180-DEGREE THREE PHASE INVERTER WITH SIMULATION

The inverters are used to convert DC power into AC power. There are several types of inverter available. Click on below link to know more about different types of inverter.

https://circuitdigest.com/tutorial/different-types-of-inverters

According to the mode of operation, three-phase inverters are classified into two types; 180-degree mode and 120-degree mode. Before going to classification, revise the circuit diagram of a three-phase inverter and it is as shown in below figure.

Circuit diagram of three-phase inverter:

As we can see in the circuit diagram, three phase star connected load is connected and output is measured at the load. One arm for each phase means we have a total of three arms. For switching purpose, we can use MOSFET or IGBT or any other power electronics switches according to the application.

When the upper switch of any phase is in conduction mode (ON), at that time positive half cycle of that phase appears in the output. When the lower switch of any phase is in conduction mode (ON), at that time negative half cycle of that phase appears in the output. In one arm two switches are available and these two switches never conduct at the same time. If this happens than DC source will short-circuit.

In the 180-degree mode of operation of the three-phase inverter, all switches conduct for 180-degree. In other words, MOSFET will ON for 180-degree and OFF for 180-degree of one cycle and the pulse width is 50% of the period.

Simulation in MATLAB/SIMULINK:

This is the simulation of three-phase inverter. The circuit diagram of 180-degree and 120-degree is same. The only difference is in the GATE pulse of switches. Here in this blog, we will discuss 180-degree inverter.

The gate pulse of 1 & 4, 3 & 6 and 5 & 2 are totally opposite in this mode of operation. When MOSFET-1 is ON at that same time MOSFET-4 is OFF.

Switching of MOSFET:

How to generate this gate pulse?

The screenshot of MATLAB for gate pulse generator is below image.

Gate pulse generator:

Vab = Va0 – Vb0

Vbc = Vb0 – Vc0

Vca = Vc0 – Va0

The peak value of phase voltage (Va0) is 2Vs/3 and the peak value of line voltage is Vs. where Vs is the DC supply voltage. The waveform of the 180 degrees is as shown in below figure. This is a book image. We will compare this image with simulation results at the end of this blog.

Waveform of 180-degree inverter:

How to set pulse width in the pulse generator. You have to create gate pulse only for MOSFET 1,3 and 5 and toggle it with the help of NOT gate and give it to MOSFET 6, 2 and 4. Now, the question is how to generate gate pulse for 1, 3 and 5. For that, we will use the PULSE GENERATOR. For 50Hz output signal, the time period is 20 msec and amplitude is 1 for all pulse generator. For 180-degree inverter mode, the pulse width is 50%.

Phase delay for one signal is 0 sec and this signal is for phase-A and MOSFET-1. It is clear in the above table that, there is 120-degree phase delay between these two signals. So, phase delay for phase B and MOSFET-3 is 120-degree or (2e-3)*2/3. Now for the third signal, the phase delay is 60-degree or (2e-3)*1/6 and this signal is for MOSFET-5.

By this way, we have three signals. For the other three signals we will use NOT gate and inverse this signals as shown in the above screenshot.                                      

Output waveform:

from above figure we can see that, for DC supply of Vs = 100V, the peak phase voltage Va0 is (2*100)/3=66.6V and the peak phase voltage Vab = 100V. This result is same as book waveform. If you need this simulation you can contact me or comment in this blog.

Thanks for reading.

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CLOSE LOOP INVERTER

Close loop inverter:

What is the difference between open loop and closed loop inverter?

* In Open loop inverter, the output is varying with respect to the input. Suppose, the source of the inverter is not the constant source and input of the inverter is not constant, its varying with respect to time. In this condition, if you have used an open loop inverter than the output of the inverter is also varying. This varying supply can damage the equipment. So, we need a constant output. In another word, the output voltage of inverter must remain constant when supply voltage varies.

