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Lecture 2
Diode Applications
Simple Diode Application: The Diode Switch
When the diode is forward biased it behaves like a closed switch, conducting current through the
circuit. When the diode is reverse biased it behaves like an open switch and disconnects the AC
supply from the load. This switching action permits only the positive half cycles of the AC input
voltage to appear across the load.
Simple Diode Application: The Rectifier
The AC input voltage to be rectified passes through the diode and then onto the resistive load.
During the positive e half cycle of the AC input the diode remains forward biased and conducts
current in the circuit. As a result the positive half cycle of the input voltage appears across RL. During
the negative half cycle of the input AC voltage the diode is reverse biased and does not conduct
current in the circuit. Therefore no voltage appears across RL. So the total result is an output of
positive half cycles of the AC input while the negative half cycles are suppressed.
Half-Wave Rectifier
The difference compared to the above circuit is that the AC input is supplied through a transformer.
This has two advantages. It allows us to step up or step down the AC input voltage according to the
requirement of the load. The transformer also isolates the rectifier circuit from the power line and
therefore reduces the risk or electric shock.
The AC voltage across the secondary wiring, AB, changes the polarity after every half cycle. During
the positive half cycle the diode is forward biased and the AC voltage appears across the load. During
the negative half cycle of the AC input the diode is reverse biased blocking any current from passing
through the load. Therefore the current through RL is always in the same direction, giving a DC
output across the load. The output across the load is pulsating DC and we can smooth this out using
a filter circuit.
The disadvantages of the half wave rectifier are:
 The pulsating current in the load contains an alternating component which has a basic
frequency equal to that of the supply. Therefore filtering is required to obtain a true DC
output.
 The AC supply delivers power only half the time. Therefore the efficiency of the circuit is low.
Full-Wave Rectifier
In the full-wave rectifier current flows through the load in the same direction for both positive and
negative half cycles of the AC input voltage. This can be achieved with two diodes working
alternately. There are two circuits that are commonly used for full-wave rectification:
1. Centre-tap full-wave rectification
2. Full-wave bridge rectifier
Centre-Tap Full-Wave Rectifier
There are two diodes D1 and D2, a centre tapped secondary winding AB and a resistive load RL.
During the positive half cycle of the secondary voltage, the end A of the secondary winding becomes
positive and the B end becomes negative. This makes the diode D1 forward biased and D2 reverse
biased. The current will flow through D1, RL and the upper half of the secondary winding. During the
negative half cycle, end A of the secondary winding becomes negative and end B becomes positive.
This makes the diode D2 forward biased and D1 reverse biased. The current will flow through D2, RL
and the lower half of the secondary winding. The current through RL is in the same direction during
both half cycles of the input voltage.
If Vm is the maximum voltage across half the secondary winding, at the instant that the secondary
voltage reaches its maximum value, diode D1 is conducting while D2 is non-conducting. Therefore the
whole secondary voltage appears across the non-conducting diode. As a result the peak inverse
voltage is twice the maximum voltage across the secondary winding.
PIV = 2Vm
The disadvantages of this setup are:
 It is difficult to locate the centre tap on the secondary winding.
 The DC output is small as each diode utilises only one half of the transformer secondary
voltage.
 The diodes used must have high peak inverse voltage.
Full-Wave Bridge Rectifier
The full-wave bridge rectifier contains four diodes D1, D2, D3 and D4. During the positive half cycle of
the secondary voltage the P end of the secondary winding becomes positive while the Q end is
negative. D1 and D3 are forward biased while D2 and D4 are reverse biased. Since D1 and D3 are
conducting, the current flows through these two diodes and RL. The two diodes are in series with
load and the current flow through the load is in the AB direction.
During the negative half cycle of the secondary voltage, end P becomes negative and Q becomes
positive. This makes D2 and D4 forward biased, which will be in series with RL. Current flows through
these two diodes and the load and the current through the load is in the AB direction. Therefore a
DC output is obtained through the load.
When the P end is positive D1 conducts and D2 doesn’t conduct. Therefore the endtire secondary
voltage appears across D2 in the reverse direction. Therefore PIV of each diode is equal to the
maximum secondary voltage.
