Working of SCR

SCR (Silicon Controlled Rectifier) :-

  • SCR  is three terminal device Anode ,Cathode ,Gate .


  • Anode and Cathode are connected in series with the power circuit and the Gate signal is applied from the cathode.
  • The silicon control rectifier is the oldest and most popular thyristor. It is an extremely reliable device and can be expected to deliver billions of operations .
  •  The SCR (Silicon Controlled Rectifier) is built in one PNP and one NPN transistor.The basic structure of SCR is below.


  • N1 layer is the thickest of all layer and it is very less doped.
  • N2 layer is very thinest of all layer an highly doped.
  • P1 & P2  layer are nearly same , thicker than N2 and less highly doped than N2 .
  • Due to above , the Junction  J3 has very low breakdown voltage in either direction.
  • So, J3 cannot support high voltage.

Working :-

  • If SCR  is in Forward Blocking mode then i.e. input to  anode is +ve and cathode is -ve then SCR  works consists of three diode – D1- (P1 N1) , D2 – (N1 P2) ,D3 – (P2 N2).
  • D1 will be Forward Biased, D2 will be Reverse Biased , D3 will be Forward Biased     So,depletion layer at Junction J2 will increased and which will block the SCR.
  • To trigger the SCR we need to decrease the depletion layer of J2 .This is done by applying the gate signal which will decrease the depletion layer of J2  and  trigger the SCR. That’s why gate terminal is kept near junction J2.

This is the normal mode of operation for of  thyristors.

  •              An external gate pulse or signal is used to switch ON  the SCR.
  • Gate signal used  only for starting of  SCR but does not have control to make SCR OFF.
  • That’s the reason for the use of the Gate terminal .

There are many more ways by which SCR can turn ON which are as follows:-

  1. By avalanche: When the anode is made much more positive then the cathode, Forward break over occurs and latch the device on.
  2. By rate of change: If the forward bias voltage across the device increases very quickly, a current will flow to charge the collector-base capacitance of the PNP transistor. This charging current represent base for the NPN transistor and turns it on
  3. By high temperature: Reversed-biased silicon junction show a leakage current that approximately doubles for every 8° C temperature rise.At some temperature, the leakage current will reach a level that latches the SCR ON.
  4. By light Energy: Light entering the junction area will release electron-hole pairs and letch the SCR on.

The Transistor structure of a SCR :- 



The equivalent two transistor structure is below:-


SCR volt-ampere characteristics curve:-



  •  Suppose Ig=0 and SCR  is F.B. therefore J1 &J3 are in F.B. and  J2 is in R.B. so,the entire voltage will is blocked by  J2 .
  • If applied voltage is greater then junction voltage of J2 (VBO) then device goes into conduction mode .
  • From 0 to VBO  the current flowing is very small only leakage current  and once device is in conduction mode then the current is limited by the  load .
  • Ig=0 so the voltage applied should be higher than VBO   .



  • By suppling +ve gate current device goes into conduction mode and the voltage  at which device goes into conduction mode also reduces.
  • But the Gate signal should be present till the through the device is higher than the latching current.
  • When device goes into conduction mode then gate current should be removed because it do not control the device at this time.

This is all about some basic working of the SCR .







What are Lightning Arresters ?

 lightning arrester

Lightning can strike the overhead power line which enters our house, or a main power line that is blocks away from our home.Lightning can strike an object near our home such as a tree or the ground itself and cause a surge.

Voltage surges can be created by cloud to cloud lightning near our home. A highly charged cloud which passes over our home can also induce a voltage surge.Voltage surges can also be caused by standard on and off switching activities of large electric motors or pieces of equipment.

When a voltage surge is created, it wants to equalize itself and it wants to do it as quickly as possible. These things seem to have very little patience. The surges will do whatever it takes to equalize or neutralize themselves, even  it can short circuiting all of our electronic equipment.

The method of providing maximum protection for equipment is done by providing grounding. Which is efficiently done by using Lightning Arrester(or Surge Diverts).


Lightning arrester is device which provides an easy conducting path or low impedance path for the flow of current when system voltage increases more than the design value and regains its original properties of an insulator at normal voltage.

Working Principle of Lightning Arrester:-

The earthing screen and ground wires can well protect the electrical system against direct lightning strokes but they fail to provide protection against traveling waves, which may reach the terminal apparatus. The lightning arresters or surge diverts provide protection against such surges. A lightning arrester or a surge diverted is a protective device, which conducts the high voltage surges on the power system to the ground.


It consists of a spark gap in series with a non-linear resistor. One end of the diverter is connected to the terminal of the equipment to be protected and the other end is effectively grounded. The length of the gap is so set that normal voltage is not enough to cause an arc but a dangerously high voltage will break down the air insulation and form an arc. The property of the non-linear resistance is that its resistance increases as the voltage increases and vice-versa.


Types of Lightning Arresters:-

  There are several types of lightning arresters in general use. They differ only in constructional details but operate on the same principle, providing low resistance path for the surges to the ground.

  • Rod arrester.
  • Horn gap arrester.
  • Multi gap arrester.
  • Expulsion type lightning arrester.
  • Valve type lightning arrester.
  • Silicon Carbide Arrestor.
  • Metal Oxide Arrestor.

This  is all about the basics of  Lightning Arrester.

What is Stepper Motor ?



Industrial Motors are used to convert electrical energy to mechanical energy. They are neither precision speed nor precision positioning devices.For an automated systems, precise speed and precise positioning are required.To fulfill these limitations stepper motors are used.

Stepper motor rotates through a fixed angular step in response to each input current pulse received by its controller. So it is named as stepper motor. 


The Stepper Motor becomes very popular because it can be controlled directly by computers, microprocessors and programmable controllers.

The only moving part in a stepper motor is its rotor. The rotor has no windings, commutator or brushes. Because of these feature the motor is quite robust and reliable.


Step angle:
The angle through which the motor shaft rotates for each command pulse is called the step angle.

The relationship between steps per revolution and step angle is given by the following formula:-
                      Step angle = 360° / [No. of steps per revolution]
Smaller the step angle, greater the number of steps per revolution and higher the resolution or accuracy of positioning obtained.

Some stepper motor operates upto 20,000 steps per second. Still it remain fully in synchronism  with the command pulses.


When the pulse rate is high, the shaft rotation looks like continuous. Such an operation of stepper motor at high-speed is called slewing’.

1. Variable Reluctance Stepper Motor.
2. Permanent Magnet Stepper Motor.
3. Hybrid Stepper Motor.



1.Stepper Motors are used in a variety of automation applications in which a relatively small amount of torque is needed.

2.Typical applications include rotary table control, wire-harness assembly, laser or pen positioning, and office peripheral equipment control.

3.Stepper motors can be used for precise positioning, without the need for a complicated position indicating feedback system.