Safety
requirements
Do not wear loose garments in the workshop. One must know how to start electrical equipment and how to stop it.
Before working on electrical system, the main switch and specific switch for the particular system must be known.
Electrical connecting cable should not be left loose, it may cause failure of supply or shorting. Ensure the cable condition, its connecting points, should be correct and properly connected. In dc circuit positive and negative points must be marked and proper connection is given. Ensure for security of mounting of the equipment and suitable cables, etc.
Check for visual defect of any component before energising it.
Ensure that after utilising the electrical circuit, the main switch must be put off, if not required.
First aid box must be available.
All electrical tools must be maintained in a fit and safe condition.
The handless hammers, files and screw drivers should be of approved length and made of hard wood.
Each tools should be used only for the work it is intended for. Electrical tools fitted with insulating handles serve as a main protective measure for working on live parts with voltages up to 1000 Volts.
Tools must be inspected for fitness once in a six months. The handles of the above tools must be smooth and be made from a tough and moisture resistance insulating material free from cracks.
In case of electric shock, rush the victim to the nearest available doctor.
All cables in an electrical installation are to be of highly insulated preferably of vulcanised rubber insulated type. It should be taped and braided or protected with tough and braided rubber compound.
Joints are to be as few as possible and must be mechanically and electrically sound. All single pole switches must be fitted in the live wire only.
Careful attention must be given to plugs, sockets and lamp holders. Plug pins should be kept clean and free from oxidation.
Earthing is of vital importance for safety from electrical shock. It provides protection of connecting to earth the parts of electrical equipment which normally operates without any potential. It does this through an earthing system or electrodes are buried or directly driven in contact with the soil. The earthing system or electrodes are needed to ensure intimate electrical contact with the soil. Earthing safe all metallic parts of electrical equipment (frames) and enclosure of electrical machines, switch-gear operating mechanism, switch board frame works, metal cable sheets, which must not operate at an electrical potential but which can become alive because of a failure in the insulation. 3-pin plugs and sockets with proper pins connected to earth are only to be used in the workshop. Isolate the branch of wiring by pulling out CB/ Fuse and switches before commencing working on the branch. In case of electrical fire do not use water to extinguish the fire. Isolate the wire and use Halon/ CTC fire extinguisher.
Electron
theory
Matter is anything that occupies space. It is universally accepted that matter is composed of molecules, which in turn are composed of atoms. The smallest particle into which any compound can be divided and still retains its identity is called a molecule. An atom is the smallest possible particle of an element. An element is a single substance that cannot be separated into different substances except by nuclear disintegration. There are more than 100 elements some of which are radio active. Some of the common elements are iron, O2, H2, Al, Cu, Pb, Gold, Silver and so on. The smallest division of any of these elements will still have the properties of that element. A compound is a chemical combination of two or more different elements, and the smallest possible particle of a compound is a molecule. An atom consists of infinitesimal particles of energy known as electrons, protons and neutrons. All matter consists of two or more of these basic components. The simplest atom is that of hydrogen, which has one electron and one proton. Oxygen has eight protons, 8 neutrons and 8 electrons. The protons and neutrons from the nucleus of the atom. Protons have positive charges and the electrons negative charges. When the charges of the nuleus is equal to the combined charges of the electrons, athe atom is neutral. But if the atom has the shortage of electrons, it will be positively charged, callednegative ion. The protons remain in the atom, only the electrons are removed or added to an atom.
These tiny elementry particles of matter are similar to tiny solar systems with a nucleus consisting of positively charged protons and uncharged neutrons. Negatively charged electrons circle the nucleus and are held to it by a strong arrractive force. The centrifugal force of the spinning electron exactly balances this force of attraction, and the atom is considered balanced.The protons and the neutrons provide the weight of the atom and its positive charge. The negative electrical charges of the electrons exactly balance the positive charges of the protons, but the mass of the proton is 1847 times that of electron. Since they have the same charge, the electron has to be much larger. The diameter of an electron is about 1800 times that of a proton. For example, a Copper atom has 29 electrons, but only one on its outer shell, movement of this electron is called current flow. The electrons move around the nucleus in round or elliptical paths formins an imagenary shell. When an atom has more than 2 electrons it must have more than one shell or orbit, since the first shell can accommodate only 2 electrons. The no. of shells in an atom depends upon total no. of electrons surrounding the nucleus. Certain elements, like metal are known a sconductors because give up or receive electrons easily. The electrons that move from one a atom to another are called free electrons. Free electrons randomly drift through the atoms of any conductor. But when these free electrons move in the same direction due to a potential difference a current flow is created. The outermost orbit is known as valence orbit, and the electtons belonging to this orbit are known as valence electrons. The fewer valence electrons in an atom, the easier it will accept extra electrons. Atom with fewer than half of theit valence electrons tend to easily accept (carry) the moving electrons of an electric current flow. Such materials are called conductors. Materials with more than half of their valence electrons are called insulators. Insulators will not easily accept extra electrons. Materials with exactly half of their valence electrons are semiconductors. 2 best conductors are gold and silver, their valence orbit is nearly empty (one each). But 2 insulators, neon and helium, they have full volume orbits. Common conductors are Cu and Al, common insulators are air, plastic fibre glass, rubber. Common semiconductors are germanium and silicon (4 valence electrons each), Less than 4 valence electrons-conductors, more than 4 valence electrons-insulators.
