oscillation of the
alternating current p.d. and the constant p.d. of a direct current.
Diagram 2. shows what happens if
pass an ac current through a diode - 'before and after' trace after
the dc output has been smoothed.
In some devices in the home the output from e.g. the transformer in your computer
power supply, is rectified to convert it from ac to a dc supply.
Diodes are used as
rectifiers, signal limiters, voltage regulators, switches, signal
modulators, signal mixers, signal demodulators, and oscillators etc. etc. in
other words - rather useful in electronic circuits.
Diodes are used radio transmitters and
receivers.
LED light emitting diode
An LED emits light when a current flows through it in the forward direction.
You should know that there is an increasing use of LEDs for lighting, as they use a much smaller current than other forms of lighting.
They have a much greater efficiency in
converting electrical energy into visible light.
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What next?
Electricity and
magnetism revision
notes index
1.
Usefulness of electricity, safety, energy transfer, cost & power calculations, P = IV = I2R,
E = Pt, E=IVt
2.
Electrical circuits and how to draw them, circuit symbols, parallel
circuits, series circuits explained
3. Ohm's Law, experimental investigations of
resistance, I-V graphs, calculations V = IR, Q = It, E = QV
4. Circuit devices and how are they used? (e.g.
thermistor and LDR), relevant graphs gcse physics revision
5. More on series and parallel circuits,
circuit diagrams, measurements and calculations
gcse physics
6. The 'National Grid' power supply, environmental
issues, use of transformers
gcse
physics revision notes
7.
Comparison of methods of generating electricity
gcse
physics revision notes (energy 6)
8. Static electricity and electric fields, uses
and dangers of static electricity gcse
physics revision notes
9.
Magnetism
- magnetic materials - temporary (induced) and permanent magnets - uses
10.
Electromagnetism, solenoid coils, uses of electromagnets gcse
physics revision notes
11. Motor effect of an electric current,
electric motor, loudspeaker, Fleming's left-hand rule, F = BIL
12.
Generator effect, applications e.g. generators
generating electricity and microphone
gcse
physics
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