100 PROBLEMS IN ELECTRICAL ENGINEERING - PART – 02 – 05 - PROBLEMS ON POWER DISSIPATION

PROBLEM – 01

A 250 V electric cooking range has three positions of switches high, medium and low. The effective resistance in three positions are 10 ohms, 20 ohms and 40 ohms respectively. There are two identical coils in the ranges. Show how the three position are obtained by proper connection of the coils. Find the power consumed in each position.

PROBLEM – 02

Three resistors Ra, Rb and Rc are connected in series to a 240 V source. If Ra = 2Rc, Rb = 3Rc and total power taken by the circuit is 800 watts. (a) Calculate Ra, Rb and Rc (b) the power consumed by each resistor.

PROBLEM – 03

Four resistors are connected as shown in figure. If the power dissipated in 10 ohm resistor is 50 watts find the value of R and total power supplied by the battery.

PROBLEM – 04

Fiver resistors are connected as shown in figure, values of resistors are in ohms. If power dissipated in 5 ohm resistor is 125 watts. Find the voltage drop across AB, total resistance in the circuit and the current flow through the 15 ohm resistor.

PROBLEM – 05

Figure represents the load resistance Rl = 80 ohms being loaded through a slide wire rheostat. The rheostat has a total resistance of 120 ohms uniformly wound. Calculate the power dissipated to Rl when the slide is in (a) position A, (b) moved down 1/3 from position A (c) moved 2/3 from position A.

100 PROBLEMS IN ELECTRICAL ENGINEERING - PART – 01 - PROBLEMS ON WORK, POWER AND ENERGY

TEN PROBLEMS 

1. How much time is required for 25 x 10 ^18 electrons to leave the negative terminal of the battery, if current provided by the batter is one amperes.

2. Determine the potential energy of 12V battery that has 6000 coulombs of charge stored in it.

3. What is the power rating of an electric device that converts 1880 joules of energy in 20 sec?

4. How much energy is required to operate 100 W incandescent lamp for 30 minutes?

5. An electric iron operates 220 V outlet draw 5 ampere current at rupees 2.25 per kWh. How much does it cost to operate the iron for two hours?

6. What is the efficiency of 0.75 hp motor that requires an input of 800 W of electric power?

7. A strip of aluminium has a length of 5 metre and cross section of 16 mm x 2.5 mm. find the resistance of the strip. Resistivity of aluminium is 2.83 x 10^-8 ohm-metre.

8. A bread toaster has a Nichrome unit resistivity 112 x 10^-8 ohm-metre of resistance 35 ohms. What is the resistance of copper conductor of equal area of cross section but 10 times as long as the Nichrome wire. Resistivity = 1.7 x 10^-8 ohm-metre.                  

9. The field coil of a motor has a resistance of 200 ohms at 15 degree centigrade. Find the resistance at 45 degree centigrade. Assume that the temperature coefficient as 0.00428 per degree centigrade at zero degree centigrade.

10. An incandescent lamp takes 2.2 amps at 220 V at the instant of switching it on. At the normal operating temperature the current drops to 0.2 A. Find the temperature of the heated filament room temperature is equal to 20 degree centigrade and the corresponding temperature coefficient of the filament is 0.005 per degree centigrade.

ELECTRIC LAMPS – PART – 10 – LIGHT EMITTING DIODE (LED)

INVENTOR OF LED
The early years of the 1960s witnessed of a 'race' in the field of semiconductors. LED was 'discovered' in the year 1961 by James R. Biard and Gary Pittman. In 1962 – Nick Holonyak, Jr. a consulting scientist for General Electric invented first visible-spectrum LED.
At that time the production cost of one LEDs was $200. These LEDs used a semiconductor combining gallium, arsenic and phosphorus - GaAsP. This type produced red light, but the efficiency of the devices was very low. In 1987 the Hewlett Packard (HP) being produced Aluminium Gallium Arsenide (AlGaAs) diodes which were bright enough for the first applications within lighting. In 1998 Aluminium Indium Gallium Phosphide (AlInGaP) diodes were manufactured by HP which are superior to Aluminium Gallium Arsenide (AlGaAs) diodes, giving double the light output. In 1993 HP started to use Gallium Phosphide (GaP) to provide high output green LEDs and HP also developed high output orange lamps.

WORKING PRINCIPLE
LED work on the principle of Electroluminescence (EL).
Electroluminescence is an optical phenomenon and electrical phenomenon, in which a material emits light in response to the passage of an electric current or to a strong electric field.