Static electricity-The study of the behavior of static electricity is called electrostatics. The
word static means stationary or at rest, and the electric charges that are at rest are called static electricity. A material with atoms containing equal nos. of electrons and protons is electrically neutral. If the no. of electrons should increase or decrease, the material is left with a static charge. An excess of electrons creates a negatively charged body. This excess or deficiency of electrons can be used by the friction between the two dissimilar substances or by contact bya neutral body and charged body. If friction produces the static charge, the nature of the charge is determined by the types of substances. Following chart is known as electric series and the list is so arranged that each substance is positive in rotation to anyone that follow it, when the two are in contact. (Fur, Flannel, Ivory, Crystals, Glass, Cotton, Silk, Eather, The body, Wood, Metals, Sealing wax, Resins, Gutta percha, Gun cotton). If for example, a glass rod is rubbed with fur, the rod becomes negatively charged, but if it rubbed with silk, it becomes positively charged. The force that is created between two charged bodies is called the electrostatic force.The electrostatic charge between those two charged bodies is inversely propotional to square of the distance between those two bodies. That is, as the distance becomes twice as large between the bodies, the electrostatic force is one fourth as great. As well as across certain types of automobile seat covers with our clothing being of synthetic materials, as we slide across the seats, we assume a charge d ifferent from that the seat. Both the seats and our clothes are poor conductors & there will be little tendency for these charges to neutralize until we reach for the door handle, then we get a good zap as the electrons flow between the handle and our hand. Static charges on a/c control surfaces have been a source of radio noise for years and various steps have been adopted this interferences. A conductive bonding braid is attached between all movable control surfaces and the main portion of the a/c structure. Static charge from the air passing over the surface will not have to build up enough to bridge the gap through the more poorly conductive hinges, but will neutralize through the braids. Another method of neutralizing static electricity is the lightning which occurs in electrical storms. The motion of air creates charged conditions among the clouds. These charges build up into values of thousands of volts and eventually become so strong they jump from cloud to cloud or between the cloud and ground. A larger or smoother surface of the aircraft, more electrons will be stored for a given pressure. It is for this reason that static electricity into the air before it can build up enough quantity to cause radio interferance. The sharp points of the dischargers concentrate the
By friction-In this method, electrons in an insulator can be separated by the work of rubbing to
produce opposite charges that remain in the dielectric. Examples of how static electricity can be generated include combing the hair, walking across a carpeted room or sliding two pieces of plastic across each other. An electrostatic discharge (ESD) occurs when one of the charged objects comes into contact with another dissimilarly charged object. The electrostatic discharge is in the form of a spark. The current from the discharge lasts for only a very short time but can be very large.
From chemical energy-Wet or dry cells and batteries are the applications. Here a chemical
reaction produces opposite charges on two dissimilar metals, which serve as the negative and positive terminals.
Electromagnetism-Electricity and magnetism are closely related. Any moving charge has an
associated magnetic field, also, any changing magnetic field can produce current. A motor is an example of how current can react with a magnetic field to produce motion, a generator produces voltage by means of a conductor rotating in a magnetic field.
Photo electricity-Some materials are photoelectric, that is, they can emit electrons when light
strikes the surface. The element Caesium is often used as a source of photoelectrons. Also, photovoltaic cells or solar cells use silicon to generate output voltage from the light input. In another effect, the resistance of the element Selenium changes with light. When this is combined with a fixed voltage source, wide variations between dark current and light current can be produced. Such characteristics are the basis of many photo- electric devices, including television camera tubes, photoelectric cells, and phototransistors.
Thermal emission-Some materials when heated can "boil off" electrons from the surface. Then
these emitted electrons can be controlled to provide useful applications of electric current. The emitting electrode is called a cathode, an anode is used to collect the emitted electrons.
Emf & Potential difference-Electro motive force or potential of a body is the work done in
joules to bring a unit electric charge from infinity to the body. If an excess of electrons with a negative charge exist at one end of a conductor and a deficiency of electrons with a positive charge at the other, an electrostatic field exists between the two charges. Electrons are repelled from the negatively charged point and are attracted by the positively charged point. The flow of electrons (Charge on an electron= 1.6 x 10-19C, 1 Coulomb= 6.28 x 1018 electrons) from a negative point to a positive point is called an electric current, this current flows because of a difference in electric pressure between the two points. The force that causes the movement of electron from a point of excess electrons to a point deficient in electron is the potential difference or the electromotive force. The unit of measurement of e.m.f or potential difference is the volt.
Current-An electric current is the result of the movement of electrons through a conductor.
Since, a negatively changed body has an excess of electrons and a positively charged body a deficiency of electrons thus the electrons flow will be form the negatively charged body to the positively charged body when the two are connected by a conductor. It can be therefore said that electricity flows from negative to positive. Since current is the movement of charge, the unit for stating the amount of current is defined as the rate of flow of charge. When the charge moves at the rate of 6.25x1018 electrons flowing past a given point per second, the value of current is one ampere, this is the same as one coulomb of charge per second.
