June- July VI Class
1. Identification of various types of electrical accessories and components.
2. Awareness and recognition of Electrical appliances.
Electrical Accessories are such as Wiring Accessories, Conduit Fittings, Terminal Blocks, Electrical Parts, Electrical Components and Electrical Switch Parts.
Accessories
Earthing Accessories
HRC Fuse Connectors
Insert For PVC Moulding Parts and Conduit Fittings
Neutral Links and Terminal Blocks
Electrical Switch Parts
General Brass Electrical Accessories
Panel Board Accessories
Earthing Accessories
HRC Fuse Connectors
Insert For PVC Moulding Parts and Conduit Fittings
Neutral Links and Terminal Blocks
Electrical Switch Parts
General Brass Electrical Accessories
Panel Board Accessories
Test Clamp
Oblong Test Clamp
Square Tape Clamp
DC Tape Clip
Tee Clamp
Rod to Tape Coupling
Our products range in electrical switch parts
Oblong Test Clamp
Square Tape Clamp
DC Tape Clip
Tee Clamp
Rod to Tape Coupling
Our products range in electrical switch parts
includes the following :
5 Amp Two Pin Socket with Spring
5 Amp Two Pin Socket Heavy Type
5 X 5 Amp Universal Combine Socket
5 Amp Earthing Socket with Spring
10 Amp Live Part Socket
15 Amp Earthing Socket
5 X 15 Amp Universal Combine Socket
10 X 15 Amp Universal Combine Socket
Moulding Inserts
6 Amp Riveting TC Piller
16 Amp Riveting TC Piller
Riveting Piller
16 Amp Side Part
16 Amp Center Part
32 Amp Side Part
32 Amp Center Part
5 Amp Top Pin with Spring
Ceiling Rose
5 Amp Two Pin Socket with Spring
5 Amp Two Pin Socket Heavy Type
5 X 5 Amp Universal Combine Socket
5 Amp Earthing Socket with Spring
10 Amp Live Part Socket
15 Amp Earthing Socket
5 X 15 Amp Universal Combine Socket
10 X 15 Amp Universal Combine Socket
Moulding Inserts
6 Amp Riveting TC Piller
16 Amp Riveting TC Piller
Riveting Piller
16 Amp Side Part
16 Amp Center Part
32 Amp Side Part
32 Amp Center Part
5 Amp Top Pin with Spring
Ceiling Rose
Electrical Appliances
very household, every business, and just about every building in the world has some major appliances in use. Clothes Washers, Dish Washers, Clothes Dryers, Refrigerators, Microwave Ovens, Stoves and disposals are some of the more common ones. They break down occasionally and need to be repaired. Is there anyone you know who doesn't own some kind of Major Appliance?
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June- July Class VII
Matter
Matter is everything around you. Matter is anything made of atoms and molecules. Matter is anything that has a mass. Matter is also related to light and electromagnetic radiation. Even though matter can be found all over the universe, you usually find it in just a few forms. Matter is anything that
occupies space and has mass
occupies space and has mass
Atom
An atom is made up of three particles including proton, neutron and electron. The mass of the atom is the smallest, indivisible particle of an element. Atoms of the same element are exactly alike and are different from the atoms of all other elements. Smallest particle of an element which shows all properties of element is called atom.
Some characteristics of "atoms" are as follows:
Atom takes part in chemical reactions independently.
Atom can be divided into a number of sub-atomic particles.
Fundamental particles of atom are electron, proton and neutr
Molecule
Molecules are the smallest particles of an element or compound that are made up of two or more atoms. Ions are particles that are charged due to loss or gain of electrons.
Properties of atom, Proton, Electron, Neutron.
