The Stroboscopic Effect in Fluorescent lamp is a phenomenon which causes running or moving equipment to appear stationary or appear to be operating slower than they actually are.

In an AC supply, the voltage drops 100 times a second to zero volts as the supply frequency is 50 Hz.  When a Fluorescent lamp is operating with an AC supply, the light intensity drops 100 times a second.  This flicker is not noticeable to the human eye due to the persistence of vision. 

When a worker in a factory observes a running machine, say a flywheel under the illumination of a fluorescent light, the flywheel may appear to be stationary or to be operating at reduced speed.  This can result in accidents and is highly dangerous.

A sewing machine whose needle moves up and down may appear to be stationary and the operator can prick the fingers.

These are some examples where the stroboscopic effect in the Fluorescent lamps can prove to be dangerous.

When using fluorescent lamps around rotating or moving machinery, two lamps powered by two different phases should be used.  This ensures that both the lamps do not flicker due to the zero crossing at the same time. 

If another phase is not available, a capacitor can be added in series to one lamp.  This ensures that there is a phase lag between the two lamps. 

The Stroboscopic effect can be eliminated by  using electronic ballasts where the supply to the lamps is of a very high frequency of the order of kiloHertz.




Optical Cables are used extensively in the field of telecommunication.  They have numerous advantages over conventional communication on wires.  They are efficient, quick and secure.  Optical Communication Cables are designed to provide high efficiency of transmission.  They are also designed to withstand the challenges of the external environment such as corrosion, heat and physical stress.
The Optical Cable has the following main components. 
Core
The Core provides the pathway for the light to travel.  It is made of glass or transparent plastic material. 
Cladding
The cladding is the layer that covers the core.  The function of the Cladding is to reflect the light which may come out of the core back into the core.  This results in Total Internal Reflection which ensures that there is no loss of the light signal. 
Buffer
The Buffer is a coating which is outside the Cladding.  The Buffer serves to protect the optical fibre. 
Aramid Yarn Protection
The Aramid yarn which surrounds the buffer provides crush protection to the cable.
Protective Jacket
The protective jacket offers mechanical protection to the cable. 


A Ballast in a fluorescent light is necessary to get the light glowing.  The Ballasts generates a high voltage by means of the would coil which acts as an inductance.  When the starter interrupts the supply to the inductance, a powerful voltage is generated.  These are called magnetic ballasts. 

However, magnetic ballasts take time to get the lamps to start.  They also have an undesirable hum.  They also produce a flicker before the tube lights up continually.

Electronic eliminate the problem of initial flicker and hum.  They also reduce power consumption.
Electronic Ballasts work by converting the AC supply into DC first.  This rectified DC is then chopped by a chopper circuit to generate high voltages to start the discharge in the lamp.  The chopped AC waveform is t a very high frequency of the order of kHz.  This ensures that the flicker is reduced to a minimum.

The efficiency of the lamp is also increased.  Electronic Ballasts are 10% more efficient than magnetic ballasts. Electronic Ballasts can generate harmonics.  This, however, is insignificant as the amount is very small.


Conductivity in metals is due to the presence of free electrons in the atomic lattice.  When the metal is heated, the atoms in the lattice vibrate.  This results in reduced movement of the electrons as they hit against the vibrating atoms.

This results in an increase in resistance of the metals. 

Most Metals have positive temperature coefficient of resistance. There are , however, exceptions such as carbon and semiconductor metals such as Silicon and Germanium.  Some Alloys have zero temperature coefficient of temperature which means that the resistance does not change with increase of temperature.  Manganin is an example.




When a semiconductor is heated, the conductance increases and the resistance decreases.  Semiconductors, thus, have a negative temperature coefficient of resistance.
When heat is applied to a semiconductor material, the outermost electrons in the atom gain energy.  These electrons are able to overcome the attraction of the nucleus and leave the atom.  Thus they become free electrons which can conduct. 
The number of electrons increase exponentially and this results in a large drop in the resistance. 
Electronic devices will behave erratically above a certain temperatures.  Hence, all electronic devices such as laptops will have a safe temperature beyond which they cannot function. 
The effect of vibration of the atomic lattice on the mobility of the electrons is offset by the large numbers of electrons which enter the conduction band. 