The electric charges that are at rest set up static electricity. Static electricity can be produced by contact, friction or induction. As an example of the friction method, a glass rod rubbed with fur becomes negatively charged, but if rubbed with silk, becomes positively charged. Some material that build up static electricity easily are silk, hard rubber and glass. The force created between two charged bodies is called the electrostatic force. This force can be attractive or repulsive, depending on the object’s charge. Like charges repel each other, unlike charges attract each other. Charge on a hollow sphere, which is made of conducting material, shows the inner surface is neural. This phenomenon is used to safeguard operating personnel of the large Van de Graff generators used for atom-smashing. The safest area for the operators is inside the large sphere, where millions of volts are being generated. Static interference in the aircraft communication systems and the static charge created by the aircraft’s movement through the air are examples of problems created by static electricity. Parts of the aircraft must be bonded or joined together to provide a low-resistance or easy path for static discharge, and radio parts must be shielded. Static charges must be considered in the refuelling of the aircraft to prevent
possible igniting of the fuel, and provision must be made to ground the aircraft structure, either by static-conducting tires or by a grounding wire.
Resistance-The property of a conductor of electricity that limits or restricts the flow of electric
current. Resistance may also be termed as electrical friction because it affects the movement of electricity in a manner similar to the effect of friction on mechanical objects. The unit to measure resistance is the Ohm. The reciprocal of resistance is known as conductance and is a measurement of the ease with which the current will flow through a substance. The unit conductance is Mho or Siemens.
Fuse & Current limiter-A fuse is a strip of metal that will melt when current in excess of its
carefully determined capacity flows through it. The fuse is installed in the circuit so that all the current in the circuit passes trough it. In most fuses, the strip of metal is made of an alloy of tin and bismuth. Other fuses are made of copper and are called current limiters, these are used primarily to sectionalise an aircraft circuit. A fuse melts and breaks the circuit when the current exceeds the rated capacity of the fuse, but a current limiter will stand a considerable overload for a short period of time. Since the fuse is intended to protect the circuit, it is quite important that its capacity match the needs of the circuit in which it is used. When a fuse is replaced, the applicable manufacturer's instructions should be consulted to be sure a fuse of the correct type and capacity is installed. Fuses are installed in two type fuse holders in aircraft. "Plug-in holders" are used for small type and low capacity fuses. "Clip" type holders are used for heavy high capacity fuses and current limiters.
Circuit breaker-A circuit breaker is designed to break the circuit and stop the current flow
when the current exceeds a predetermined value. It is commonly used in place of a fuse and may sometimes eliminate the need for a switch. A circuit breaker differs from a fuse in that it "trips" to break the circuit and it may be reset, while a fuse melts and must be replaced. There are several types of circuit breakers in use in aircraft systems. One is a magnetic type. When excessive current flows in the circuit, it makes an electromagnet strong enough to move a small armature which trips the breaker. Another type is the thermal overload switch or breaker. This consists of a bimetallic strip which, when it becomes overheated from excessive current, bends away from a catch on the switch lever and permits the switch to trip open. Most circuit breakers must be reset by hand. When the circuit breaker is reset, if the overload condition still exists, the circuit breaker will trip again to prevent damage to the circuit.
Thermal protector-A thermal protector or switch is used to protect a motor. It is designed to
open the circuit automatically whenever the temperature of the motor becomes excessively high. It has two positions open and closed. The most common use for a thermal switch is to keep a motor from overheating. If a malfunction in the motor causes it to overheat. the thermal switch will break the circuit intermittently. The thermal switch contains a bimetallic disk or strip, which bends and breaks the circuit when it is heated. This occurs because one of the metals expands more than the other when they are subjected to the same temperature. When the strip or disk cools the metals contract and the strip returns to its original position and closes the circuit.
Relays-Relays, or relay switches, are used for remote control of circuits carrying heavy
currents. A relay is connected in the circuit between the unit controlled and the nearest source of power (or power bus bar) so that the cables carrying heavy current will be as short as possible. A relay switch consists of a coil, or solenoid, an iron core, and both fixed and movable contacts. A small wire connects one of the coil terminals (which is insulated from the housing) to the source of power through a control switch usually located in the cockpit. The other coil terminal is usually grounded to the housing. When the control switch is closed, an electromagnetic field is set up around the coil. In one type of relay switch, an iron core is fixed firmly in place inside the coil. When the control switch is closed, the core is magnetized and pulls a soft iron armature towards it, closing the main contacts. The contacts are spring-loaded to the open position. When the control switch is turned off, the magnetic field collapses and the spring opens the contacts. In another type of relay switch, part of the core is movable. A spring holds the movable part a short distance away from the fixed part. When the coil is energised, the magnetic field tries to pull the movable part of the coil. This pull overcomes the spring tension. As the core moves inward, it brings the movable contacts, which are attached to but insulated from it, down against the stationary contacts. This completes the main circuit. When the control switch is turned off, the magnetic filed collapses and the spring returns the movable core to its initial position, opening the main contacts. Relays vary in constructional details
continuously, while others are designed to operate only intermittently. The starter-relay switch is made to operate intermittently and would overheat if used continuously. The battery-relay switch can be operated continuously because its coil has a fairly high resistance which prevents overheating. In a circuit carrying a large current, the more quickly the circuit is opened the less it will arc at the relay and the less the switch contacts will be burned. Relays used in circuits with large motors have strong return springs to open the circuit quickly. Most of the relays used in a.c circuitry of an aircraft are energised by d.c. current.