* In close loop inverter, a controller is used to control the output voltage. In open loop inverter, the output voltage is not controlled. So, no need to use a controller. But in case of closed loop inverter, we need a controller which can control the output voltage at our reference voltage.

* Cost of open loop inverter is less compared to close loop inverter.

* Open loop inverter is easy to design. But close loop inverter is very difficult to design.

* Close loop inverter is used more in real life application.

How to make a closed loop inverter?

To make a closed loop inverter, we need an open loop inverter. Now we will apply control strategy to this inverter.

1) measure output voltage (Vact) of the inverter.

2) compare Vact to a reference voltage (Vref).

3) difference between output voltage and the reference voltage is called an error and this error is given to the controller (PI, PID)

4) the output of the controller is the modulation index.

5) The modulation index changes with respect to actual output. This modulation index is multiplied with sine wave signal.

6) this sine wave signal is compared with a triangular signal and according to that gate pulse for IGBT is generated. This method is known as the SPWM technique to control the output.

If you need this simulation, comment your mail ID.

For simulation of closed loop inverter:

https://youtu.be/oAAlrNxoGmE

For simulation of open loop inverter:

https://youtu.be/YOrpGGn4mbY

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SINGLE PHASE INVERTER WITH SIMULATION (MATLAB)

Single phase inverter:

Single phase inverters are used in low power applications like home, office etc. This type of inverter is not use in industries to run heavy load.

There are two types of supplies from utility company; one is single phase supply and other is three phase supply. This type of inverters is use where single-phase utility supply is available. The combination of inverter and battery are used for backup power. In solar power plants, solar energy is generated by PV panels. This power is unregulated DC power. So, this power is not useful, we cannot directly use this power for our home appliances because, this equipment is made for AC supply. So, it requires pure AC supply which is directly not available from PV panels. In this case, the inverters are plays an important role and it is used to convert unregulated DC power in to regulated AC power. This is the main function of inverter.

Operation of single phase inverter:

                In single phase inverter, four IGBTs are used for switching. For upper half cycle of sine wave two IGBT (1&4) will operate and for lower half cycle remaining two IGBT (2&3) will operate. By this way, we can get AC waveform across the load. Now, we need gate pulse to ON and OFF IGBT. As we seen, IGBT 1 & 4 operate at same time. So, same gate pulse is required for this both IGBT. Similarly, IGBT 2 & 3 operate at same time. IGBT 1 & 4 operates simultaneously, both switches cannot operate at same time. So, pain 1-4 and 2-3 has exactly opposite gate pulse. This is how we can get square wave AC output. But our appliances are not suitable with this supply. So, we need to convert this square wave output into the pure sine wave output. For that we will use LC filter or LLC filter.

  

Simulation of single-phase inverter in MATLAB:

Open loop single-phase inverter:

                In MATLAB, IGBT/diode is used for switching purpose. The pulse generator is used to generate gate pulse for all IGBT. [A] is gate pulse for IGBT 1-4. [B] is gate pulse for IGBT 2-3. Connect all components as shown in below picture and for more info watch video.

circuit diagram (simulation in MATLAB):

 Waveform without filter:

Waveform with filter:

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SINGLE PHASE OPEN LOOP INVERTER

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INTRODUCTION OF FACTS DEVICES

FACTS (Flexible AC Transmission System)

Introduction:

FACTS devices are used in AC transmission system. These devices are used to improve the stability and controllability of the transmission network. The FACTS is a static device, which is based on the power electronics. The power transfer capacity is fixed for any transmission network. If we transfer power beyond this limit, equipment may get damage. We can increase power transfer capacity by changing the layout of the power system but this is not a feasible way. So, in the actual power system, we use FACTS devices to increase power transfer capacity.

In simple word, FACTS is a variable impedance device like inductors and capacitors. In an ideal transmission system, the active power is equal to the apparent power. It means that the Power factor is unity. The power factor is defined as the ratio of the active power to the apparent power. This is the main role of FACTS devices, it is trying to keep power factor as near as possible to unity.