In many instances, optical cables are needed to be jointed with one another.  Jointing in optical cables is different from jointing in electrical wires and cables.  The connection should be made such that there is minimum loss of the light energy. 
There are different methods of jointing optical cables
Fusion Jointing
In the method, the surface of the two optical fibres to be connected are heated and fused together.  This ensures that the light is conveyed from one fibre to another efficiently
Mechanical jointing
In this method, the surfaces of the ends are held firmly to make proper contact.  An gel or epoxy is used to match the different reflective indices of the materials.  The fibres are held together by mechanical splices. 


A cleaver is a tool used to cleave optical fibres prior to splicing.  Optical fibres should have a plain and clear surface when they are cut.  The cut should be perpendicular to the longitudinal axis of the fibre.  This is essential to avoid loss of light or distortion.    The cleaving tool is used to cleave the fibre. 
Cleaving Tools generally use diamond tips and blades to cleave the fibres.
Certain mechanical cleaving tools use a diamond blade to make a wedge in the fibre and the twist the fibre to produce a clean break.  The cleave angle should be perfectly 90 degrees. 
The blade in the cleaver may have to be replaced after a certain number of splices.  Most manufacturers provide replacement blades. 
The cleave made is examined by the splicing machine prior to fusion


Optical fusion Splicers are used to join two optical fibres using fusion.  The splicing is done by first heating the ends to be joined.

The fibres after being cleaved are fed into the splicer.  The two fibres to be joined are held against each other and the alignment is checked after which the fusion is done.  The heating is done by means of an electric arc or a laser.  The heating is done in about 15 seconds and the fusion is then carried out.  The device is battery operated. 

Splicers can be programmed for different types of fibre optic cables.  Most Splicers are portable and have a rugged construction.  The life of the heating electrodes is specified after which the electrode may have to be replaced. 


An optical time domain reflectometer is a device which is used to check the integrity of an optical fibre system detect and locate faults in optical cables. 
The optical time domain reflector sends out a series of optical pulses from one end of the cable.  The light which is reflected is analyzed.  This gives information about the state of the optical cable and its terminations.  If there is a drop in the quality of the light of if the distorted, it can indicate a problem such as a cut or an improper splice joint. 
The optical time domain reflectometer can also measure the attenuation of signals through the optical fibre.



The Q factor of an inductor is a very important parameter.  The Q factor tell us how close the inductor to an ideal inductor. 

An ideal inductor is an inductor which has no losses.  That is, its series resistance is zero.  It is not possible to construct an ideal inductor as all inductors are made of wires which have resistances. 

Q factor is the ratio of the inductive reactance to the series resistance of the inductor  at a given frequency.

Q= XL/ R

The Q factor is an crucial parameter when designing resonant circuits as it will affect the damping.  Higher the Q factor, higher is the efficiency of the inductor. 




Moulded chokes are chokes which are moulded in a polymer or synthetic material.  These chokes are used in applications such as Led lighting, automotive electronic components, mobile  phones etc. 
Moulded chokes are small in size and highly compact. 
These chokes have very small inductance values from 10 microhenries to 1000 micro henries.


A choke is an inductor which is used to block AC voltages from a circuit.  Thus, it "chokes off" the AC currents. Chokes usually have fixed values.

Chokes can be classified into
  • Audio Frequency chokes which function at the power and audio frequency and
  • Radio Frequency chokes which function at high frequencies.
Audio frequency chokes  have toroidal cores made of ferrite material. Radio frequency chokes have iron powder or ferrite materials.


Optical Fibre Metallic Wire
Not a lightning hazard as it is non conducting Can attract and transmit lightning
Lighter in weight Heavier in weight
Not affected by Interference Affected by interference
High data bandwidth Lower data bandwidth
Lower data loss Data loss is more
Faster data transmission Relatively slower data transmission
Unauthorised tapping of data is difficult Easier to tap data without authorization.
Difficult to terminate Easier to terminate
High initial cost Lower initial cost
Less affected by chemicals and pollution More prone to effects of pollution
No risk of sparking.  Hence, can be used in petroleum and chemical industries. Risk of sparking and fire.  Hence, cannot be used in hazardous environments. 