Solenoid-A coil of wire conductor with more than one turn is generally called a solenoid. An
ideal solenoid, however, has a length much greater than its diameter. Like a single loop, the solenoid concentrates the magnetic field inside the coil and provides opposite magnetic poles at the ends. These effects are multiplied, however, by the number of turns as the magnetic field lines aid each other in the same direction inside the coil. Outside the coil, the field corresponds to a bar magnet with north and south poles at opposite ends.
Thermocouples-When two metals having different work functions are placed together, a
voltage is generated at the junction which is nearly proportional to the temperature. The junction is called a thermocouple. This principal is used to convert heat energy to electrical energy at the junction of two conductors The e.m.f produced is proportional to the temperature and hence to the r.m.s. value of the current. Therefore the scale of a permanent magnet moving coil instrument can be calibrated to read this current. The thermocouple type of instruments can be used for both d.c & a.c applications. The most attractive feature of thermocouple instruments is that they can be used for measurements of current and voltage even at very high frequencies.
Switches-Switches control the current flow in most aircraft electrical circuits. A switch is used
to start, to stop or to change the direction of the current flow in the circuit. The switch in each circuit must be able to carry the normal current of the circuit and must be insulated heavily enough for the voltage of the circuit. Knife switches are seldom used on aircraft. They are included here to simplify the operation of the toggle switch. Toggle switches operate much the same as knife switches, but their moving parts are enclosed. They are used in aircraft circuits more than any other kind of switch. Toggle switches, as well as some other type of switches are designated by the number of poles, throws and positions they have. A pole of a switch is its movable blade or contactor. The number of poles is equal to the number of circuits or paths for current flow that can be completed through the switch at any one time. The throw of a switch indicates the number of circuits, or paths for current, that it is possible to complete through the switch with each pole or contactor. The number of positions a switch has is the number of places at which the operating device (toggle, plunger etc.) will come to rest and at the same time open or close one or more circuits.
A toggle switch that is spring-loaded to the OFF position and must be held in the ON position to complete the circuit is a momentarily contact two-position switch. One that will come to rest of two positions, opening the circuit in one position and closing it in another, is a two-position switch. A toggle switch that will come to rest at any one of the three positions is a three-position switch. A switch that stays open, except when held in the closed position, is a normally open switch (usually identified as NO). One that stays closed, except when it is held in the open position is a normally closed switch (NC). Both kinds are spring-loaded to their normal position and will return to that position as soon as they are released. When it is possible to complete only one circuit through a switch, the switch is a single-pole-single-throw (SPST) switch. A single-pole switch through which two circuits can be completed (not at the same time) is a single-pole-double-throw (SPDT) switch.
Single Pole Single
A switch with two contactors, or poles, each of which completes only one circuit, is a double-pole-single-throw (DPST) switch. A double-pole switch that can complete circuits, one circuit at a time through each is a double-pole-double-throw (DPDT) switch.
Double Pole Single
Throw Knife Switch Double Pole Double Throw Knife Switch Single Pole Single Throw Toggle Switch Double Pole Double Throw Toggle Switch
A switch must be capable to carry a greater load than the nominal running load of the circuit in which it is installed. Accordingly, derating factors are applied in determining the capacity of a switch for a particular installation. The derating factor is a multiplier which is used to establish the capacity a switch should have in order to control a particular type of circuit without damage.
Nominal system voltage Type of load Derating factor
24 Lamp 8 24 Inductive 4 24 Resistive 2 24 Motor 3 12 Lamp 5 12 Inductive 2 12 Resistive 1 12 Motor 2
Push-button switches-Push-button switches have one stationary contact and one movable
contact. The movable contact is attached to the push button. The push button is either an insulator itself or is insulated from the contact. The switch is spring-loaded and designed for momentary contact.
Micro-switches- A micro-switch will open or close a circuit with a very small movement of the
tripping device. These are usually push-button switches. They are used primarily as limit switches to provide automatic controls of landing gear, actuator motors, and the like. When the operating plunger is pressed in, the spring and the movable contact are pushed, opening the contacts and the circuit.
Rotary selector switch-A rotary selector switch takes the place of several switches. When the
knob of the switch is rotated, the switch opens one circuit and closes another. Ignition switches and voltmeter selector switches are typical examples of this kind of switch.
Rotary Selector Switch
Switches should always be installed in panels so the lever will be moved up or forward to turn the circuit on. For switches which operate movable parts of the aircraft, the switch should be installed so the switch lever is moved in the same direction that the aircraft part will be moved. The landing gear switch should be installed so the switch lever will be moved down to lower the landing gear and up to raise the gear. The same principle should apply for using flap operation.
Capacitor & Capacitive reactance- A capacitor, sometimes called a condenser, is a device
that stores electrical energy in the electrostatic fields that exist between two conductors that are separated by an insulator, or a dielectric. The capacitance is directly proportional to area of the plates & the dielectric constant & is inversely proportional to the distance between the plates. The unit of capacitance is Farad. The capacitive reactance is the resistance offered by a capacitor to alternating current. This is expressed as Xc=1/(2πfC), where f is the frequency & C
Inductor & Inductive reactance- Inductance is the ability of a conductor to induce a voltage
into itself when a change in current is applied to the inductor. The inductance of a single straight wire is usually negligible. However, if the wire is wound into a coil, the inductance value increases significantly. This is due to the relatively strong magnetic field produced by the conductor flowing through the coil of wire. It is the increase or decrease of this magnetic field that produces the coil's inductance. The inductance of a coil is measured in a unit called the Henry (H). One Henry is the inductance of a coil, when a change of current of one ampere peer second will induce an e.m.f of one volt. The symbol for inductance is the letter L. The Henry is too large a unit for most applications, and so a smaller unit called the millihenry is used. The faster the current changes, the higher the induced voltage because when the flux moves at a higher speed, it can induce more voltage. Since inductance is a measure of induced voltage, the amount of inductance has an important effect in any circuit in which the current changes.