Atom Properties
The atom above, made up of one proton and one electron, is called hydrogen (the abbreviation for hydrogen is H). The proton and electron stay together because just like two magnets, the opposite electrical charges attract each other. What keeps the two from crashing into each other? The particles in an atom are not still. The electron is constantly spinning around the center of the atom (called the nucleus). The centrifugal force of the spinning electron keeps the two particles from coming into contact with each other much as the earth's rotation keeps it from plunging into the sun. Taking this into consideration, an atom of hydrogen would look like this:
Proton Properties
Charge: Proton is a positively charged particle. Magnitude of charge: Charge of proton is 1.6022 x 10-19 coulomb. Mass of proton: Mass of proton is 1.0072766 a.m.u. or 1.6726 x 10-27 kg. Comparative mass: Proton is 1837 times heavier than an electron. Position in atom: Protons are present in the nucleus of atom. |
Electron
Charge: It is a negatively charged particle. Magnitutide of charge: Charge of electron is 1.6022 x 10-19 coulomb. Mass of electron: Mass of electron is 0.000548597 a.m.u. or 1.1 x 10-31 kg. Symbol of electron: Electron is represented by "e". Location in the atom: Electrons revolve around the nucleus of atom in different circular orbits. Neutron charge: It is a neutral particle because it has no charge. Mass of neutron: . Mass of neutron is 1.0086654 a.m.u. or 1.6749 x 10-27 kg. Comparative mass: Neutron is 1842 times heavier than an electron. Location in the atom: Neutrons are present in the nucleus of an atom. *****____***** |
June July – Class VIII
Fuse
An electrical fuse is a current interrupting device which protects an electrical circuit in which it is installed by creating an open circuit condition in response to excessive current. The current is interrupted when the element or elements which carry the current are melted by heat generated by the current. Fuse terminals typically form an electrical connection between an electrical power source and an electrical component or a combination of components arranged in an electrical circuit. A fusible link is connected between the fuse terminals, so that when electrical current flowing through the fuse exceeds a predetermined limit.
TYPES OF FUSES
Fuses may be classified as fast-acting or time delay and as current-limiting or non-current limiting. Fast-acting fuses are designed to respond quickly to overload currents, while time delay fuses are required to carry an overload current for a predetermined amount of time. This permits time-delay fuses to carry starting current and other temporary overloads. Fuses that limit the maximum peak current that could flow during a short circuit are classified as current limiting fuses. Whether the fuse is classified as
fast-acting or time-delay, current-limiting fuses
will open quickly during short-circuit conditions.
A fuse unit essentially consists of a metal fuse element or link, a set of contacts between which it is fixed and a body to support and isolate them. Many types of fuses also have some means for extinguishing the arc which appears when the fuse element melts. In general, there are two categories of fuses viz.
I) Low voltage fuses.
II)High voltage fuses.
Usually isolating switches are provided in series with fuses where it is necessary to permit fuses to be replaced or rewired with safety. In absence of such isolation means, the fuses must be so shielded as to protect the user against accidental contact with the live metal when the fuse is being inserted or removed.
LOW VOLTAGE FUSES Low voltage fuses can be further divided into two classes namely
I) Semi-enclosed or Rewireable type.
II)Totally enclosed or Cartridge type.REWIREABLE FUSES
The most commonly used fuse in 'house wiring' and small current circuit is the semi-enclosed or rewireable fuse.(also sometime known as KIT-KAT type fuse). It consist of a porcelain base carrying the fixed contacts to which the incoming and outgoing live or phase wires are connected and a
porcelain fuse carrier holding the fuse element, consisting of one or more
strands of fuse wire, stretched between its terminals. The fuse carrier is a
separate part and can be taken out or inserted in the base without risk, even without opening the main switch. If fuse holder or carrier gets damaged during use, it may be replaced without replacing the complete unit. The fuse wire may be of lead, tinned copper, aluminum or an alloy of tin-lead. The actual fusing current will be about twice the rated current. When two or more fuse wire are used, the wires should be kept apart and aerating factor of 0.7 to 0.8 should be employed to arrive at the total fuse rating. The specification for rewireable fuses are covered by IS: 2086-1963.Standard ratings are 6, 16, 32, 63, and 100A. A fuse wire of any rating not exceeding the rating of the fuse may be used in it that is a 80 A fuse wire can be used in a 100 A fuse, but not in the 63 A fuse. On occurrence of a fault, the fuse element blows off and the circuit is interrupted. The fuse carrier is pulled out, the blown out fuse element is replaced by new one and the supply can is resorted by re-inserting the fuse carrier in the base. Though such fuses have the advantage of easy removal or replacement without any danger of coming into the contact with a lie part and negligible replacement cost but suffers from following disadvantages:1. Unreliable Operations.