CONSTRUCTION
1. LED is basically a specialized type of PN junction diode, made
    up of a very thin layer of moderately doped semiconductor
    material.
2. LEDs are made from exotic (exotic means strikingly strange or
    unusual) semiconductor compounds such as Gallium Arsenide
    (GaAs), Gallium Phosphide (GaP), Gallium Arsenide Phosphide
    (GaAsP), Silicon Carbide (SiC), Gallium Indium Nitride
    (GaInN) which are together at different ratios to produce a
    distinct wavelength of colour.
3. Different LED compounds emit light in specific regions of the
    visible light spectrum and therefore produce different
    intensity levels.
4. The PN junction of an LED is surrounded by a transparent,
    hard plastic epoxy resin hemispherical shaped shell or body
    which protects the LED from both vibration and shock.
5. The epoxy resin body is constructed in a such way that the
    Photons of light emitted are focused upwards through the
    domed top of the LED.
6. The epoxy resin acts like a lens concentrating the amount of
    light.
7. All LEDs are not made with a hemispherical shaped dome for
    their epoxy shell.
8. Some LEDs have a rectangular or cylindrical shaped
    construction with a flat surface on top or their body which is
    shaped into a bar or like an arrow.
9. The most common colours of LEDs are RED, AMBER, YELLOW 
    and GREEN and are highly used as visual indicators and as
    moving light displays.
10. Blue and white coloured LEDs are also available but they are
      more expensive than standard colour LEDs.

OPERATION

1.Under forward biased condition, the LED emit light due   to  the

   recombination of holes and electrons within the device,    

   releasing    energy as photons which are called as photon

   electrons.

2.The electrons dissipate energy in the form of heat for ordinary

   diodes. But in LED the electrons dissipate energy by emitting

   photons. 

3.The charge carriers recombine in a forward P-N junction as

   the electrons cross from the N-region and recombine with the

   holes existing in the P-region.

4.Free electrons are in the conduction band of energy levels,

   while holes are in the valence energy band.

5.Thus the energy level of the holes will be lesser than the

   energy levels of the electrons.

6.Some part of the energy must be dissipated in order to

   recombine the electrons and  the holes.

7.This energy is emitted in the form of heat and light.

8.The majority of the light is produced from the area of the

   junction nearer to the P-type region.

9.If the semiconductor is translucent, the junction becomes the

   source of light as it is emitted, thus becoming a light emitting

   diode (LED).

10.LED will not emit light when it is reverse biased and at the

    same time it also get damaged.

TYPES OF LEDS AND ITS COLOURS

1. GaAs            - infra-red

2. GaAsP          - red to infra-red, orange

3. AlGaAsP       - orange-red, orange, and yellow

4. GaP              - red, yellow and green

5. AlGaP           - green

6. GaN              - green, emerald green

7. GaInN           - near ultraviolet, bluish-green and blue

8. SiC               - blue as a substrate

9. ZnSe            - blue

10. AlGaN         - ultraviolet

ADVANTAGES

1. Energy efficient source of light for short distances and small

    areas.

2. Miniature in size and hence light weight.

3. Low voltage and current are enough to drive the LED and

    require only 30-60 milliwatts to operate.

4. Durable and shockproof unlike glass bulb lamp types.

5. The response time is very less – only about 10 nanoseconds.

6. It can withstand shock and vibrations because it is rugged.

7. Lumens per watt: 28 - 150 (depends on environment)

8. Lamp life: 25,000 - 100,000 hours [more than 20 years]

9. Available in 0.01 - 3 W

10. LEDs long life, rich color, and easily-controlled features with integrated electronics offer a scalable lighting solution. As technology continues to bring rapid improvements in luminous efficiency and as cost compression persists, applications expand rapidly.

DISADVANTAGES

1. A slight excess in voltage or current can damage the device.

2. Unreliable in outside applications with great variations in

    summer/winter temperatures.

3. Reduced lumen output over time.

4. Much wider bandwidth compared to the laser.

5. The temperature affects LEDs radiant output power and

    wavelength.

APPLICATIONS

1. One mobile phone takes two LED backlight sources and six

    SMD (Surface Mount Diode) LED key lights. As a result, mobile

    phones create a demand for 3.2 billion LEDs per year.

2. Interior usage of automobiles include indicator lights on    

    dashboard gauges, audio status lights, security status lights

    and warning signals and exterior usage includes third brake

    lights, left and right rear lamps, turn signals, etc.

3. The LED screen has become a new display medium for

    advertising and information.

4. Today, LEDs have been integrated as warning lights and

    indicators on most electronic applications.

5. LEDs are being used in advertising billboards, illumination of

    commercial building exteriors, landmark buildings, bridges,

    roads, town centers, airports, subways, hotels, shopping

    centers and landscape lighting because of numerous advantages

    they offer.