The active power is known as true power and it is useful for work in the system. The reactive power is not useful for the load. The reactive power is caused by the energy storage equipment. If we want to achieve the maximum power transfer capacity, we have to reduce the amount of reactive power. The reactive power reduces the power transfer capacity. The techniques used to reduce reactive power is known as the compensation technique.

In order to improve the efficiency of the power system, the compensation technique is supplying the inductive and reactive power to the power system. By these techniques, we can improve the quality of power, stability, voltage regulation, power factor.

Classification:

According to the type of connection; series compensation, shunt compensation.

Series compensation:

This type of compensation technique, FACTS devices are connected in series with the power system network. By series compensation, we can improve the stability and dynamic response of the system. Generally, the capacitors are connected in series with the transmission network. The amount of compensation is controlled by power electronics switches. For better control, more than one capacitor (capacitor bank) is connected.

                Example of series compensation:

                                Thyristor Controlled Switched Capacitor (TCSC)

                                Static Synchronous Series Compensator (SSSC)

                                Fixed Series Capacitor (FSC)

                                Thyristor Switched Series Capacitor (TSSC)

                                Thyristor Switched Series Reactor (TSSR)

Shunt compensation:

in this type of compensation technique, FACTS devices are connected in parallel or shunt with the transmission network. In this technique, the current is injected into the system from the FACTS devices and by this way the reactive component is reduced. This technique is used to improve voltage regulation and reduce losses of the power system. There are two types of shunt compensation; shunt capacitive compensation and shunt inductive compensation. The shunt capacitive compensation technique is used to improve the power factor. The shunt inductive compensation technique is used to charging the transmission line in low load condition or no-load condition.

                Example of Shunt compensation:

                                STATic Synchronous COMpensator (STATCOM)

                                Static VAR Compensator (SVC)

                                                Thyristor Controlled Reactor (TCR)

                                                Thyristor Switched Reactor (TSR)

                                                Thyristor Switched Capacitor (TSC)

In the next article, we will discuss briefly on all types of compensation device.

Please review this article, at the bottom of this page.                                      

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INVERTER

What is the function of Inverter?

There are two types of electrical energy; AC power and DC power. In our regular life, generally we use AC power for our household equipment and in office also we need AC power. But we cannot store electrical energy in the form of AC. We can store only DC power. For that, we use a battery. The batteries are run on very low voltage. The battery cannot be used to run household appliances. Power generated from a diesel generator or power supplied by the utility is AC power and most of all appliances are also designed to run on AC power.

Now you think what is the function of the inverter in office or home. The inverter is used to supply back up power when utility fails to supply power or during maintenance. The inverter takes DC power from the battery and converts it into AC power. This is the main function of an inverter.

Types of inverter:

(A) According to the output characteristic (shape of AC wave they produce), inverters are classified into three types.

                1) The square wave inverter

                2) The modified sine wave inverter or quasi-sine wave inverter

                3) The pure sine wave inverter

        The output waveform of this inverter is a square wave. This type of inverter is not used because the shape of the output waveform is not suitable for most running appliances. Construction of square wave inverter is easy and cheap in cost. but square wave inverters are obsolete from the market because of fewer applications.

        In this type of inverter, the output waveform is not a pure sine wave and not a square wave. The shape of the output power is similar to the staircase. This shape is useful for appliances like television, radio etc. but not useful for sensitive devices like a battery charger, variable speed motor and heaters. In this waveform some amount of noise is present. It is cheap compared to pure sine wave inverter. So, it depends on the application, if you require inverter for selected household application only then go for this inverter otherwise go for pure sine wave inverter.

        This is the best inverter for all type of applications and efficient for all nature of the load. Generally, if cost is not considerable than always use pure sine wave inverter. The shape of output power is a pure sine wave. This shape is the same as utility power. Filters are used to getting a pure sine wave. The pure sine wave inverters are the costliest inverter compared to all another inverter.