2. Lack of Discrimination.
3.Small time lag.
4.Low rupturing capacity.
5.No current limiting feature.
6.Slow speed of operations.
4.Low rupturing capacity.
5.No current limiting feature.
6.Slow speed of operations.
2.TOTALLY ENCLOSED OR CARTIDGES TYPE FUSE.
The fuse element is enclosed in a totally enclosed container and is provided
with metal contacts on both sides. These fuses are further classified as
I) D-type.
II)Link type.
Link type cartridges are again of two types viz. Knife blade or bolted type.
D- Type Cartridges Fuses
It is a non interchangeable fuse comprising s fuse base, adapter ring, cartridge and a fuse cap. The cartridge is pushed in the fuse cap and the cap is screwed on the fuse base. On complete screwing the cartridge tip touches the conductor and circuit between the two terminals is completed through the fuse link. The standard ratings are 6, 16, 32, and 63 amperes. The breaking or rupturing capacity is of the order of 4k A for 2 and 4ampere fuses the 16k A for 63 A fuses.
Link type Cartridge or High Rupturing Capacity(HRC)
Where large number of concentrations of powers are concerned, as in the modern distribution system, it is essential that fuses should have a definite known breaking capacity and also this breaking capacity should have a high value. High rupturing capacity cartridge fuse, commonly called HRC cartridge fuses.
KNIFE BLAD TYPE HRC FUSE
insulated fuse puller.
BOLTED TYPE HRC LINK FUSE
it has two conducting plates on either ends. These are bolted on the plates of the fuse base. Such a fuse needs an additional switch so that the fuse can be taken out without getting a shock. Preferred ratings of HRC fuses are 2 ,4, 6, 10, 16, 25, 30, 50, 63, 80, 100, 125, 160, 200, 250, 320, 400, 500 ,630,800, 1000 and 1,250 amperesPossible accidents without fuse.
Fuse is a safety device. It is inserted into an electrical circuit to protect the equipment and wiring and to cause great loss against any excessive current flow. It is a short thin piece of wire which becomes hot and melts when the current through it is greater than its rated value. The thicker the wire, the more current is needed to melt it and the higher is its rating.
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June – July Class IX
Earthing System
Introduction: The earthing system, sometimes simply called ‘earthing’, is the total set of measures used to connect an electrically conductive part to earth. The earthing system is an essential part of power networks at both high- and low-voltage levels. A good earthing system is required for:
‹ Protection of buildings and installations against lightning
‹ Safety of human and animal life by limiting touch and step voltages to safe
values
‹ Electromagnetic compatibility (EMC) i.e. limitation of electromagnetic
disturbances
‹ Correct operation of the electricity supply network and to ensure good power
quality.
All these functions are provided by a single earthing system that has to be designed to fulfill all the requirements. Some elements of an earthing system may be provided to fulfill a specific purpose, but are nevertheless part of one single earthing system. Standards require all earthing measures within an installation to be bonded together, forming one system.
The main objectives of the earthing are to:
1) Provide an alternative path for the fault current to flow so that it will not
an danger the user
2) Ensure that all exposed conductive parts do not reach a dangerous potential
3) Maintain the voltage at any part of an electrical system at a known value so as
to prevent over current or excessive voltage on the appliances or equipment .
The qualities of a good earthing system are:
1) Must be of low electrical resistance
2) Must be of good corrosion resistance
3) Must be able to dissipate high fault current repeatedly
1) PLATE EARTHING • In major power stations and major sub-stations 12 mm thick, 1200 m long, 1200 mm wide Cast Iron plates are used.
