What is Rectifier?
Rectifier is an electronic circuit that converts AC voltage into DC voltage. It is the most important circuit in electronics because almost every electronic circuit operates on DC power supply and the universal supply of electricity is AC in nature. So, AC to DC conversion is the fundamental need of almost every electronic circuit and appliances. Although, there are different DC power sources like batteries and DC generators present but these are limited and are not economical.
For the conversion of AC signal into DC signal, a rectifier circuit employs different types of electronic devices which have unidirectional current conduction property like Diode, SCR, MOSFET etc.
The reason behind using those
types of switches which have unidirectional current conduction property is the
one or constant directional nature of the DC signal.
As we know, an AC waveform is a
sinusoidal waveform that changes its magnitude and direction periodically and
its average value is zero. And a DC signal has constant magnitude and
direction, and in case of DC we deal with average value. So, for the conversion
of AC signal into DC signal, it is required that the circuit conducts only in
one direction so that a bidirectional AC waveform can efficiently convert into
a unidirectional waveform.
Therefore, we can also say that the
main function of a rectifier circuit is to convert a bidirectional waveform
like an AC signal, which has zero average value and constantly changing
magnitude into a unidirectional waveform, which has a non-zero average value
and constantly changing magnitude.
The process of conversion of a bidirectional waveform like AC voltage waveform into a unidirectional waveform like DC signal is known as rectification.
If we see the output waveform in
the above figure then we will find that it is pulsating in nature, means this
waveform changes its magnitude periodically but never changes its direction.
This pulsating waveform is considered nearly equal to DC because it has non
zero average value and constant direction like a DC.
Although this output waveform is
not as smooth as a pure DC signal and also it contains some harmonic content.
But the smoothness of this rectified DC output waveform can be improved
with the help of freewheeling the filter capacitance.
Types Of Rectifiers
Generally, rectifiers are
classified into two types i.e. Half Wave Rectifier and Full Wave Rectifier. This classification is based on the rectification of the AC
input wave.
Half Wave Rectifier
As the name implies “half wave rectifier”, this circuit converts / rectifies only positive half (or we can say only one half) of the AC supply into pulsating DC. For this conversion, the half wave rectifier circuit employs only one switch per phase.
In a half wave
rectifier circuit, the switch is connected in such a way that it conducts only
for the positive half of the AC input signal and does not conduct for the
negative half of the AC input.
Given figure shows
the generalized half wave rectifier circuit diagram. In this figure we see that
switch S1 is connected in series with load for half wave rectification. Here in
this circuit switch S1 can be any of the electronic devices which has
unidirectional current conduction property like Diode, MOSFET, SCR etc.
Full Wave Rectifier
Full wave rectifier
circuit converts a full cycle of AC input signal into DC Signal. For that
conversion this circuit employs one or more than one switch (Diode, SCR, MOSFET
etc) per phase. So that it efficiently converts both positive as well negative
half cycle of AC input waveform.
For full wave
rectification we use two types of configurations
- Full Wave Bridge Rectifier
- Full Wave Centre Tapped Rectifier
Centre Tapped Rectifier
A Center Tapped Full
Wave Rectifier is a type of full-wave rectifier that uses a center tapped
transformer and one switch per phase configured as given figure to convert AC
input voltage into DC voltage.
Above figure shows
the generalized Centre Tap Full Wave Rectifier Circuit diagram, in this figure
we see that switch S1 & S2 are connected with the terminal A & B of the
transformer secondary respectively in the same configuration. And the load is connected with the other
terminals of switch S1 & S2 combinedly and tapped to the center of the
transformer secondary winding. Due to this center tap connection of transformer’s
secondary winding and load this configuration is named as Center Tapped Full
Wave Rectifier.
In the given circuit
diagram, switch S1 conducts when the terminal A is positive with respect to B,
means for the positive half of AC input and switch S2 conducts when the
terminal B is positive with respect to A, means for the negative half of AC
input. In that way a full wave Centre-Tapped rectifier converts both positive
as well as negative half of the AC waveform.
Full Wave Bridge Rectifier
A Full Wave Bridge
Rectifier configuration uses four switches S1, S2, S3, and S4, configured in a
bridge-like arrangement to convert AC waveform to DC signal as shown in the
given figure.
In this type of rectifier circuit, two diagonally opposite switches S1 & S3 conduct for one half of ac input supply and the other two switches S2 & S4 conduct for negative half of AC input supply.
A Full wave bridge rectifier circuit configuration is the most preferably used rectifier circuit configuration among all the rectifier circuits because it has certain advantages over other rectifier circuits. Some of the advantages are listed below.
- The center-tapped rectifier uses a transformer with a center tap on the secondary winding. Due to the presence of this transformer the center tapped rectifier is bulkier and costlier.