(B) The output of the inverter is AC power and there are two types of AC power; single phase and three phase. According to output power inverter can classify into two types;

                1) single phase inverter

                2) three phase inverter

        This is depending on the utility connection. For a small load, single phase connection is provided and for large load three phase connection is provided. Generally, for domestic load single phase connection is used and for industrial load three phase connection is used.

(C) According to a source of input;

                1) current source inverter (CSI)

                2) voltage source inverter (VSI)

        In CSI, the input is a current source. This type of inverters is widely used in the medium voltage industrial application, where high-quality current waveform are compulsory. But CSIs are not popular.

        In VSI, the input is a voltage source. This type of inverter is widely used in all applications because it is more efficient and have higher reliability and faster dynamic response. VSI is capable of running motors without de-rating.

Applications:

1) An uninterrupted power supply (UPS) supplies AC power with the help of inverter and battery combination.

2) Speed control of electric motor: An inverter is designed to produce a variable output voltage and by controlling the output voltage of the inverter, we can control the speed of an electric motor.

3) In the compressor of refrigerator and air-conditioner: In the simple inverter, single speed compressor switches on and off periodically. But in inverter compressor, VFD (variable frequency drive) is used to control the speed of compressor motor and according to this control of cooling is done.

4) Solar: Output of solar PV panels is DC. So, we have to convert it to AC for that inverters are used.

5) HVDC transmission line: For bulk power transmission, HVDC transmission line has more advantages than HVAC transmission line. In this transmission network, power is generated in AC and convert in DC with the help of rectifier and transmit this DC power for long distance. At receiving end power is converted into AC with the help inverter.

Simulation of simple three-phase inverter:

https://articleelectrical.blogspot.com/2018/11/simulation-of-three-phase-inverter.html

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SIMULATION OF SIMPLE THREE PHASE INVERTER

SIMPLE THREE PHASE INVERTER:


FIND BLOCKS FOR BELOW LISTED COMPONENTS FROM SIMULINK LIBRARY,


COMPONENTS:

1) POWERGUI

2) IGBT OR MOSFET -6

3) PULSE GENERATOR -6

4) DC VOLTAGE SOURCE

5) THREE PHASE Y RESISTIVE LOAD

6) THREE PHASE V-I MEASUREMENT

7) SCOPE


CONNECT ALL COMPONENT AS SHOWN IN VIDEO.


DC VOLTAGE SOURCE: 200V

RESISTIVE LOAD: 50 OHM

PULSE GENERATOR:

Period    : 2E-3 MEANS 20mS FOR 50Hz SIGNAL.

Pulse Width: 50%

Phase delay: FOR IGBT-1: 0

FOR IGBT-2: 2e-3*(60/360)

FOR IGBT-3: 2e-3*(120/360)

FOR IGBT-4: 2e-3*(180/360)

FOR IGBT-5: 2e-3*(240/360)

FOR IGBT-6: 2e-3*(300/360)


HERE WE CAM SEE THAT TIME REQUIRED FOR ONE CYCLE IS 20E-3 MEANS 20 mS AND THIS IS FOR 50Hz SIGNAL.

SO HERE WE HAVE 50Hz POWER OUTPUT.

BUT WE THERE ARE SOME PROBLEMS.


PROBLEMS:

1) YOU CAN NOT USE OUTPUT OF THIS INVERTER DIRECTLY BECAUSE

THIS IS NOT PURE SINE WAVE OUTPUT. 

FOR SINE WAVE OUTPUT WE REQUIRE A FILTER, WE WILL DESIGN

A FILTER IN NEXT VIDEO.


2) IN THIS INVERTER, OUTPUT IS FIXED. IF YOU WANT VARIABLE 

OUTPUT, THEN YOU HAVE TO USE CONTROL CIRCUIT.