• For minor sub-stations 18 mm broad, 50 x 50 cm. G.I. plates are used.
• These plates are dug vertically in the pit. Coal, sand and salt are filled in the pit each of 150 mm layer.
• The plate should be dug deep so that soil will be wet from all sides.
Pipe earthing:
• The pipe type earthing is generally provided outside the base of the tower.
• earthing is given in A hole of the required diameter and depth is augured in the earth for the earthing pipe.
• The earthing pipe is then put inside the hole.
• A mixture of coke and salt is filled in the hole in which the earthing pipe is provided.
• The earthing strip which was fitted to the stub of the tower leg is then connected to the earthing pipe.
• The Railway authorities specify that the size of the pipe used for earthing should be of 38 mm diameter. Therefore, for towers on both sides of the Railway crossing, 2 pipes connected together are to be used for earthing.
• In case of difficult locations, the pipe may be laid horizontally or slanting and within the tower base or foundation pit.
METHODS OF REDUCING EARTH RESISTANCE
• By adding mixture of salt and water to the earth pit.
• By adding salt, charcoal and sand mixture to the pit.
• By using a bigger grounding plate
• By burying the ground plate as deep as possible
• By having parallel ground plates with a distance of 10m between grounds
• By using salt, charcoal etc., to reduce resisitvity
Reference: http://www.seminarprojects.com/Thread-elecrical-earthing-system#ixzz1Nme6NAfE
2) Pipe Earthing
pipe earthing is the best form of earthing and is very cheap in cost.
in this method a galvanised iron pipe of approved length and diameter is placed up right in a permanently wet soil. the size of the pipe depends upon the current to be carried and type of soil usually the pipe used for this purpose is of 38mm and 2.5m in length the depth at which the pipe must be buried depends upon the moisture of the ground. the pipe is placed at a depth of3.75m. the pipe is provided with a tapered casing at the lower end in order to facilitate the driving
in this method a galvanised iron pipe of approved length and diameter is placed up right in a permanently wet soil. the size of the pipe depends upon the current to be carried and type of soil usually the pipe used for this purpose is of 38mm and 2.5m in length the depth at which the pipe must be buried depends upon the moisture of the ground. the pipe is placed at a depth of3.75m. the pipe is provided with a tapered casing at the lower end in order to facilitate the driving
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June- July Class X.
Electrical Fundamentals
Electrical Fundamentals
Potenial
A potential may refer to the scalar potential or to the vector potential. In either case, it is a field defined in space, from which many important physical properties may be derived.
Potential difference
A familiar way to understand potential difference is to think of a stone on
the top of a hill. The stone has a lot of gravitational potential energy which it can lose if it is allowed to fall to the ground. In a battery, or
power supply, the electric charges at one terminal (by convention the positive terminal) have MORE energy (electrical potential energy) than they will when they get to the other terminal, i.e. they can fall down hill from the positive terminal to the negative terminal. For the charges to lose their energy they require a conductive path to allow them to fall down the electrical hill. This path is an ELECTRIC CIRCUIT that provides a way for the charges to lose their excess energy as they travel around the circuit . This is just like the rock on the hill. If it is allowed to, it can fall to the "ground" and lose the stored energy it had.
The energy the charges have to lose is referred to as the 'potential difference'. The potential difference tells you how much energy per coulomb the charges have to lose. Potential difference is measured in volts, symbol V, where 1 volt is equivalent to an energy difference of
1 joule for each coulomb of charge. Potential difference is commonly referred to asvoltage.
Examine the circuit diagram below in which a voltmeter is used to measure the potential difference across a resistor. The voltmeter will measure the potential difference between X and Y.
Electric current (I)
This is the rate at which the electric charges flow through the circuit, i.e. How quickly the electric charges are moving through the circuit. An ammeter is used in a circuit to record the current. Electric current has the symbol I and is measured in ampere (symbol A)where 1 ampere is equivalent to a flow of 1 coulombper second. Although current was originally thought to be a flow of positive charge, we now know that in most cases it is only the very tiny negatively charged electrons that flow through wires as a current.