Terminating resistors are used in communication cables to prevent reflection of the transmitted signal.  The reflected signal can cause interference which may affect data transmission.  Hence, to prevent this resistances are connected in parallel.

The value of the impedance will match the wave impedance of the line.  Thus a communication line with an impedance of 120 ohms will have a 120 ohm resistor connected across it.  Short cables can function without terminating resistors. 




A Band Stop Filter is a filter which blocks a set of frequencies in a specific band and permits frequencies above and below that range to pass through. 

Functionally, the Band Stop filter does the opposite of the Band Pass Filter. 

The series LC circuit is connected in parallel.  At the resonant frequency of the LC circuit, the reactance is minimum.  Thus a specific frequency is shorted across the input and prevented from reaching the output.

The Band Stop Filter can be made by connecting a Low Pass Filter and a High Pass Filter in parallel. 



A Band pass filter combines the characteristics of the High Pass and Low Pass Filters. 

The Band pass Filter, as the name suggests, allows only signals of a particular band or range of frequencies to pass through.  All other signals are blocked or shorted.  Band Pass Filter

Band pass Filters can be made by connecting a high pass filter in series to a low pass filter or vice versa. 

A Band pass filter can be made by connecting an inductor in parallel to filter the low frequency components which lie below the desired frequency.  Another inductor in series will then block the high frequency components which lie above the desired frequency.

Likewise, the Band pass filter can also be made using capacitors as in the second figure.  Here, the first capacitor filters the high frequency components and the second capacitor in series blocks the low frequency components. 




High Pass FilterA High Pass Filter is a filter which permits high frequency signals to pass through and blocks only low frequency signals.  The high pass filter has a relatively simple construction.  The filter can be constructed by either providing a low impedance path to high frequency signals from the input to the output or by providing a low impedance path to low frequency

If a capacitor is connected in series between the input and the output, it will provide low impedance to high frequency signals and high impedance to low frequency signals.  High frequency signals alone will be able to pass the capacitor.

Alternatively, if an inductor is connected in parallel to the input, it will offer low impedance to low frequency signals which will get shorted across the input.  High frequency signals will alone reach the output.




A Low pass filter is a filter which permits only low frequency signals to pass through.  High frequency signals are blocked or shorted across the input.  The low pass filter offers low impedance to low frequencies and high impedance to high frequencies.Low Pass Filter

There are two ways of constructing a Low Pass Filter. 

The first method is to connect an inductor in series to the output.  The reactance of the inductor is so chosen that it offers high reactance to high frequency signals.  Thus, high frequency voltages are blocked.  At low frequency, the reactance is low and thus low frequency signals are allowed to pass.

Another method is to connect a capacitor in parallel to the input.  The capacitor provides low reactance to high frequency signals which are shorted across the input.  The low frequency signals see a high reactance in the capacitor and they alone reach the output.  The series resistance serves to limit the current.




Air Termination Rods are used to provide lightning protection to parts of a building which protrude from the superstructure.  Air termination rods can fixed to terraces, beams, etc.  Air termination rods consist of a rod or any pointed surface which is connected to the lightning protection system of the building. 

Air Termination Rods are also used for equipment which are kept in exposed flat surfaces such as solar panels and pipelines. 

Air Termination Rods may need to be supported against winds by means of suitable support structures.




Bonded Connections are used to connect a conductor to another body such as a pipe, gate, door or a railing.  All metallic objects in a building or a facility need to be connected to the earth.  This is necessary to prevent accidental voltage getting induced in them due to contact

Bonded Connectors are usually galvanized to prevent corrosion.  They have flat surfaces which increase the contact area with the object to be bonded.  They are made of a material similar to the conductor material such as copper or aluminium. 




Voltage limiting devices are used in Electric Traction systems, to prevent dangerous voltages from appearing the insulated tracks and the earthed components of the installation. 

Overvoltages can occur due to lightning or due to short circuits.  Voltage limiting devices typically use a MOV (metal oxide varistor) and and air gap mechanism to conduct the high voltage impulse to the ground.

If case of minor overvoltages, the MOV operates and diverts the surge.  It then returns to its normal non-conducting state.  In case of severe overvoltages, a permanent short circuit occurs between the protective electrodes and the device has to be replaced.