The inductance is an additional characteristic of the circuit besides its resistance. The characteristics of inductance in –
AC circuits-Here the current is continuously changing and producing induced voltage. Lower
frequencies of alternating current require more inductance to produce the same amount of induced voltage as a higher- frequency current. The current can have any waveform, as long as the amplitude is changing.
DC circuits-in which the current changes in value-It is not necessary for the current to reverse
direction. One example is a dc circuit being turned on or off. When the direct current is changing between zero and its steady value, the inductance affects the circuit at the time of switching. This effect with a sudden change is called the transient response. A steady direct current that does not change in value is not affected by inductance, however, because there can be no induced voltage without a change in current. The effect of inductance in an ac circuit is called inductive reactance, and is measured in ohms because it resists the flow of current in the circuit.
Inductive reactance is the opposition to current flow created by inductors in an ac circuit. The inductive reactance in a circuit is proportional to the inductance of the circuit and the frequency of the alternating current. As the inductance is increased, the induced voltage (which opposes the applied voltage) is increased, hence the current flow is reduced. Similarly, when the frequency of the circuit is increased, the rate of current change in the inductance coil is also increased, hence the induced (opposing) voltage is higher an the inductive reactance is increased. As inductive reactance increases, current in the circuit is reduced. This is expressed as XL=2πfL where, XL=inductive reactance in ohms, f = frequency in Hz & L=inductance in henry.
D.c source of
electricity
Primary cells & Secondary cells-The term “Battery” means an assembly of voltaic primary or
secondary cells. Batteries of secondary cells are also known as storage batteries or accumulators. In primary as well as secondary cells, the electrical energy is produced from the chemical energy liberated as a result of the chemical reactions taking place in the cell. In these reactions ions play an active role. Certain ions tend to react with the electrons from ions to electrode or vice-versa. As the reaction proceeds by closing the external circuit to which the battery is connected, the transfer of electrons from one electrode to the other gives rise to an electric-current flowing in the external circuit. In both types the individual cells consists of a positive and negative electrode, immersed in an electrically-conducting fluid called the electrolyte and generally separated by a porous insulating diaphragm, called the separator. The electrode must be electrically conducting. In dry cells outer metal container may constitute one of the electrode.
An electric storage battery differs from a primary cell, in that the latter depends for its functioning upon the consumption of a metallic electrode, usually zinc, by the action of electrolyte and can not be electrically recharged, whereas a storage battery after being exhausted by discharge can be brought back to a full state of change by passing a current through it and this can be repeated number of times. In storage batteries, the energy is actually stored in chemical-form.
The voltaic cell-When two different conducting materials are immersed in an electrolyte, the
chemical action of forming a new solution results in the separation of charges. It is also called a galvanic cell. The potential difference resulting from the separated charges enables the cell to function as a source of applied voltage. The voltage across the cell’s terminals forces current to flow in the circuit.
Current outside the cell-Electrons from the negative terminal of the cell flow through the
external circuit with load resistance and return to the positive terminal. The chemical action in the cell separates charges continuously to maintain the terminal voltage that produces current in the circuit. The current tends to neutralize the charges generated in the cell. For this reason, the process of producing load current is considered discharging of the cell. However, the internal chemical reaction continues to maintain the separation of charges that produces the output voltage.
Current inside the cell-The current through the electrolyte is a motion of ion charges. The
current inside the cell flows from the positive terminal to the negative terminal. This action represents the work being done by the chemical reaction to generate the voltage across the output terminals. The negative terminal is considered to be the anode of the cell because it forms positive ions for the electrolyte. The opposite terminal of the cell is its cathode.
Lead-acid battery-In a lead acid battery, the positive electrode is lead peroxide and the
negative electrode is spongy lead. The electrolyte is dilute sulphuric acid. In these batteries each cell contains grid shaped lead plates, which are filled with chemically active material. The negative plate looks grey (spongy lead), the positive plate brown (lead peroxide). Since, the capacity depends on the size of the plate there are always several plates each combined to one element by connecting post-straps and feature of terminal post for outside connection. To prevent any risk of touching of plates being slid in one another, separators are installed, a thin wooden sheet and a corrugated perforated plastic-sheet. The container and the cell-covers consist of acid-proof insulating material, hard-rubber or also plastic. The plates rest on ridges in the bottom. In aircraft batteries, the cell openings are closed with vented screw-in-type caps which have lead-weights inside them to close the vent when the battery is tipped. This prevents the electrolyte spilling in unusual flight-attitudes.
Nickel cadmium battery-Nickel Cadmium batteries are made up of individual removable cells.
It consists of positive and negative plates, separators, electrolyte, cell vent and cell-container. The positive plates are made from a porous plaque on which Nickel Hydroxide has been deposited. The negative plates are made from similar plaques on which Cadmium Hydroxide is deposited.
