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Transcript
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© Boardworks Ltd 2009
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What is a diode?
A diode is a component made
from a semiconducting material
such as silicon.
Diodes have the unique
property of only allowing
current to pass through them
in one direction.
diode and its symbol
The arrowhead on the symbol
shows the direction of current flow.
Some diodes emit light when they
conduct electricity. These are called
Light Emitting Diodes (LEDs).
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Investigating diodes
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V–I graph for a diode
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Conduction in metals
For materials to conduct electricity, they must be able to
transfer a charge when a potential difference is applied
across them.
metal ions: fixed positive charge
Metals conduct electricity due to
delocalized electrons. These
electrons are not bound to a
single atom, and act as a source
of mobile negative charge.
Despite this free charge, metals
have a net neutral charge. As free
electrons are released from the
metal atoms, the atoms are left
with a balancing positive charge. electrons: mobile negative charge
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Conduction in semiconductors
Some semiconductors also contain a pool of delocalized
electrons which allow current to flow.
However, semiconductors can also contain groups of
atoms with an unusually low number of electrons.
Such regions contain holes.
Holes are positively charged, and are able to flow through a
semiconductor, just like delocalized electrons. Similarly, their
formation leaves an opposing fixed charge behind.
Semiconducting materials can be artificially modified to
adjust their conductive properties, adding extra holes or
electrons to their structure.
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How diodes work
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Diodes summary
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Types of current
Current can be either:
Direct current (DC):
In a direct current the electrons flow
in one direction around a circuit.
The voltage remains constant,
producing a flat line on an oscilloscope.
Alternating current (AC):
In an alternating current the direction
of electron flow around a circuit is
continuously switching.
The voltage oscillates between negative
and positive, producing a waveform.
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From AC to DC
AC is used to carry electrical
energy around the country as:
 Its voltage is easy to change
using transformers.
 It requires a simple generator.
Most modern appliances in our home
now use microchips.
Most microchips require DC with a
near constant voltage to function.
This means we must convert AC from the mains, to DC in
order to use our appliances.
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Rectification
There are two major steps required to convert an AC signal
to a DC signal:
 The current must be made to flow in a constant direction.
 The variations in voltage must be removed.
Rectification is the process which makes an AC signal flow in
a constant direction.
The signal produced is imperfect DC, as its voltage still varies.
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Half wave rectification
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Full-wave rectification
By using a combination of four diodes, called a bridge
rectifier, an AC signal can be fully rectified.
input
output
This output signal always has a positive voltage, meaning
that, unlike half-wave rectification, no energy is lost.
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Bridge rectifier – how does it work?
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Rectification summary
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What is a capacitor?
A capacitor is a component that
stores charge.
capacitor
symbol
The amount of charge a capacitor is
able to store, or its capacitance, is
measured in farads (F) or
microfarads (µF).
Capacitors contain two metal plates
with an insulator. The larger the
plates, the larger the capacitance.
The charge stored on a capacitor’s
plates can be discharged into a circuit,
maintaining a current for a short time.
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How does a capacitor charge?
When a discharged capacitor is connected to a circuit, it will
begin to charge.
electrons forced
The high potential
onto plate
difference across the
circuit forces electrons
onto one plate, and pulls
them from the other.
electrons forced
from plate
This increases the
potential difference
across the capacitor.
The capacitor is fully charged when it has the same potential
difference as the battery.
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Charging a capacitor
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How does a capacitor discharge?
When a conductor is
connected across a charged
capacitor, it will discharge.
Electrons flow from the negative
plate to the positive plate.
Its high potential difference
forces electrons from one
plate to the other.
This decreases the potential
difference across the capacitor,
and causes a current to flow,
lighting the bulb.
The current lights the bulb.
The bulb will only light for a few seconds as the capacitor
does not store a great deal of charge.
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Discharging a capacitor
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Uses for capacitors
Capacitors are used in
defibrillators to store
electric charge.
Initially the capacitor is
charged from a power
source. There is no power
to the paddles.
Once the paddles are positioned on the patient’s chest, the
circuit between the paddles and capacitor is completed.
The capacitor can now release the charge in a very short
space of time, via the patient’s heart.
This results in a large current which can restart the heart.
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Capacitor summary
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Perfect DC
Rectification allows the production of a DC signal from an
AC input. However, the DC is imperfect, as it still has a
great level of variation in its voltage.
Microchips require a perfect DC supply of around 5 V.
A rectified signal is not good enough
to continuously run a chip in a circuit,
due to these voltage fluctuations.
In order to run most modern
devices, this fluctuation in voltage
must be removed.
This process is called smoothing.
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Smoothing
Smoothing turns the bumpy rectified signal into a smooth,
near-perfect direct current, which can be used by microchips.
A capacitor is used to smooth the signal.
9V
0V
input: rectified
output: smoothed
How do you think a capacitor does this?
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How do capacitors smooth signals?
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Converting AC to DC
The combination of a bridge rectifier and a capacitor can
convert a mains AC signal to a near-perfect DC signal.
load
The electricity entering this circuit will normally pass through
a transformer first. This will bring its voltage down to a more
appropriate level to run appliances containing microchips.
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Varying voltage
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Glossary
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Anagrams
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Multiple-choice quiz
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