- In case of bridge rectifiers two switches conduct for one half of Ac input and in case of centre tap rectifiers only one switch conducts for the one half of Ac input. That’s why switches used in bridge rectifier circuits have low PIV (Peak Inverse Voltage) as compared to the switch used in center tap rectifiers. And High PIV (Peak Inverse Voltage) switches are costlier as compared to low PIV switches so cost is reduced in bridge rectifiers.
Classification of rectifier on the basis of switch used
As we know that there are various Electronic and Power Electronic devices which we use in different electrical and electronic circuits. In these devices some are uncontrolled, some are fully controlled and some are semi controlled.
Uncontrolled switches
are those switches in which we cannot control neither on state nor off state
ex. Diode.
Controlled switches are those switches in which we can control both on state and off state ex. BJT, MOSFET, GTO, IGBT etc.
Half controlled switches are those switches in which we can control only one state either on state or off state example SCR & RCT.
Rectifier circuits
can be designed by using any of the above electronic devices or the combination
of above devices. With the help of the application of the right switch we can
get variable DC output Voltage.
So according to the
application of switches and controlling the output waveform, Rectifiers are
classified in three types.
- Un-Controlled Rectifier
- Controlled Rectifier
- Half-Controlled Rectifier
Un-Controlled Rectifier
As the name implies,
an uncontrolled rectifier converts AC input voltage into fixed DC voltage. An
Uncontrolled Rectifier cannot provide variable DC output Voltage. Only
uncontrolled switches like diode are used to design this type circuit. Ex:- Diode Bridge Rectifier
Controlled Rectifier
A Controlled Rectifier converts AC input voltage into variable DC voltage. A controlled
rectifier circuit employs full controlled switches or half-controlled switches
such as SCR, MOSFET etc.
In full converters we can get a variable DC output voltage. The DC output Voltage can be controlled by controlling the switch’s on state and off state with their respective parameters.
For example, in case of SCR rectifier circuits we can get the desired output voltage by varying the firing angle of the SCR.
A Controlled Rectifier Circuit can be worked as an inverter. This happens when a sufficiently high voltage DC Source is connected to the load terminal of the rectifier circuit and the controlling parameter of control switch adjust in such a way that the average output voltage of the rectifier circuit negative, in case of SCR Rectifier circuit we can achieve this by adjusting the firing angle greater than 90. (We discussed the working of the rectifier circuit as an inverter in detail in full wave controlled rectifiers).
Half-Controlled Rectifier
A half controlled
rectifier circuit employs the combination of both SCRs (half controlled switch)
and
A half controlled rectifier circuit is also known as semi-converter, and has certain advantages over Full Converter. That's why for some precise application semi converters are preferred over full converters.
The performance parameters of semi-converters are better than full converters.
The semi converter circuit required the combination of SCR and Diodes but a full converter needs either controlled switch or half controlled switches, which are expensive.
Semi Converters have better power factor then full converter.
Classification Of Rectifiers on The Basis of Pulse Number
Pulse numbers
indicate the number of output pulses for one complete cycle of AC input Supply.
As we know, the fundamental time period of an Ac signal is 2Ï€, which means an Ac signal completes its one cycle in 2Ï€ times. So, how many pulses we get for this one complete cycle of AC input i.e. in 2Ï€ time indicates the pulse number.
On the Basis of Pulse number rectifiers are classified as follows.
One Pulse Converter
Rectifiers which give
one output pulse for one complete cycle of input voltage are called one pulse
converters. Single Phase Half Wave Rectifier is the example for one pulse
converter. In given figure we can see the fundamental time period of the output
waveform is 2Ï€.
Two Pulse Converter
Rectifiers which give
two output pulses for one complete cycle of input voltage are called two pulse
converters ex. Single Phase Full Wave Controlled Rectifier and Single Phase
Semi-Converter. In these types of converters, the fundamental time period of the
output waveform is π.
So, in the discussion of the above two types of converters i.e. one pulse converter and two pulse converter we see that the fundamental time period of the output waveform of two pulse converter is less as compared to one pulse converter.
So, we can say that if the pulse number increases then the fundamental time period decreases. If the fundamental time period decreases then the frequency of pulse increases consequently smoothness of the waveform increases and also the ripple and harmonic content of the waveform decreases.
That’s why full wave rectifiers are most widely used in modern electronics over half wave rectifiers.
Similarly, three
phase full wave rectifiers are preferred over three phase half wave rectifiers.
Because in case of three phase full wave rectifier pulse number increases and
consequently smoothness of the waveform increases and also the ripple and harmonic content of the waveform decreases.
related posts
#_Rectifiers: Circuit Diagram and Working
#_SCR (Silicon Controlled Rectifier)
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