In both cases, the porous plaque is obtained by sintering nickel powder at a high temperature. After the active positive and negative materials are deposited on the plaque, it is formed and cut into the proper plate size. A nickel tab is then welded to a corner of each plate and the plates are assembled with the tabs welded to the proper terminals. A continuous strip of porous plastic separates the plates from each other. The electrolyte used is 30% by weight of KOH (Potassium Hydroxide) in distilled water. The specific gravity of the electrolyte remains between 1.240 and 1.300 at room temperature. No appreciable change occurs in the electrolyte during charge or discharge. As a result, the battery charge can not be determined by a specific gravity check of the electrolyte. The electrolyte-level should be maintained just above the tops of the plates.
Operation-When a charging current is applied to a Ni-Cd battery, the negative plates lose
oxygen and begin forming metallic Cd. The active material of the positive-plates, Nickel Hydroxide, becomes more highly oxidized. This process continues while the charging current is applied or until all the oxygen is removed from the negative-plates and only Cadmium remains. Towards the end of the charging cycle, the cells emit gas. This will also occur if the cells are overcharged. This gas is caused by decomposition of the water in the electrolyte into hydrogen at the negative plates and oxygen at the positive plates. The voltage used during charging, as well as the temperature, determine when gassing will occur. To completely charge a Ni-Cd battery, some gassing, however-slightly, must take place, thus, some water will be used. The chemical action is reversed during discharge. The positive plates slowly give up oxygen, which is regained by the negative plates. This process results in the conversion of the chemical energy into electrical energy. During discharge, the plates absorb a quantity of the electrolyte. On recharge, the level of the electrolyte rises and at full charge, the electrolyte will be at its highest
buildup of internal resistance and subsequent voltage-drop during high-rate discharges. The Ni-Cd battery has a very low internal resistance and so its voltage remains constant until it is almost totally discharged. This low resistance will accept high charging currents without damage. While high charging currents are possible, there are dangers involved. These dangers begin with a breakdown of the cellophane-like material that separates the plates in the Ni-Cd cell. This breakdown is usually the result of high temperatures resulting from high rates of charge. The breakdown of the cell separator creates a short-circuit and current-flow creates heat, the heat will cause further breakdown of the separator-material and the cycle continues. This process is known as vicious-cycling or thermal runaway.
Cell imbalance-In Ni-Cd battery the negative plate controls the cell's voltage characteristics.
This, with slightly lower charge efficiency in the positive-plates, results in an imbalance between the negative and positive plates in each cell. Constant-Voltage-Charging is unable to recognize this condition, for voltage-wise the battery appears to be fully charged. As long as the battery stays on a constant-voltage-charger, the imbalance condition will become a little worse each time the battery is cycled, until the battery's available capacity will be too small to crank engines or supply emergency power. The cell-imbalance condition is reduced by more sophisticated techniques, such as pulse charging or terminating the constant-potential-charging, when the battery is close but below full charge. Then, proceed to complete the charge at a constant-current rate of approximately 10% of the ampere-hour capacity of the battery. This technique when compensated for temperature will drive the negative-plates into a controlled overcharge, which allows the positive plates to be brought to full-charge without generating so much gas as to damage the gas barrier.
Alkaline cell-A popular type is the manganese-zinc cell which has an alkaline electrolyte. It is
available as either a primary or a secondary cell but the primary type is more common. Output is the same 1.5 V as a carbon-zinc cell but the alkaline cell lasts much longer. The electrochemical system consists of a powdered zinc anode and a manganese dioxide cathode in an alkaline electrolyte. The electrolyte is potassium hydroxide, which is the main difference between the alkaline and Leclanche cells. Hydroxide compounds are alkaline with negative hydroxyl (OH) ions, whereas an acid electrolyte has positive hydrogen (H) ions. Voltage output from the alkaline cell is 1.5 V.
The alkaline cell has many applications because of its ability to work with high efficiency with continuous high discharge rates. Depending on the application, an alkaline cell can provide up to seven times the service of a Leclanche cell. As examples, in a transistor radio an alkaline cell will normally have twice the service life of a general-purpose carbon-zinc cell, in toys the alkaline cell typically provides seven times more service. The outstanding performance of the alkaline cell is due to its low internal resistance. Its internal resistance is low because of the dense cathode material, the large surface area of the anode in contact with the electrolyte, and the high conductivity of the electrolyte. In addition, alkaline cells will perform satisfactorily at low temperatures.
Zinc chloride cells-This type is actually a modified carbon-zinc cell. However, the electrolyte
contains only zinc chloride. The zinc chloride cell is often referred to as the heavy duty type. It can normally deliver more current over a longer period of time than the Leclanche cell. Another difference is that the chemical reaction in the zinc chloride cell consumes water along with the chemically active materials, so that the cell is nearly dry at the end of its useful life. As a result, liquid leakage is not a problem.
Cells connected in series & parallel-When cells are connected in series, the total voltage
available across the combination is the sum of all the individual voltages of the cells, though the current delivery capacity remains the same. When the cells are connected in parallel, the total voltage across the combination equals the voltage of an individual cell but the total current delivery capacity is the sum of the currents given out by all the cells.
