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. A controlled rectifier is also known as full converter.

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 Diodes (Uncontrolled Switch). This type of rectifier circuit offers one quadrant operation which means this type of circuit cannot be worked as an inverter like controlled rectifier.

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 2π, which means an Ac signal completes its one cycle in 2π times. So, how many pulses we get for this one complete cycle of AC input i.e. in 2π 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 2π.

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.