Recurrent Surge Oscillation is done on the windings of large generators such as turbo alternators.  The Recurrent Surge Oscillation Test (RSO) helps identify shorts in the winding.

Shorts in the winding occur as the insulation between turns deteriorates and fails.  Shorts can cause localised heating and arcing which can further damage the alternators.  Shorts can also become earth faults in course of time.  Multi-turn shorts can also result in a drop in the voltage.

The Recurrent surge oscillation tests is done by sending voltages of low voltage and high frequency through the winding and checking the waveform at the other terminal.  If the waveform has suffered any distortion, it may indicate an abnormality.  The waveform can give information such as the location of the fault and its severity.

Some short circuits may not be obvious when the rotor is at rest.  The conductors will come in contact with each other only during the running condition, due to the centrifugal force.  To identify such faults, the rotor is made to rotate and the test is conducted.




The Half Wave Rectifier functions using a single diode.  It rectifies only half of the sine wave.  Half Wave Rectifier

During the positive half cycle, the diode D1 is in forward bias and current flows to the load.  In the negative half cycle, when the voltage is applied in the opposite direction, the diode is in reverse bias and no current flows.

The Half wave rectifier is a simple device which requires only one diode to be put in series with the load.

The half wave rectifier has a low power output. 

Besides, the ripple content of the rectified DC supply is very high.  This can damage the loads.

It has very few applications and is only used in emergencies as a temporary measure.




Full Wave Rectifier with Centre Tapped Transformer

This Full Wave rectifier has only two diodes.  Each diode conducts during one half cycle.  In the first half cycle, diode D1 is forward bias and the current flows into the load and returns through the centre tap of the transformer. 

During the negative half cycle, diode D2 is in conduction.  The current flows through the load in the same direction and returns through the centre tap.  Thus the current is in the same direction through the load.

This means of rectifier has generally been replaced by the bridge rectifier as the centre tap is not always available.




Controlled Rectifiers are rectifiers which have thyristors in place of a diode.  A thyristor is a three terminal device which can be switched on by applying a suitable gate voltage.  Controlled Bridge Rectifier

In a Controlled Rectifier, generally two of the diodes are replaced with a thyristor.  The thyristor enables the enables switching on the output of the rectifier at the desired time.  This allows the output voltage and current to be controlled.

A rectifier with two thyristors is called a half controlled rectifier while a rectifier where all the diodes have been replaced with thyristors is called a fully controlled rectifier.

Controlled Rectifiers are also known as converters.




A Bridge Rectifier is a very popular and widely used circuit.  The Rectifier converts an AC supply into a DC supply.  The circuit requires only four diodes. 

The four diodes operate two at a time.  That is, during the positive half cycle, diodes D1 and D4 are in forward bias and conduct.   Diodes D2 and D3 are in reverse bias. Circuit Diagram Bridge Rectifier

In the negative half cycle of the AC supply, diodes D2 and D3 are in conduction while D1 and D4 are in reverse bias. 

Thus the current from the rectifier flows in only one direction.

Bridge rectifiers are used in Alternators for excitation of the field.  They are also used in welding machines for DC welding and in battery Chargers

Bridge Rectifiers are also used in measurement circuits to measure the amplitude of an alternating signal. 

Controlled rectifiers contain thyristors instead of diodes.  This enables control of the output supply.




The Class B Chopper is typically used in applications which require transfer of power from the load to the source.  An example would be regenerative braking in trains, where the power from the driving motor is sent to the power mains.  This is also known as inverting operation. Class B Chopper_Circuit_Diagram

In the Class B chopper, the output voltage is positive while the output current is negative.

In a class B chopper, a diode, in reverse bias,  blocks power from the source to the load.  The chopper is connected parallel to the load and the source.  The load voltage is the back-emf of the winding of a DC motor.  When the chopper is in the ON condition, the current due to the back-emf flows through the inductance and the resistance through the chopper.  The diode does not conduct as the voltage across the chopper is zero as the chopper is 'ON'. No Current flows into the source.Chopper B Graph

When the chopper is switched OFF, the voltage across the chopper increases and this biases the diode in the forward direction.  The diode conducts and the power reaches the source.  The source may be a battery or any other power source. 




There are wide range of choppers which are used for different applications.  These circuits differ in the voltage level, method of functioning and the output waveform.