Remember the positive charge is carried by the proton and they are all held very tightly in the nucleus of an atom.
Resistance
The electrical resistance of a circuit component or device is defined as the ratio of the voltage applied to the electric currentwhichflows through it:
R = V
I
If the resistance is constant over a considerable range of voltage, then Ohm's law, I = V/R, can be used to predict the behavior of the material. Although the definition above involves DC current and voltage, the same definition holds for the AC application of resistors.
Whether or not a material obeys Ohm's law, its resistance can be described in terms of its bulk resistivity. The resistivity, and thus the resistance, is temperature dependent. Over sizable ranges of temperature, this temperature dependence can be predicted from a temperature coefficient of resistance.
SPECIFIC RESISTANCE OR RESISTIVITY
Specific resistance, or resistivity, is the resistance in ohms offered by a unit volume (the circular-mil-foot or the centimeter cube) of a substance to the flow of electric current. Resistivity is the reciprocal ofconductivity. A substance that has a high resistivity will have a low conductivity, and vice versa. Thus,the specific resistance of a substance is the resistance of a unit volume of that substance.Many tables of specific resistance are based on the resistance in ohms of a volume of a substance 1foot in length and 1 circular mil in cross-sectional area. The temperature at which the resistancemeasurement is made is also specified. If you know the kind of metal a conductor is made of, you canobtain the specific resistance of the metal from a table. The specific resistances of some commo
Material | "K" | Material | "K" |
Brass | 43.0 | Aluminum | 17.0 |
Constantan | 295 | Monel | 253 |
Copper | 10.8 | Nichrome | 600 |
German Silver 18 % | 200 | Nickel | 947 |
Gold | 14.7 | Tantalum | 93.3 |
Iron (Pure) | 60.0 | Tin | 69.0 |
Magnesium | 276 | Tungsten | 34.0 |
Manganin | 265 | Silver | 9.7 |
"K" = Specific Resistance | |||
Calculation of resultant resistance in a circuit.
The resistance of a conductor of a uniform cross section varies directly as the product of the lengthand the specific resistance of the conductor, and inverselythe cross-sectional area of the conductor.Therefore, you can calculate the resistance of a conductor if you know the length, cross-sectional area,and specific resistance of the substance. Expressed as an equation, the "R" (resistance in ohms) of aconductor is
IN SERIES, so that the same current flows through all the components but a different potential difference (voltage) can exist across each one.
The formula for combining a total of "n" series resistors into a single equivalent resistor "Req" is:
Req = R1+R2+....Rn
That is, all the series resistor values are simply added.
Req = R1+R2+....Rn
That is, all the series resistor values are simply added.
For the example shown below: Req = 50Ω+100Ω + 150= 300Ω
IN PARALLEL, so that the same potential difference (voltage) exists across all the components but each component may carry a different current.
The equation for combing a total of "n" resistors in parallel is:
Req = 1/{(1/R1)+(1/R2)+(1/R3)..+(1/Rn)}
Req = 1/{(1/R1)+(1/R2)+(1/R3)..+(1/Rn)}
For the example below there are 3 resistors (n = 3) with R1 = 20Ω, R2 = 30Ω, and R3 = 30Ω. Using the above formula we find the total equivalent resistance for all 3 resistors in parallel is:
Req = 1{(1/20)+(1/30)+(1/30)}
= 1/{(3/60)+(2/60)+(2/60)}
= 1/(7/60)=60/7 Ω = approximately 8.57Ω.
Req = 1{(1/20)+(1/30)+(1/30)}
= 1/{(3/60)+(2/60)+(2/60)}
= 1/(7/60)=60/7 Ω = approximately 8.57Ω.
Transister
A transistor is a semiconductor device used to amplify and switch electronic signals. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much more than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.
Principle
The design of a transistor allows it to function as an amplifier or a switch. This is accomplished by using a small amount of electricity to control a gate on a much larger supply of electricity, much like turning a valve to control a supply of water.