Connecting batteries in series increases the total voltage but not the ampere-hour capacity. In multiengine aeroplanes, where more than one battery is used, the batteries are connected in parallel, increasing the ampere-hour capacity. The voltage is equal to that of one battery but the ampere-hour capacity is increased. The total capacity is the sum of the ampere-hour ratings for the individual batteries.
Internal resistance & its effect on a battery -Any source that produces voltage output
continuously is a generator. It may be a cell separating charges by chemical action or a rotary generator converting motion and magnetism into voltage output, for common examples. In any case, all generators have internal resistance.
The internal resistance is important when a generator supplies load current because its internal voltage drop subtracts from the generated e.m.f, resulting in lower voltage across the output terminals. Physically, the internal resistance may be the resistance of the wire in a rotary generator or in a chemical cell internal resistance is the resistance of the electrolyte between electrodes. More generally, the internal resistance is the opposition to load current inside the generator. Since, any current in the generator must flow through the internal resistance, internal resistance is in series with the generated voltage,. It may be of interest to note that, with just one load resistance connected across a generator, they are in series with each other because the load resistance is in series with the internal resistance. If there is a short circuit across the generator, its internal resistance prevents the current from becoming infinitely high. As an, example, if a 1.5- V cell is temporarily short-circuited, the short-circuit current could be about 15 A. Then the internal resistance equals 1.5 A/15V or 0.1 ohm, for the internal resistance. These are typical values for a carbon-zinc D-size cell.
Chemical changes during charging & discharge-When a conductor connects the positive
and negative terminals of the battery, electrons flow from the lead to the lead peroxide. When electrons leave the lead, it leaves behind positive ions which attract the negative sulphate radicals from the sulphuric acid in the electrolyte. This combination forms lead sulphate on the negative-plate. The electrons arriving at the positive plate drive the negative oxygen radicals from the lead-peroxide. This oxygen joins up with the hydrogen in the electrolyte that has lost its sulphate radical and this now becomes water. The lead that was left on the positive plate attracts sulphate radicals from the electrolyte and becomes lead sulphate. Now, with lead sulphate on both the positive and negative plates and with the electrolyte diluted by the water that has formed in it, the battery is discharged and electrons no longer flow.
Five hour discharge rate-The standard rating used to specify the capacity of a battery is the
five-hour discharge rating. This is the number of ampere-hours of capacity of the battery. When there is sufficient current-flow to drop the voltage of a fully charged battery to 1.75 volts per cell at the end of five hours. If a discharged battery is attached to a source of direct current having the proper voltage and the positive plates of the battery connected to the positive terminal of the source, electrons will be drawn from the positive plate and forced into the negative plates. Electrons arriving at the negative plates drive the negative sulphate radicals out of the lead sulphate back into the electrolyte, where they join with the hydrogen from the water to form sulphuric acid. When the electrons flowed from the positive plates, they left behind positively charged lead atoms which attract oxygen from the water in the electrolyte to form lead peroxide (PbO). Now, when the battery is fully charged, the positive plate has again become lead peroxide, the negative plate has become lead, and the electrolyte again has a high concentration of sulphuric acid. All during the charging process, as the electrolyte is being changed back into sulphuric acid, hydrogen gas is released in the form of bubbles. As the charge is completed, the bubbling increases.
Condition of charge
Specific gravity-The open circuit voltage of a lead-acid battery remains relatively constant, at
about 2.1 volt per cell and so does not indicate the state of charge of the battery. The electrolyte of a fully charged battery will have a specific gravity of between 1.275 and 1.3 with an electrolyte temperature of 80º F. A specific gravity reading between 1.300 and 1.275 indicates a high state of change, between 1.275 and 1.240, a medium state of charge and between 1.240 and 1.200, a low state of charge.
Specific gravity= Weight of the substance/ Weight of an equal volume of water or, Density of the substance/ Density of water
When the battery is discharged until its specific gravity is down to 1.150, there is not enough chemical strength in the electrolyte convert the active materials into lead sulphate and the battery is considered to be discharged.
Voltage -The open circuit voltage of a lead-acid battery is 2.10 volt per call when the
electrolyte has a specific gravity of 1.265. The physical size of the cell or the number of plates has no effect on this voltage. When a load is placed on the battery, the active material begins to convert into lead sulphate which has a higher resistance than the fully-charged plates. This increased internal resistance will cause the closed-circuit terminal voltage to drop and when it is down to about 1.75 volts per cell, the battery is, for all practical purposes, discharged.
Ampere-hour capacity-The capacity of battery is its ability to produce a given amount of
current for a specified time and is expressed in ampere-hours. Theoretically, a 100 Ampere-hour battery will be able to produce 100 amps for one hour, 50 amps for two hours or 20 amps for five hours. The amount of active material, the area of the plates and the amount of electrolyte determine the capacity of a battery.
Lead acid battery charging method-A storage battery may be charged by passing
direct-current through the battery in a direction opposite to that of the discharge direct-current. Because of the internal resistance in the battery, the voltage of the external charging source must be greater than the open-circuit voltage e.g. the open circuit voltage of a fully-charged 12-cell lead-acid battery is approximately 28 volts are required to charge it. This larger voltage is needed for charging because of the voltage-drop in the battery caused by the internal resistance. Hence, the charging voltage of a lead acid battery must equal the open-circuit voltage plus the IR drop within the battery.