Advantages of the full-wave bridge rectifier are:
 The need for a centre-tapped transformer is eliminated.
 The output is twice that of the centre tap circuit for the same secondary voltage.
 The PIV is half of that of the centre tap circuit.
Disadvantages of this setup are:
 Four diodes are required
 Since two diodes are utilised during each half cycle, the diode voltage drop and their internal
resistance will be twice as great as that of the centre tap circuit. This becomes a problem
when the secondary voltage is small.
Zener Diodes
When the reverse bias on a diode is increased a critical voltage called the breakdown voltage is
reached. At this voltage the reverse current increases sharply to a very high value. The breakdown
region is the knee of the reverse characteristic, and to honour C. Zener who explained this
phenomena for the first time in a satisfactory manner, the breakdown voltage is also referred to as
the zener voltage and this current is known as the zener current.
The breakdown or zener voltage depends on the amount of doping in the semiconductor material. If
the diode is heavily doped the depletion layer will be thin and therefore the breakdown of the
junction will occur at a lower reverse voltage. Similarly a lightly doped diode has a higher breakdown
voltage. A properly doped crystal diode which has a sharp breakdown voltage is known as a zener
diode.
The zener diode has the following characteristics.
 It is like an ordinary diode but is properly doped to have a sharp breakdown voltage.
 A zener diode is always reverse biased.
 A zener diode has a sharp breakdown voltage called the zener voltage, VZ.
 The zener diode is not immediately destroyed when it enters the breakdown region. As long
as the rest of the circuitry ensures that the diode current is below the ‘burn out’ value the
diode will remain operational.
As we increase the reverse voltage across a zener diode beyond the breakdown voltage VZ, the
current increase very sharply. This means that the voltage across the zener diode remains constant
at VZ though the current changes.
Zener Diode as a Voltage Regulator
A zener diode can be used as a voltage regulator to provide a constant voltage from a source whose
voltage may vary over a sufficient range.
In this circuit the zener diode of zener voltage VZ is reverse connected across the load RL across
which a constant output is needed. The series resistance R absorbs the output voltage fluctuations,
to maintain a constant voltage across the load. The zener diode maintains a constant voltage of V Z
across the load as long as the input voltage does not fall below VZ.
If the input voltage increases, since the zener is in the breakdown region, the output voltage will
remain constant at VZ. The excess voltage is dropped across the series resistance R, causing an
increase in the total current I. The zener will conduct the increase in current, while the load current
remains constant.
If the input voltage is constant and the load resistance decreases, there will be an increase in load
current. The extra current cannot come from the source since the drop in R will not change as the
zener is within the regulating range. The additional load current will come from decreasing the
current through the zener diode, IZ.
Light Emitting Diodes (LED)
An LED converts a forward current into light. The light emitted by the diode is proportional to the
forward current, therefore as the current increases the brightness of the LED increases.
LEDs are very popular in different applications such as various displays (even televisions), bulbs,
traffic lights etc. They can be made to produce light in different colours.
The symbol for the LED is:
Photodiode
If a reverse biased photodiode is exposed to incident light causes a current to flow across the pn
junction. This current is known as the photocurrent and is proportional to the intensity of the
incident light. These diodes can be used to convert light signals into electric signals.
The photodiode is an important component of a growing family of circuits known as optoelectronics
or photonics. Such circuits use electronics and optics for signal processing, storage, transmission.
Examples include fibre-optic transmission of telephone and television signals, optical storage in CDROM computer disks. Optical transmission provides very wide bandwidth and low signal attenuation.
Optical storage allows large amounts of data to be stored reliably in a small place.
The symbol for the photodiode is:
Optoisolators
Combining an LED with a photodiode in the same package results in an optoisolator. The LED
converts an electrical signal applied to the optoisolator into light, which the photodiode detects and
converts back to an electrical signal at the output. Optoislotaors provide electrical isolation between
the input and output circuitry connected to it. This can reduce electrical interference on signal
transmission within a system. These devices are also useful in medical equipment to prevent
electrical shocks to patients.