Transistors are composed of three parts – a base, a collector, and an emitter. The base is the gate controller device for the larger electrical supply. The collector is the larger electrical supply, and the emitter is the outlet for that supply. By sending varying levels of current from the base, the amount of current flowing through the gate from the collector may be regulated. In this way, a very small amount of current may be used to control a large amount of current, as in an amplifier. The same process is used to create the binary code for the digital processors but in this case a voltage threshold of five volts is needed to open the collector gate. In this way, the transistor is being used as a switch with a binary function: five volts – ON, less than five volts OFF.
Transistors are composed of three parts – a base, a collector, and an emitter. The base is the gate controller device for the larger electrical supply. The collector is the larger electrical supply, and the emitter is the outlet for that supply. By sending varying levels of current from the base, the amount of current flowing through the gate from the collector may be regulated. In this way, a very small amount of current may be used to control a large amount of current, as in an amplifier. The same process is used to create the binary code for the digital processors but in this case a voltage threshold of five volts is needed to open the collector gate. In this way, the transistor is being used as a switch with a binary function: five volts – ON, less than five volts OFF.
Construction
The very first transistors were known as point-contact transistors. Their construction is similar to the construction of the point-contact diode covered in chapter 1. The difference, of course, is that the point-contact transistor has two P or N regions formed instead of one. Each of the two regions constitutes an electrode (element) of the transistor. One is named the emitter and the other is named the collector, as shown in figure
Point-contact transistors are now practically obsolete. They have been replaced by junction transistors, which are superior to point-contact transistors in nearly all respects. The junction transistor generates less noise, handles more power, provides higher current and voltage gains, and can be mass-produced more cheaply than the point-contact transistor. Junction transistors are manufactured in much the same manner as the PN junction diode discussed earlier. However, when the PNP or NPN material is grown (view B), the impurity mixing process must be reversed twice to obtain the two junctions required in a transistor. Likewise, when the alloy-junction (view C) or the diffused-junction (view D) process is used, two junctions must also be created within the crystal.
Although there are numerous ways to manufacture transistors, one of the most important parts of any manufacturing process is quality control. Without good quality control, many transistors would prove unreliable because the construction and processing of a transistor govern its thermal ratings, stability, and electrical characteristics. Even though there are many variations in the transistor manufacturing processes, certain structural techniques, which yield good reliability and long life , are common to all processes: (1) Wire leads are connected to each semiconductor electrode; (2) the crystal is specially mounted to protect it against mechanical damage; and (3) the unit is sealed to prevent harmful contamination of the crystal.
Types of Transistors
The transistor is an arrangement of semiconductor materials that share common physical boundaries. Materials most commonly used are silicon, gallium-arsenide, and germanium, into which impurities have been introduced by a process called “doping.” In n-type semiconductors the impurities or dopants result in an excess of electrons, or negative charges; in p-type semiconductors the dopants lead to a deficiency of electrons and therefore an excess of positive charge carriers or “holes.”
The Junction Transistor
The n-p-n junction transistor consists of two n-type semiconductors (called the emitter and collector) separated by a thin layer of p-type semiconductor (called the base). The transistor action is such that if the electric potentials on the segments are properly determined, a small current between the base and emitter connections results in a large current between the emitter and collector connections, thus producing current amplification. Some circuits are designed to use the transistor as a switching device; current in the base-emitter junction creates a low-resistance path between the collector and emitter. The p-n-p junction transistor, consisting of a thin layer of n-type semiconductor lying between two p-type semiconductors, works in the same manner, except that all polarities are reversed.
The Field-Effect Transistor
A very important type of transistor developed after the junction transistor is the field-effect transistor (FET). It draws virtually no power from an input signal, overcoming a major disadvantage of the junction transistor. An n-channel FET consists of a bar (channel) of n-type semiconductor material that passes between and makes contact with two small regions of p-type material near its center. The terminals attached to the ends of the channel are called the source and the drain; those attached to the two p-type regions are called gates. A voltage applied to the gates is directed so that no current exists across the junctions between the p- and n-type materials; for this reason it is called a reverse voltage. Variations of the magnitude of the reverse voltage cause variations in the resistance of the channel, enabling the reverse voltage to control the current in the channel. A p-channel device works the same way but with all polarities reversed.
