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.  

Why should the dc series motor never be run without any load ?

The dc series motor develops a very heavy torque during start-up. The motor relies on the connected load to restrain the speed.

Hence, if the dc series motor is run without any load. it may result in overspeeding which can cause serious damage to the motor .

Comparison of Single Phase vs Three Phase Motors

Single Phase Motors	Three Phase Motors
Can be operated anywhere as Single phase supply is Widely Available	Can be used only when three phase supply is available.
Efficiency is lower as a single winding has to carry all the current.	Efficiency is Higher as Three windings carry the current
Torque is not uniform	Torque is uniform
Larger in Size for the same kW rating	Smaller in Size
Cost is cheaper at the fractional kW range due to high volume of production	Cost is higher at low kW range.
Cost is higher at high kW ratings	Cost is cheaper at lower kW range.

What is the permisible amount of current which can pass through the human body

When a current passes through a human body, it can cause damage in the form of burns.  The burns can also happen in the internal organs as the current passes through the body.  The resistance offered by the body is only by the skin.  Blood, being an electrolyte,  is a good conductor of electricity.

The normal limits of current and the effects they produce are as follows.

1mA - Slight Tingling Sensation

1mA -6mA- This is the let-go current when it is possible to let going of a live object.

9mA to 25 mA - Loss of Muscular control.  In this range, it is not possible to let go of the live object

>25mA - Muscles controlling breathing are affected.

50 - 100mA - Fibrillation.  The electric rhythm of the heart is disturbed. The heart stops.  This is a life threatening condition.

Semi Conductor Fuse - an overview

Semiconductor fuses serve to product semiconductor components such as diodes and triacs. These fuses are made specially to act quickly within milliseconds to isolate the electronic component.  

Semiconductor fuses are made of silver strips.  The cross section of these strips determines the fuse limits.  A number of strips may be connected in parallel to provide the desired fuse rating. The strips are enclosed in a tubing containing quartz which serves to quench the arc. 

Semiconductor fuses are designed to act at overloads of 5 to six times the rated capacity.   These fuses are available at MV ratings with a capacity of several thousand amperes.