Bridge Rectifiers - An Overview

A Bridge Rectifier is a very popular and widely used circuit.  The Rectifier converts an AC supply into a DC supply.  The circuit requires only four diodes. 

The four diodes operate two at a time.  That is, during the positive half cycle, diodes D1 and D4 are in forward bias and conduct.   Diodes D2 and D3 are in reverse bias.

In the negative half cycle of the AC supply, diodes D2 and D3 are in conduction while D1 and D4 are in reverse bias. 

Thus the current from the rectifier flows in only one direction.

Bridge rectifiers are used in Alternators for excitation of the field.  They are also used in welding machines for DC welding and in battery Chargers

Bridge Rectifiers are also used in measurement circuits to measure the amplitude of an alternating signal. 

Controlled rectifiers contain thyristors instead of diodes.  This enables control of the output supply.

Class B Chopper

The Class B Chopper is typically used in applications which require transfer of power from the load to the source.  An example would be regenerative braking in trains, where the power from the driving motor is sent to the power mains.  This is also known as inverting operation.

In the Class B chopper, the output voltage is positive while the output current is negative.

In a class B chopper, a diode, in reverse bias,  blocks power from the source to the load.  The chopper is connected parallel to the load and the source.  The load voltage is the back-emf of the winding of a DC motor.  When the chopper is in the ON condition, the current due to the back-emf flows through the inductance and the resistance through the chopper.  The diode does not conduct as the voltage across the chopper is zero as the chopper is 'ON'. No Current flows into the source.

When the chopper is switched OFF, the voltage across the chopper increases and this biases the diode in the forward direction.  The diode conducts and the power reaches the source.  The source may be a battery or any other power source. 

How are Choppers Classified

There are wide range of choppers which are used for different applications.  These circuits differ in the voltage level, method of functioning and the output waveform.

Choppers can be classified into the following types

Step Up or Step Down Choppers

Step up Choppers, as the name suggests, step up the voltage.  These choppers are used when the voltage has to increased to a higher level.

AC and DC Choppers

Choppers can be classified into AC and DC choppers depending on the supply

Circuit Operation

On the basis of Circuit Operation, Choppers can be classified into

• First Quadrant
• Second Quadrant and
• Fourth Quadrant

On the Basis of Commutation

• Impulse Commutated Choppers
• Voltage Commutated Choppers
• Current Commutated Choppers
• Load Commutated Choppers

Depending on the Direction of Current

Class A

Class B

Class C

Class D and

Class E

In recent times, Choppers are usually classified based on their application such as switched mode power supplies, Class D Electronic Amplfiers, etc.

Choppers-An Overview

A Chopper is an electronic circuit which controls or reduces a dc supply.  Their function can be compared to an ac transformer.  In an AC transformer, voltage is controlled by changing the turns ratio of the transformer.  In a chopper, voltage is varied by connecting and disconnecting the load from the source many times in a second. 

The chopper is essentially a switching circuit which switches off and on many times.  The output of a chopper is a square wave form while the input is a unidirectional dc waveform. 

Choppers can be used in motor speed controls.  They are increasingly being used in electric automobile technology. 

Choppers are used widely in Electronics in circuits in solar power conversion, speed control of motors in the industry.  They are used to reduce DC voltage to different levels in machines and other electronic equipments

Choppers have high efficiency and can be designed to have very fine control.

Band Theory of Conduction

Electrical conduction in materials occurs due to the free electrons which drift about the atomic lattice.  In an atom, the electrons in the outer most orbit are called the valence electrons.  If the electrons have sufficient energy , they can break free of the atom and flow through the lattice when a voltage is applied.

If the energy levels are graphically represented, we will get a band diagram.

In the Band Diagram, there is the box representing the Conduction band and the box representing the valence band. 

Valence Band

The Valence band is the range of energy levels of the electrons in the outermost orbit of the atom. 

Conduction Band

The conduction band is the range of energy levels all electrons which are involved in conduction. 

In conductors, the valence and the conduction bands overlap.  In  Insulators, the valence and conduction bands are far apart. 

In semiconductors, the distance between the valance and the conduction bands are small.  When external energy in the form of heat or light is applied to the semiconductors, the electrons get excited and jump from the valence to the conduction band. 

The difference between the valence and the conduction band is called the energy gap.

Germanium in Electronics-An Overview

Germanium is used in specific applications such as communication, spectroscopy, etc.  They have largely been replaced with silicon.  Germanium diodes are more expensive compared to silicon.

Germanium diodes have a lower forward bias voltage compared to silicon 0.15 volts.  This enables the use of the Germanium diode at low voltages where silicon cannot be used.

Germanium is also used in photoelectronics application.  Germanium diodes have a smaller band gap  0.66 eV.  This means that the electrons can be excited even by near-infrared radiation. 

Germanium is used in solar cells to capture the energy in near-infrared regions of the light spectrum.  Germanium based sensors are used spectroscopy to detect light radiation at low frequencies.

Silicon Diodes-An Overview

Silicon diodes are diodes in which the P and N materials are made of silicon.  Silicon Diode have a a forward bias voltage of 0.7 volts.  That is, the diode conducts when the voltage across the it in the forward bias is 0.7 volts or greater. 

They are the most widely used diodes in the industry.  Other diodes such as Germanium diodes are used at voltages below 0.7 volts.

The diode can withstand a voltage of 50V or more in the reverse direction.  This is known as the peak inverse Voltage.