The metal-oxide semiconductor field-effect transistor (MOSFET) is a variant in which a single gate is separated from the channel by a layer of metal oxide, which acts as an insulator, or dielectric. The electric field of the gate extends through the dielectric and controls the resistance of the channel. In this device the input signal, which is applied to the gate, can increase the current through the channel as well as decrease it.
Uses of Transiror
There are two main uses of transistors.
Amplifiers: In this situation, a relatively weak signal fed into the base gets amplified into a much larger current flowing from the emitter to the collector. This amplification, which can be up to 100 times or more, is used, for example, in stereo systems to amplify the relatively weak electrical signal coming from a tape reader into a signal strong enough to drive a speaker.
Switches: A transistor can also be used as a switch: a certain type of signal into the base will cut off the current flowing from the emitter to the collector. Removing this signal will allow this current to flow again. Although relatively simple, this is the basic use of a transistor in computers, of which we now discuss.June- July Class XI
Resistance & Color code
Electrical resistances are mass-produced by mixing graphite (a conductor) with clay (an insulator), molding the compound with a binder (glue) with metal wires imbedded for electrical contacts. The resistance is governed by the ratio of clay to graphite. Possible values of electrical resistance in these devices ranges from 10 Ohms to about 20 Million Ohms (2 x 107) Ohms. The physical size of the resistor determines how much power the resistor can absorb without overheating. The most common physical size available in the lab will dissipate ¼ Watt. Composition resistors are color coded to indicate resistance values or ratings. The color code consists of various color bands, which indicate the resistance values of resistors in ohms as well as the tolerance rating.
Color Code
The fifth band is rare (esp. on older resistors used in instrument amps). Unfortunately there have been two separate systems which used 5 bands: in the older system the 5th band represents the percentage the resistance may change per 1000 hours of operation. In the newer system, the 3rd band becomes the third significant digit, the 4th band becomes the multiplier, and the 5th band denotes tolerance. Hopefully you won't have to deal with any of these. (I have seen 5 band resistors with bands which appeared to be centered along the resistor's length, making it difficult (in some case impossible) to know which way to read the value. In these cases you will have to measure the resistor's value with an ohmmeter.)
Calculate the Resistances by seaing color Bands
Resistance
Resistance is the property of a component which restricts the flow of electric current. Energy is used up as the voltage across the component drives the current through it and this energy appears as heat in the component. Two elements are said to be in series whenever the same current physically flows through both of the elements. The critical point is that the same current flows through both resistors when two are in series.
1 Ohm is quite small for electronics so resistances are often given in kilo Ohm and Mega Ohm.1 kilo ohm = 1000 Ohms 1 M Ohm = 1000000 Ohms. Resistors used in electronics can have resistances as low as 0.1 ohm or as high as 10 Mega Ohms
Resistors connected in Series
When resistors are connected in series their combined resistance is equal to the individual resistances added together. For example if resistors R1 and R2 are connected in series their combined resistance, R, is given by:
Combined resistance in series: R= R1 + R2
This can be extended for more resistors: R= R1+ R2 + R3 + R4 ...
Mega Ohm.
Resistors connected in Parallel
When resistors are connected in parallel their combined resistance is less than any of the individual resistances. There is a special equation for the combined resistance of two resistors R1 and R2:Combined resistance of two resistors in parallel: | R= R1 x R2 |
R1 + R2
For more than two resistors connected in parallel a more difficult equation must be used. This adds up the reciprocal ("one over") of each resistance to give the reciprocal of the combined resistance, R:
1 | = | 1 | + | 1 | + | 1 | + ... |
R | R1 | R2 | R3 |
The other common connection is two elements in parallel. Two resistors or any two devices are said to be in parallel when the same voltage physically appears across the two resistors. Schematically, the situation is as shown below.