WATCH UPCOMING VIDEO TO SOLVE THIS PROBLEMS.

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DC DC BUCK BOOST CONVERTER

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AC to DC converter in MATLAB/SIMULINK

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Basic of PCB -2

In previous article, we learn about basics of PCB. 
https://articleelectrical.blogspot.in/2017/12/basics-of-pcb-1.html

In this article, we will see different parts of PCB, material used in PCB and different PCB designing software.

Different parts of PCB

Pad: The pad is a piece of copper. Lead of components are mounted on pad and on which soldering are done. Pad provides the mechanical support to the components.

Silk layer: The silk layer is used for printing line, text or any art on the surface of PCB. It can be used in top and/or bottom layer of PCB. Which is known as silk screen TOP and silk screen BOTTOM. The epoxy ink is used for screen printing. 

Layers: User can choose the layer of PCB according to application. But for choose number of layer user have keep in mind the cost and available space of circuit. The single layer PCB is easy to design and used in routine life and construction of this PCB is simple compare to double layer and multi layer PCB. The double layer PCB or Multi-layer PCB is most prefer compared to single layer PCB cost and available space of circuit for large and complicated circuit. In multi-layer PCB 10-12 layer can be connected. In multi layer PCB most critical thing is to make communicatation between the component in different layer.

Top and bottom layer: All components are mounted in this part of PCB. This layer is green or blue coloured, but it is not a rule you can choose any colour. 
PCB is coated with green colour layer in top and/or bottom layer. This layer is known as solder mask. All components are soldered through the hole and lead of components is known as bottom layer of PCB. 

Trace: The components are not connected with the help of wires in PCB. All components are connected with a conducting material like copper. This copper part of PCB which is used to connect all components that is known as trance.

PCB materials: 
The main material of PCB is dielectric substrate. The dielectric substance is rigid or flexible and this is used according to the application. This dielectric substrate is used with conducting material like copper. The glass epoxy laminates or composite materials are used as dielectric material. 

FR4: FR is stand for FIRE RETARDENT. For all type of PCB manufacturing, most common glass laminated material is FR4. The FR4 is a composite material based on woven glass-epoxy compounds. FR4 is most used dielectric substance because it provides very good mechanical strength.

FR-1 and FR-2: FR1 and FR2 is made up from the paper and phenol compounds. This material is used for only single layer PCB. FR1 and FR2 has similar characteristic. The glass transition temperature is different for both dielectric substance. FR1 has higher glass transition temperature compared to FR2. These material is sub divided in standard, halogen free and non-hydrophobic.

CEM-1: CEM1 is made up from one layer of the paper and two layer of the woven glass epoxy and phenol compounds. CEM-1 can used instead of FR4, but price of CEM1 is higher than FR4. This material is used for Single sided PCB only. 

CEM-3: CEM3 material is white coloured and glass epoxy compound material. This material is mostly used in double layer PCB. CEM-3 has lower mechanical strength compared to FR4. But CEM3 is cheaper than FR4. CEM3 is a good alternative of FR4.

Polyimide: The polyimide is a flexible dielectric substance. So, this material is used in flexible PCB. The polyimide is made up from kepton, rogers and dupont. This material has good electrical properties, felicity, wide range of temperature and high chemical resistance. Working temperature of this material is -200 ͦC to 300 ͦC.

Prepreg: Prepreg is stands for pre-impregnated. Prepreg is a Fibre glass impregnated with resin. These resins are pre-dried. This resin flows, sticks and completely immersed when hetated. The Prepreg has adhesive layer. This adhesive layer gives strength similar to FR4. According to resin content, there are many versions of this material, SR- standard resin, MR- medium resin and HR- high resin. This is chosen according to required thickness, layer structure and impedance. This material also available in high glass transition temperature and halogen free.

