July 2017 – APSEEE

Month: July 2017

Methods of Triggering of Thyristor

Methods of Triggering of Thyristor

The method by which a thyristor is turned on is called triggering of Thyristor. In this process the thyristor brings in conducting state from non-conducting state. A thyristor can be turned on by any one of following techniques. In this article, we will discuss about different methods of triggering of thyristor or SCR.

READ – Construction and working of Silicon Control Rectifier  or Thyristor

  • Forward voltage triggering
  • Gate triggering
  • Thermal triggering
  • dv/dt triggering
  • Light triggering
  1. Forward Voltage Triggering

When a thyristor is forward biased and the forward voltage applied across the anode and cathode, the width of the depletion layer, starts decreasing. At a breakover voltage the depletion layer vanishes and thyristor starts conducts. The triggering of the device is caused by the cathode, that is why it is called forward voltage triggering methods.

  1. Gate Triggering

In this method of triggering, source (say battery) connected across gate terminal and cathode. This junction makes forward biased with respect to cathode. Small gate signal applied to a thyristor turned on it. Gate triggering is most common method of triggering. The firing angle of device can be controlled by varying the gate signal.

methods of triggering of thyristor gate triggering

The Gate triggering methods can be classified as.

  • DC gate triggering
  • AC gate triggering
  • Pulse Gate triggering or UJT triggering

AC triggering may be Resistance r triggering or resistance capacitance rc triggering of scr or thyristor.

Why gate triggering is preferred?

 In gate triggering, we can control the output of an SCR or thyristor by gate signal which is applied to the junction J2 that helps to make junction J2 forward biased and small gate signal  can be triggered the SCR or Thyristor that is why gate triggering is preferred.

  1. Thermal Triggering

In this method, heat energy is applied to the thyristor to trigger it. We known semiconductor materials have negative temperature coefficient of resistance. As the external heat energy is applied to thyristor the resistance of the device is caused by using heat, hence the name is thermal triggering method.

  1. dv/dt Triggering

In this method, the triggering of thyristor is caused by using high rate of rise of voltage is called dv/dt triggering.

  1. Light Triggering

In this method, light energy fall on the thyristor, which results in electron hole pairs are generated in the device. This increase the flow of current with in the device which in turn causes triggering light activated silicon controlled switch are the examples of used light triggering method.

 

{ Add a Comment }

Push-Pull Amplifier

Push-Pull Amplifier

The Push-Pull Amplifier commonly used in power amplifier. It is employed in the output stages of the circuits. It is used to get high output power at high efficiency. The audio power amplifiers used in transistor receivers, tape recorders etc. Distortion is greatly reduced by using push-pull operation employing two transistor in a single stage. There systems are usually operated by batteries.

READ – What is Transistor?

Circuit Arrangement

Two transistor Q1 and Q2 placed back employed. The emitter terminals of the two transistor Q1 and Q2 connected together. Both transistors are operated in class B operation. The input signal is applied to the input of two transistors through centre tapped step up transformer. The input transformer is called Driver transformer. Driver transformer supplies equal and opposite voltages to the base circuits of two transistors.

push-pull amplifier

The output transformer has the centre tapped primary winding. The load speaker is connected across the secondary of the output transformer.

Working or Operation

The primary of driver transformer connected to AC supply. The input signal appears across the secondary AB of the driver transformer. During first half of the input signal A becomes positive and end B negative. Here we have used two NPN transistors. This will make the base emitter junction of Q1 forward biased and that of Q2 forward biased. Now the current will conduct through Q1 while Q2 remains in non conducting state.

working of push-pull amplifier

The current conduction through transistor Q1 shown by solid arrow. The amplified signal appears in the upper half of the output transformer.

During Negative half cycle of the supply the terminal A becomes negative and B becomes positive. This will make the case emitter junction of Q1 reverse biased and that of Q2 forward biased. The transistor Q2 and transistor Q1 remains in non-conducting state.

operation of push-pull amplifier

The amplified signal appears in the lower half of the output transformer.

The centre-tapped primary of the output transformer combines two collector currents to form a sine wave output in the transformer.

Advantageous

  1. Less distortion for a given power output due absence of even harmonics.
  2. Elimination of dc component at output.

Disadvantageous

  1. Requirement of two transistors.
  2. Need of bulky and expensive transformer.

 

{ Add a Comment }

What is Triac?

What is TRIAC?

Triac belongs to thyristor family. The triac is the most widely used device in an alternating current applications. Basically, it is a combination of two SCRs connected in anti-parallel manner. Triac can conduct in both directions positive as well as negative of an ac supply. A triac is widely used in industries for control purposes. The triac can conduct alternating current. So, it is used for ac current applications.

Construction of Triac

The triac is a three terminal, four layer bidirectional semiconductor device. The name of three terminals is MT1, MT2 and Gate(G). The same is used for the construction of triac that is used used for thyristor.

Triac

Basically, a triac consists of two SCRs connected in anti-parallel connection with a common gate.

antiparallel connected scr triac

Symbol of Triac

symbol of triac

I-V Characteristics of Triac

As we discuss above the triac has three terminals. MT1 MT2 and Gate (G). The triac can trigger by two methods.

Firstly when input voltage level reaches at breakover voltage or exceeds the breakover voltage.

Secondly, Gate signal is applied to the triac.

The first quadrant characteristics are just like an SCR but on the other hand, the third quadrant characteristics of a triac are identical to its first quadrant. Only polarities of the main terminals (such as MT1 and MT2) change.

vi characteristics of triac

The Triac can be operated in both the half-cycle of an ac supply. The output of the triac can be controlled by with the help of gate current.