ELECTRIC LAMPS – PART – 09 – OPERATION OF HALOGEN LAMP

INVENTOR OF HALOGEN LAMP
A halogen lamp is an advanced form of incandescent lamp.
1955 Frederick A. Mosby a General Electric Engineer developed an efficient halogen lamp, and adapted the lamp for use in regular lamp sockets.
1955 - Philips Engineers developed a lamp that used the halogen bromine.
In 1959, General Electric patented a commercially viable halogen lamp using iodine as the halogen gas.
Halogen bulbs produce light that is whiter and brighter, use less energy, and longer life than standard incandescent bulbs of the same wattage.

PROBLEM IN THE INCANDESCENT LAMP

The main problem in the tungsten filament incandescent lamp is evaporation from the hot filament condenses on the cooler inside bulb wall, causing the bulb to blacken.
This blackening process continuously reduces the light output over the life of the lamp.
This problem gave birth to halogen lamp.
The halogen lamp is a type of incandescent lamp which uses a halogen gas in order to increase both light output and rated life.
Iodine and Bromine are only used in halogen lamps.
It is also known as a quartz halogen and tungsten halogen lamp.
CONSTRUCTION
1. GLASS BULB
Depending on the type of halogen lamp, the bulb material is either quartz (fused silica) or aluminosilicate glass. Quartz glass has the appropriate temperature resistance for the tungsten-halogen cycle, which produces bulb temperatures of upto 900°F.
Either glass comes in the form of cylindrical tubes that are precut to the desired length or cut to length by the lamp manufacturer.
2. FILAMENT
The filament is composed of ductile tungsten and located in a gas filled bulb just like an incandescent bulb.
Filaments are also oriented in two ways, axial or transverse.
3. GAS
krypton or xenon – Helps retard the rate of tungsten evaporation and the gas in a halogen bulb is at a higher pressure (7-8 ATM).
The higher the pressure and better filling gases extend the life of the bulb.  Higher filament temperature results in better lamp efficacy.
4. REFLECTIVE COATING
Halogen lamps are also designed with a special infrared reflective coating on the outer side of the bulb to ensure that    the radiated heat, which otherwise is wasted, is reflected     back to the lamp filament.
OPERATION
1. When supply is switched ON, the filament begins to glow red
    as more current passes through it.
2. The temperature rapidly increases. The halogens boil to a gas
    at relatively low temperatures.
3. Iodine is at 184 C and Bromine is at 59 C.
4. The halogen chemically reacts with the tungsten deposit to
    produce tungsten halides (a salt of any halogen acid).
5. When the tungsten halide reaches the filament, the intense
    heat of the filament causes the halide to break down,
    releasing tungsten back to the filament.
6. This process—known as the tungsten-halogen cycle—maintains
    a constant light output over the life of the lamp.
7. Most halogen lamps range in power from 20-2,000 watts and
    low voltage types range from 4-150 watts.
8. Halogen lamps can be configured as single ended and double
    ended.
9. Double ended halogen is commonly used.
10. These lamps generally are the larger wattage lamps and are
     used for work lights, yard lights and film production lamps.

ADVANTAGES

1. Halogen Lamps are compact in size and lightweight.
2. The halogen lamp is fully dimmable unlike compact
    fluorescent lamps.
3. Does not contain mercury like CFLs (fluorescent) or mercury
    vapor lights.
4. Better color temperature than standard tungsten (2800-
    3400 Kelvin) lamp.
5. Its light is closer to sunlight.
6. Instantly switches on to full brightness and requires no warm
    up time.
7. The filament burns hotter and hence less wattage is required.
8. Life of halogen lamp is 2,000-4,000 hours (about two to four
    years).
DISADVANTAGES
1. Halogen lamps can also pose a safety threat, as the heat
    generated can range from 250-900 F (121-482°C).
    [Formula for converting F – C = [F-32] / [1.8]
2. Extremely hot (easily capable of causing severe burns if
    the lamp is touched).
3. On, explosion, the bulb is capable of blowing and sending hot
    glass shards (a broken piece of a brittle artifact) outward.
4. The life of a halogen lamp is shortened by frequent on and
    off cycles.
APPLICATIONS
1. The halogen lamp has an instant 'on' ability unlike mercury  vapor 
     or high pressure sodium, therefore they work well for  security 
     lamps that are activated by motion sensors.    
2. As portable work lights and auto headlights.

3. As home Interior Lighting (Smaller Wattage).

4. In commercial Exterior Lighting (Larger Wattage).

5. Halogen lamps are used in television and film production.