PCB designing software:

EAGLE: EAGLE is a most popular and easiest way to design PCB. EAGLE stands for "Easily Applicable Graphical Layout Editor". This software is previously developed by CadSoft Computer and currently Autodesk is developer of this software. EAGLE provides a schematic editor for designing a circuit diagram. 
EAGLE file extension is .SCH
Different parts and components are define in .LBR extension. (Library) 
Board file extension is .BRD

MULTISIM: Multisim is also very powerful and easy learning software. This software is widely used in academic and also in industry for circuit education. This software is originally developed by "Electronics Workbench". Presently this software is a division of "National Instruments (NI)". 

EasyEDA: EasyEDA used to design and simulate circuits. Most important advantage of this software is that, it is web based software and used in browser window. So, this software is independent from OS. This software is an integrated tool for schematic capture. This software provides environment for SPICE circuit simulation and PCB layout. 

Altium Designer: Altium designer is developed by Australian software company "Altium Limited". This is first software which offer 3D visualisation and clearance checking of PCB directly from PCB editor. The main feature of this software is schematic capture, 3D PCB design.

KiCad: KiCAD is developed by "jean-pierre charras". This software has tools to create BoM (Bill of Material), artwork and 3D view of PCB. Many components are available in the library of this software and there is feature that user can add their custom components. This software is support many human languages.

CircuitMaker: Circuitmaker is also developed by "Altium". This software is used to design advanced multichannel and hierarchical schematics. Schematic editor of this software includes basic component placement. All schematic is uploaded to server. These files are available to view by anyone, provided that you need a CircuitMaker account. 

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Types of switches - Electrical switch

In previous article we discussed about mechanical switch:

https://articleelectrical.blogspot.in/2017/12/types-of-switches.html

In this article we will discuss about different types of electrical switch:
Electrical Switches:

The electrical switches is a semiconductor device. These switches are useful compare to mechanical switches because of their low cost, small size and reliability. In this switch, semiconductor materials is used, i.e silicon (Si), germanium (Ge). The electrical switch is used in integrated circuits (ICs), automation, electrical motor drives, HVAC application and also widely used as digital output (DI) of controller. 

               1) Relay

               2) Bipolar transistor

               3) Power diode

               4) MOSFET

               5) IGBT

               6) SCR

               7) TRIAC

               8) DIAC

               9) GTO

1) Relay: The relay is works on the principle of electromechanical, and because of this reason relay is also known as electromechanical switch. When current pass through a coil of relay, magnetic field is induced around the coil. The amount of magnetic field depends on the amount of current passes through the coil. Arrangement of the contacts is done such a way that, if current is increased with curtain limit contacts are energised and change its position. Sometimes, relay uses bi-metallic strip to sense the temperature for safety purpose. Relay are available in wide range of voltage and current. The relay plays an important role in protection against fault in power system network. In industries also, relays are used as a protecting device.

2) Bipolar Transistor: The bipolar transistor has three terminals or lags which is known as base, emitter and collector. The transistor is works on three regions; cut-off region, saturation region and active region. Symbol of the transistor is as shown in below figure. For the switching purpose, active region of the transistor is not used. The transistor enter into saturation region if sufficient amount of current is available at the base terminal. In this region, the current will flow through collector-emitter path and transistor act as a ON switch. If the base current is not sufficient, the circuit will remains open and current cannot flow through the collector-emitter and transistor enters in to cut-off region. The transistor act as OFF switch in cut-off region. The transistors are used as an amplifier in electronics application and it also used to make a gate like AND, OR, NOT in digital circuits and the transistor is also used as a switching device in integrated circuit (ICs). The transistors are not useful in high power application because it has more resistive loss compared to MOSFET.