Choppers can be classified into the following types

Step Up or Step Down Choppers

Step up Choppers, as the name suggests, step up the voltage.  These choppers are used when the voltage has to increased to a higher level.

AC and DC Choppers

Choppers can be classified into AC and DC choppers depending on the supply

Circuit Operation

On the basis of Circuit Operation, Choppers can be classified into

  • First Quadrant
  • Second Quadrant and
  • Fourth Quadrant

On the Basis of Commutation

  • Impulse Commutated Choppers
  • Voltage Commutated Choppers
  • Current Commutated Choppers
  • Load Commutated Choppers

Depending on the Direction of Current

Class A

Class B

Class C

Class D and

Class E

In recent times, Choppers are usually classified based on their application such as switched mode power supplies, Class D Electronic Amplfiers, etc.




A Chopper is an electronic circuit which controls or reduces a dc supply.  Their function can be compared to an ac transformer.  In an AC transformer, voltage is controlled by changing the turns ratio of the transformer.  In a chopper, voltage is varied by connecting and disconnecting the load from the source many times in a second. 

The chopper is essentially a switching circuit which switches off and on many times.  The output of a chopper is a square wave form while the input is a unidirectional dc waveform. 

Choppers can be used in motor speed controls.  They are increasingly being used in electric automobile technology. 

Choppers are used widely in Electronics in circuits in solar power conversion, speed control of motors in the industry.  They are used to reduce DC voltage to different levels in machines and other electronic equipments

Choppers have high efficiency and can be designed to have very fine control.




Electrical conduction in materials occurs due to the free electrons which drift about the atomic lattice.  In an atom, the electrons in the outer most orbit are called the valence electrons.  If the electrons have sufficient energy , they can break free of the atom and flow through the lattice when a voltage is applied.

If the energy levels are graphically represented, we will get a band diagram.

In the Band Diagram, there is the box representing the Conduction band and the box representing the valence band. 

Valence Band

The Valence band is the range of energy levels of the electrons in the outermost orbit of the atom. 

Conduction Band

The conduction band is the range of energy levels all electrons which are involved in conduction. 

In conductors, the valence and the conduction bands overlap.  In  Insulators, the valence and conduction bands are far apart. 

In semiconductors, the distance between the valance and the conduction bands are small.  When external energy in the form of heat or light is applied to the semiconductors, the electrons get excited and jump from the valence to the conduction band. 

The difference between the valence and the conduction band is called the energy gap.




Germanium is used in specific applications such as communication, spectroscopy, etc.  They have largely been replaced with silicon.  Germanium diodes are more expensive compared to silicon.

Germanium diodes have a lower forward bias voltage compared to silicon 0.15 volts.  This enables the use of the Germanium diode at low voltages where silicon cannot be used.

Germanium is also used in photoelectronics application.  Germanium diodes have a smaller band gap  0.66 eV.  This means that the electrons can be excited even by near-infrared radiation. 

Germanium is used in solar cells to capture the energy in near-infrared regions of the light spectrum.  Germanium based sensors are used spectroscopy to detect light radiation at low frequencies.




Silicon diodes are diodes in which the P and N materials are made of silicon.  Silicon Diode have a a forward bias voltage of 0.7 volts.  That is, the diode conducts when the voltage across the it in the forward bias is 0.7 volts or greater. 

They are the most widely used diodes in the industry.  Other diodes such as Germanium diodes are used at voltages below 0.7 volts.

The diode can withstand a voltage of 50V or more in the reverse direction.  This is known as the peak inverse Voltage.




Silicon is the most popular and widely used of the semiconductors.    There are many factors which have made silicon the material of choice in the world of electronics.

Some of the advantages are

  1. Silicon is abundant.  Hence, it is also economical.  The extraction process form its ore is cheaper when compared to other materials. 
  2. It is strong and easy to handle.
  3. It forms a nice stable oxide.
  4. Doping is easy.  Both P type materials and N type materials can be formed.
  5. It can be easily cut into wafers.
  6. It has good mechanical strength. Hence, designing circuits in silicon is easy.
  7. Silicon has fewer free electrons in room temperature.  This means that the collector cut-off current in transistor is lower than in other semiconductor materials, such as Germanium.