Applications of Triac

  1. It is used to control power in steps.
  2. It is used as a fan regulator.
  3. It is used to control the speed of ac motors. Triac can control the speed by adjusting the gate current.

The triac are available in different sizes and ratings. They have the capacity to handle current from 0.5A to 100A and voltages upto 2000V. The size of the device depends upon its current carrying capacity. BT 151 is an example of triac.

{ Add a Comment }

Conducting Materials

Conducting Materials

Conducting materials are those materials which offers easy path to the flow of electric current. It offers low resistance to the flow of current. Silver, Copper, Aluminium, iron are the example of conducting materials. Conducting materials are subdivided into low resistivity and high resistivity materials.

Low Resistivity Materials

There are the materials which offers low resistance to the flow of electric current through them. In order words, we can say that current can easily flow in these materials.

Low resistivity materials are used in house wiring, as conductors for power transmission and distribution of an electric power, in windings of electric machines like transformer, motors and generators. Basically, these materials are used where low power losses and voltage drops are required.

List of low resistivity materials are given below

  • Copper
  • Aluminium
  • Iron

Copper and Aluminium are used commercially because these materials have low resistivity and high conductivity. The conductivity of copper is higher than aluminium but due low cost of aluminium these are used in most of applications. Iron is used basically for manufacturing of earth wire or ground wire. The resistivity of iron is very high as comparatively to copper and aluminium.

Properties of Low Resistivity Materials

  1. It should have low temperature coefficient.
  2. The mechanical strength should be high.
  3. The material should be ductile which allow it to be drawn out into a wire.
  4. These materials should be such that it is not corroded when used in out-door services.

High Resistivity Materials

These materials are used for making resistance elements for heating devices filaments of lamps, rheostats etc. High resistivity materials are used in all such applications where a large value of resistance is required.

Tungsten, Nichrome etc are the examples of high resistivity materials.

Properties of High Resistivity Materials

The high resistivity materials should have following properties.

  1. These materials should have low temperature coefficient of resistance.
  2. These materials should have high meeting point. So it can withstand high temperature for a long time without melting.
  3. It should be able to withstand high temperature for a long time without oxidation.
  4. These materials should be ductile.
  5. High resistivity materials should have high tensile strength.

{ 1 Comment }

Electrical Earthing

Electrical Earthing

Earthing means connecting of the non current carrying parts of electrical device or equipment or the neutral point of the power system to the earth through a wire or conducting materials having very low resistance. An electrical earthing is very necessary for equipment and power system. It protects the electrical equipment and power system against short circuit, fault currents and lightning. In this article, we will deals with objectives of earthing and type of earthing.

Objectives of Earthing

  1. It provides the safety of the personnel against electric shock. It insure the safety of human being during insulation failure.
  2. It protects the device or electrical equipment against damaged which may cause due to flow of overload current during short circuit.
  3. It limits the over voltages from neutral to ground or line to ground.
  4. Earthing suspress dangerous earth potential gradients.

Types of Electrical Earthing

Earthing can be divided into two types

  • Neutral Earthing
  • Equipment Earthing

Neutral Earthing

Neutral earthing deals with the earthing of the system neutral to ensure system security and protection. Neutral earthing is also known as system earthing. There are various methods for neutral earthing. The name of these methods is given below.

  1. Isolated Earthing
  2. Resistance Earthing
  3. Reactance Earthing
  4. Solid Earthing

Equipment Earthing

Equipment earthing deals with earthing of non-current carrying parts of the equipment to ensure safety of personnel and protection against lightning. Equipment earthing is also known as safety earthing. The name of different types of earthing is given below.

  • Plate Earthing
  • Pipe Earthing

How earthing protects the personnel against electrical shocks?

In this example, we will explain about protection of personnel against electric shock.

In first case, a man touches an electrical motor. If short circuit occurs in a motor the short circuit current starts flowing through a human body.

without electrical earthing

In second case, an electrical machine is earthed. When a man touch with a motor. If short circuit occurs in motor the fault current flows through a earth wire instead of flowing through human body.

with electrical earthing

{ Add a Comment }

Interconnected System

Interconnected System

The connection of many generating stations such as hydro power plant, Thermal power plant, Nuclear power plant etc running in parallel is called interconnected system. The interconnected system may be two types (i) Integrated and (ii) Uni-integrated. An integrated interconnection results in maximum overall economy. But most of the system interconnections are unintegrated. An unintegrated interconnection the identity of individual not lost. There is no central control office like integrated interconnection system.

Advantages of Interconnection or Interconnected System

The interconnected system has many advantages. There are given below.

  1. It increases the service reliability. Maintenance and replacement of any equipment is required during its operation so if the system is interconnected it will increase the reliability. An electrical energy is generated from different source, such as water, coal a nuclear etc. It the system is interconnected then we will choose that plant where will have to pay less cost. During peak load we can receive or transmit energy from large capacity plant.
  2. Load growth necessitates additional transmission facilities. If the system is interconnected them we have a provision to add and replace transmission. It makes the system more reliable.
  3. Reserve capacity required is reduced with interconnected system we do not need reverse capacity of plant.
  4. Reduction in total installed capacity. An interconnection decrease the installed capacity needed to meet the load requirement. Different areas have different demands of electrical energy. In areas required less energy and at the same other area required more electrical energy. If two such areas are interconnected, the diversity of load would cause the maximum combined demand to be less than the some of the individual maximum demands. In this way, diversity factor improves.
  5. The interconnected system improves the efficiency of plants.
  6. Economical operation of station is ensured. We can choose that plant which has lower running cost in this way the system become economical.
  7. With system interconnection capital and maintenance cost is reduced.

{ Add a Comment }