Laser Diode - An Overview

A Laser Diode is an electronic device which produces coherent light.  All the waves of the radiation are of the same phase and frequency.  The radiation may be in the visible or the infrared regions of the electromagnetic spectrum.

Laser Diodes find applications in burglar alarms, optical storage system such as compact discs, optical fibre communication, etc.  

The Laser generated by diodes is of lower power.  It is portable and handy.  It produces a conical beam which can be focused using convex lenses.

A laser diode is a P-i-N Diode in which the P type material and the N type material enclose an intrinsic semiconductor.  In the P type material, the majority carriers are holes while in the N type material, the majority charge carriers are the electrons.  The diode is designed such that the holes and the electrons recombine in the I region i.e. the region with the intrinsic semiconductor.

When an electron and a hole recombine, they get annihilated.  This results in the emission of a photon. 

The light thus emitted needs to be of the frequency and phase to become a laser.  This is done by means of a waveguide on the crystal surface. The photons emitted will travel back and forth in the waveguide and be reflected several times. When the   The light is amplified and the laser emerges.

Gallium Arsenide is a material which is widely used in the construction of Laser Diodes.

Leak Off Current in Transistors

Leak Off Current in Transistors refers to the curent  which passes through the transistor, even when it is switched off and the PN regions are not in forward bias.

Leak-off current in modern transistors is very small.  The leak off current varies inversely with the thickness of the semiconductor layer.  

Leak-off Current is caused by design imperfections in the transistors.  the Leak-off Current is of the order of micro amperes or nano amperes.  The leak-off current is considered to be significant only when the transistor is designed to be of very low power rating.

Causes of Transistor Failure

Transistor failure can occur due to a variety of reason.  The following are some of them.

Age

Aging of transistor due to temperature variations inside the components due to carrying current can cause failure.  The electrical properties of the materials inside can drift due to age.

External Causes

External causes such as spikes in the power supply, heat, mechanical damage can also result in transistor failures.  Hence, all transistors should be adequately protected against overvoltage.

Poor Circuit design.

Improperly chosen components and wrong circuit design can also result in transistor failure. Hence, all components in the circuit should be properly rated with sufficient allowance for overloading and temperature rise.  The failure of one component can lead to cascade failures of other components.

Overheating

Overheating is the most common cause of transistor failure.  Hence, it is important that transistors be provided with cooling mechanism.  Devices which contain temperature-sensitive electronic components should be kept in air conditioned environments.

Electrostatic Discharge

Electrostatic discharge can also damage transistors.  Hence proper precautions against Electrostatic Discharge Should be taken.

Transistor - An Overview

A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit.

A transistor is a three terminal semiconductor device.  It  consists of three terminals and two PN junctions.  Transistors have revolutionized the field of computing and telecommunications. Transistors are used to amplify signal.  They can be used two switch power on and off.  

The invention of the Transistor enabled the production of cheaper radios, calculators and computers.  

The transistor consists of two PN junctions.  It consists of two P materials with a N type materials sandwiched in between which is called a PNP transistor or two N-type materials with a P-type material sandwiched in between which is called a NPN transistor.  Thus, two PN junctions are formed.  

The transistor consists of three terminals as mentioned earlier.  The terminal attached to the material in the centre is called the Base.  The terminals attached to the other two materials in the sides are called the emitter and the collector.

The common transistor is also known as the Bipolar transistor.  

Mu Metal - An Overview

A Mu Metal Enclosure

Mu metal is an alloy which is known for its high permeability. It is composed of nickel(77%), Iron(16%), Copper(5%), and Chromium (2%).  Its relative permeability is between 80,000 and 100,000.  It has low hysteresis losses when used in AC circuits. Mu metal is also more malleable which makes it easier to make into thin sheets.  

The name Mu comes from the Greek Alphabet Mu which is the symbol for permeability.

Mu metal having high permeability provides a low reluctance path for static and slowly varying magnetic fields.  The Mu metal shields are annealed after they are in final form.  High frequency magnetic fields can be shielded using Faraday Cage or metal sheets.

Types of Single phase motors.

Resistance Split Phase motors

It has Low Starting current.  The starting torque is moderate.  Applications include Fans, grinders, centrifugal pumps, washing machines

Capacitor Start Motors

The starting torque is high.  Hence, it is used for applications with heavy connected loads such as pumps, conveyors, compressors, etc.  It is the most widely used of all single phase motors.  It is used for motor sizes up to 6 kW.  The capacitor is in line during starting  The capacitor is disconnected by a centrifugal switch as the motor approaches the rated speed.

Permanent Capacitor Motor

The starting torque is high.  The power factor is good due to the presence of the capacitor at all times.  The efficiency is high and the torque produced is smoother.  Ceiling Fans use this type of motor

Two Value Capacitor Motors

This motor is also a capacitor based motor.  However, here there are two capacitors instead of one.  One capacitor is in line during start of the motor while the other capacitor comes in line when the motor is running.

Shaded pole motor

The Shaded pole motor works on the principle of a shading ring which is fitted to the stator poles. This ring varies the reactance and creates two fields.  This motor is simple in construction.  It is used for fractional kW applications such as in hair dryers, table fans, etc.  It has low starting torque and lower power factor.