When a P type material and a N type material are brought in contact with each other, some of the holes in the P material migrate to the N region and combine with electrons.  Similarly, some of the electrons of  N material migrate to the P region and combine with holes. 

Thus, at the point of contact of the P and N materials, a layer is formed which has no majority charge carriers such as holes or electrons.  This region is called the depletion region as the region has been depleted of its charge carriers. 

The depletion region behaves almost like an insulator.  When a voltage exceeding the barrier potential is applied across the PN junction, current starts to flow.




Barrier Potential in a PN junction refers to the potential required to overcome the barrier at the PN junction.

When a P material and N material are brought in contact in a junction, some of the electrons of the N material near the junction cross over to the P material.  These electrons combine with the holes in the P material.  Similarly, the some of the holes of the P material near the Junction cross over to the N material and combine with the electrons.

The region in the contact area is thus depleted of holes and electrons.  This region is called the Depletion Layer.    The majority charge carriers are absent in this region.  This region almost becomes like an insulator.  Thus, there is no conduction after the depletion layer is formed.

For current to flow through this layer, a specific voltage has to be exceeded.  This is known as the barrier potential.  When an external voltage greater than the barrier potential is applied, the PN junction conducts.  




P type Materials

The P type material is obtained when a semiconductor is doped with a trivalent impurity such as Aluminium or Boron. P type material is a material which has holes as its majority carriers.  Electrons are the minority Charge Carriers in P type materials.  When a trivalent impurity is added to the crystal lattice of a semiconductor, there is a vacancy for every impurity atom added.  This vacancy is called a hole.

N type Materials

N type materials are made when a semiconductor is doped with a pentavalent impurity.  A pentavalent impurity is one whose atom has five electrons in its outermost orbit (valence electrons).   Examples of pentavalent impurities are Phosphorous, Antimony, Bismuth.  In an N type Material, electrons are the majority charge carriers while holes are the minority charge carriers.

When a semiconductor is doped with a pentavalent impurity for every impurity atom added, there is a free electron.  These electrons are responsible for conduction.




When the PN junction is formed, there is movement of the charge carriers across the junction.  The electrons move from the N material across the junction into the P material.  The holes from the P material cross into the N junction.

This movement of charge carriers results in a current across the junction. 

This current is known as diffusion current.  This current occurs in the absence of potential.




Cable Sleeves are used to enclose a single wire or a group of wires.  Cable sleeves are used to arrange a set of wires going through a panel or to an equipment like a motor.  Cable sleeves are made of different materials from braided metals to rubber and even kevlar. 

Sleeves serve to protect the wires and their insulation from sharp edges.  They can protect wires from UV radiation, moisture, oil and temperature.  cable sleeve

In automobiles, special heat resistant sleeves are used to protect the wires from hot surfaces. 

Teflon sleeves have great cut resistance.  Nylon sleeves have great abrasion resistance. 

Sleeves are available in two broad categories depending on the method of installation. 

The first is the slit sleeve which is cut and the wires are inserted into the sleeve.  The second is the wrap-around, side entry type of sleeve. 

Heat Shrinkable Sleeves

Some sleeves are heat shrinkable.  Heat shrinkable sleeves are made of a polymer which contracts when heat is applied. A stream of hot air from a blower is passed on the sleeve.  This results in the sleeves shrinking and wrapping the wires.  




Electrical Safety Mats are important safety equipment.  Safety mats protect personnel from electric shock by providing an insulated surface to work on.  If the worker comes in contact with a live conductor by mistake, he will not get an electric shock as his feet are insulated from the ground by the safety mat.Electrical Insulation Mats

Safety mats come in different colours.  They are usually made of rubber.  The surface of the mats is ribbed to provide an anti-slip surface to workers. 

Safety mats are also designed to resist aging and ozone. 

Safety mats should also be age and fire resistant.

Safety mats come in different voltage ratings.   They should also be resistant to acids and oils. 

The following is the list of mats and their working voltages

 

Class of Mats Working Voltage Colour
0 1000 Red
1 7500 White
2 17000 Yellow
3 26500 Green
4 36000 Orange

 

The most common class of safety mats is the type 0 which is rated for a working voltage of 1000 V.