3) Power diode: The power diode have two terminals which is known as an anode and cathode. The diode is made up of p (positive) and n (nagative) type of semiconductor material and make a pn-junction, which is known as diode. Symbol of power diode is as shown in below figure. When the diode is in forward bias current can flow through the circuit (switch is ON) and in reverse bias blocks current (switch is OFF). If anode voltage is positive with respect to cathode voltage, the diode is in forward bias and act as a ON switch. If he cathode voltage is positive with respect to anode voltage, the diode is in reverse bias and act as a OFF switch. The power diodes are used in power electronics application like, rectifier, voltage multiplier circuit and voltage clamper circuit, etc.

4) MOSFET: MOSFET is stands for Metal Oxide Semiconductor Field Effect Transistor. MOSFET has three lags which is known as gate, drain and source. MOSFET is works on two basic forms; Depletion type and Enhancement type. If gate-source voltage (VGS) is not sufficient, n this condition the MOSFET works in the depletion type. The depletion mode of MOSFET is similar to OFF switch. If gate-source voltage (VGS) is sufficient, MOSFET works as enhancement type and enhancement mode of MOSFET is similar to switch ON. 

5) IGBT: IGBT is stands for Insulated Gate Bipolar Transistor. The IGBT is a combination of BJT and MOSFET. IGBT has a low saturation voltage (characteristic of BJT) as well as  high input impedance and high switching speeds (characteristic of MOSFET) . IGBT has three terminals; Gate, Emitter and Collector. IGBT can control with the use of gate terminal. It can be switched ON and OFF by triggering and disabling its gate terminal. IGBT can block both +Ve and -Ve voltage same as GTO. IGBT is used in the inverter, traction motor control, induction heating and switched mode power supplies.

6) SCR: SCR stands for Silicon Controlled Rectifier. SCR has three terminals; Gate, Anode and Cathode. Working of SCR is same as power diode, The difference is only that SCR start conduction when it is in forward bias (cathode is -ve and anode is +ve) and positive clock pulse at the gate is also required. If clock pulse of gate is zero, SCR turned off by forced commutation in forward bias and in reverse bias SCR is remains in OFF state same as diode. SCRs are used in motor control, power regulators and lamp dimming.

7) TRIAC: TRIAC is same as two SCRs connected in inversely parallel with gate connected. TRIAC is bi-direction device. TRIAC has three lags; Main terminal 1 (MT), Main terminal 2 (MT2) and gate. MT1 and MT2 terminals are connected with circuit which we want to control and gate is available for triggering pulse by positive voltage or negative voltage. When MT2 terminal of TRIAC is at +ve voltage with respect to MT1 terminal and the gate is also +ve triggered, then SCR-1 of TRIAC triggers. Similarly, when MT1 terminal is at positive voltage with respect to MT2 terminal and gate is also +ve triggered, then SCR-2 of TRIAC triggers.

8) DIAC: DIAC is stands for Diode AC switch. The DIAC has two lags. DIAC can operate in both direction so, DIAC is bidirectional switch. The symbol of DIAC is as shown in below figure. DIAC works on two regions; forward blocking or reverse blocking region and avalanche breakover region. When applied voltage is less than breakover voltage DIAC works in forward blocking or reverse blocking region. In this region DIAC act as OFF switch. Avalanche breakdown occurs when applied voltage is greater than the break-over voltage and DIAC act as ON switch. DIAC cannot switch sharply for low voltage and low current application as compared to TRIAC and SCR. DIAC used in light dimming, control of universal motor and heat control circuit.

9) GTO: GTO is stands for Gate Turn-off Thyristor. The GTO has three lags which is known as Gate, Anode and Cathode. As name suggest, this device can turn OFF through gate terminal. In symbol of GTO consists of two arrows on the gate terminal, which shows the bidirectional flow of current through the gate terminal. By applying a small positive gate current GTO can turn ON and turn OFF by negative pulse from the gate terminal. GTO used in inverters, AC & DC Drives, induction heater and SVC (static VAR compensation). GTO cannot use for turning inductive loads off, without the help of the snubber circuit.

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