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P6H
The diode
A diode is a device that allows current to
flow in one direction only. You should be
able recognise and draw the symbol for a
diode.
Almost all diodes are made of silicon.
Which direction?
The electric current flows through a diode
in the direction indicated by the triangle
in its circuit symbol.
When the diode is connected so that
current flows, it is described as forward
biased.
When the diode is connected the other
way round, so that current does not flow,
it is described as reverse biased.
The diode
A diode contains two different layers, a
positive p-type layer and a negative
n-type layer.
The p-type layer is missing electrons and
contains positive holes where electrons
would have been. The n-type layer
contains extra electrons. The deletion
layer where the two different layers are
joined together acts as an insulator.
This is what happens when the diode is
connected in a circuit:
 if the positive terminal is connected
to the n-type layer, holes and
electrons move away from the
depletion layer, making it wider and
preventing current from flowing
 if the positive terminal is connected
to the p-type layer, holes and
electrons move into the depletion
layer, making it disappear and
allowing current to flow
Refraction
AC and DC
AC is alternating current and DC is direct
current. These oscilloscope traces show
the difference between them.
Current-voltage graph
The graph shows how the current varies
with voltage for a silicon diode.
AC electricity is supplied by the mains
supply.
DC electricity is supplied by batteries and
solar cells.
Many devices need a DC supply rather
than an AC supply.
A rectifier changes AC into DC. The
process is called rectification and it uses
diodes.
Half-wave rectification
A single diode can produce half-wave
rectification. One half of the AC wave is
removed because it cannot pass through
the diode.
Full-wave rectification
Half-wave rectification is achieved using
one diode, but full-wave rectification
needs four diodes in a bridge circuit.
Note that in full-wave rectification current
passes all the time, rather than just every
half cycle.
Full-wave rectification
The diagrams below show the bridge
circuit needed for full-wave rectification.
The two diagrams show how the DC
(direct current) output is only in one
direction, even when the direction of the
AC (alternating current) input is reversed
in the second diagram.
You should remember that one diode can
produce half-wave rectification, and you
should be able to recognise half-wave
rectification from a voltage-time graph (as
seen in the last diagram).
Capacitors
A capacitor is an electrical device that
stores charge temporarily.
As a capacitor discharges, the voltage
across its plates falls, so the current in the
circuit falls too. The lower the resistance
of the circuit, the faster a capacitor will
discharge.
A simple capacitor can be made using two
parallel metal plates separated by an
insulating material, and this arrangement
is reflected in the circuit symbol for a
capacitor.
You should be able to recognise and draw
the circuit symbol for a capacitor.
Smoothing circuits
The DC (direct current) output from a
rectifier is not smooth – it varies a lot with
time. A capacitor with a high capacitance
can be used to smooth the output so that
it varies very little. This is important for
some electronic devices, such as radios.
Charging and discharging
A capacitor is charged by connecting it to
a DC (direct current) supply. As charge is
stored, the voltage across the capacitor
gradually increases to a maximum. The
maximum voltage is the same as the
voltage of the DC supply.
Smoothing circuits
As the voltage in the rectifier circuit
increases, the capacitor stores charge.
When the voltage begins to fall, the
capacitor begins to discharge, keeping the
DC more constant. When the voltage rises
again, the capacitor begins to store
charge again. This process repeats and
keeps the DC supply smoother.