Constant current & constant voltage charging-Batteries are charged by either the
constant-voltage or constant-current method. In the constant-voltage method a motor-generator set with a constant, regulated voltage forces the current through the battery. In this method, the current at the start of the process is high but tapers off, reaching a value of approximately 1-ampere when the battery is fully charged. The constant-voltage method requires less time and supervision than dos the constant-current-method. In the constant-current method, the current remains almost constant during the entire charging-process. This method requires a larger time to charge a battery fully and toward the end of the process, presents the danger of overcharging, if care is not taken .
In the aircraft, the storage-battery is charged by direct-current from the aircraft-generator-system. This method of charging is the constant-voltage-method. Since the generator voltage is held constant by use of voltage regulator. When a storage battery is being charged, it generates a certain amount of hydrogen and oxygen. Since, this is an explosive mixture, it is important that steps be taken to prevent ignition of the gas-mixture. The vent caps loosened and left in place. No open flames, sparks or other source of ignition should be permitted in the vicinity. Before disconnecting or connecting a battery to the charger, always turn off the power.
Temperature is a vital factor in the operation and life of a storage battery chemical action takes place more rapidly as temperature increases. For this reason, a battery will give much better performance in tropical climates. On the other hand, a battery will deteriorate faster in a warm climate. In cold climates, the state of charge in a storage battery should be kept at a max. A fully charged battery will not freeze even under most severe weather condition, but a discharged battery will freeze very easily. When water is added to a battery in extremely cold weather, the battery must be charged at once. If this is not done, the water will not mix with the acid and will freeze. Operating a storage battery in cold weather is equivalent to using a battery of lower capacity. Specific gravity Freezing point 0C 0F 1.300 -70 -90 1.275 -62 -80 1.250 -52 -62 1.225 -37 -35 1.200 -26 -16 1.175 -20 -4 1.150 -15 5 1.125 -10 13 1.100 -8 19
For example, a fully charged battery at 80º F (26.6 ºC) may be capable of starting an engine twenty times. At 0º F (17.8ºC), the same battery may start the engine only three times. Low temperatures greatly increase the time necessary for charging a battery. A battery which could be recharged in one hour at 80ºF may require approximately five hours of charging, when the temperature is 0 ºF. These effects on a battery’s capacity are caused by the slow chemical reactions created by the cold temperatures.
Lead-acid battery testing methods-The state of charge of a storage battery depends upon
the condition of its active materials, primarily the plates. However, the state of charge of a battery is indicated by the density of the electrolyte and is checked by a hydrometer, an instrument which measures the specific gravity (weight as compared with water) of liquids. The hydrometer commonly used consists of a small sealed glass tube weighted at its lower end s0 it will float upright. Within the narrow stem of the tube is a paper scale with a range of 1.100 to 1.300. When a hydrometer is used, a quantity of electrolyte sufficient to float the hydrometer is drawn up into the syringe. The depth to which the hydrometer sinks into the electrolyte is determined by the density of the electrolyte, and the scale value indicated at the level of the electrolyte is its specific gravity.
The more dense the electrolyte, the higher the hydrometer will float, therefore, the highest number on the scale (1.300) is at the lower end of the hydrometer scale. In a new, fully charged aircraft storage battery, the electrolyte is approximately 30% acid and 70% water (by volume) and is 1.300 times as heavy as pure water. During discharge, the electrolyte become less dense and its specific gravity drops below 1.300. A specific gravity reading between 1.300 and 1.275 indicates a high state of charge, between 1.275 and 1.240, a medium state of charge and between 1.240 and 1.200, a low state of charge.
Aircraft batteries are generally of small capacity but are subject to heavy loads. The values specified for state of charge are therefore rather high. Hydrometer tests are made periodically on all storage batteries installed in aircraft. An aircraft battery in a low state of charge may have perhaps 50% charge remaining, but is nevertheless considered low in the face of heavy demands which would soon exhaust it. A battery in such a state of charge is considered in need of immediate recharging. When a battery is tested using a hydrometer, the temperature of the electrolyte must be taken into consideration. The specific gravity readings on the hydrometer will vary from the actual specific gravity as the temperature changes. No correction is necessary when the temperature is between 70°F and 90°F, since the variation is not great enough to be considered. When temperatures are greater than 90°F or less than 70°F, it is necessary to apply a correction factor. Some hydrometers are equipped with a correction scale inside the tube. With other hydrometers it is necessary to refer to a chart provided by the manufacturer. In both cases, the corrections should be added to, or subtracted from, the reading shown on the hydrometer. Electrolyte temperature Correctio n points 0C 0F 60 140 24 55 130 20 49 120 16 43 110 12 38 100 8 33 90 4 27 80 0 23 70 -4 15 60 -8 10 50 -12 5 40 -16 -2 30 -20 -7 20 -24 -13 10 -28 -18 0 -32 -23 -10 -36 -28 -20 -40 -35 -30 -44
The specific gravity of a cell is reliable only if nothing has been added to the electrolyte except occasional small amounts of distilled water to replace that lost as a result of normal evaporation. Hydrometer readings should always be taken before adding distilled water, never after. This is necessary to allow time for the water to mix thoroughly with the electrolyte and to avoid drawing up into the hydrometer syringe a sample which does not represent the true strength of the solution. Extreme care should be exercised when making the hydrometer test of a lead acid cell. The electrolyte should be handled carefully, for sulphuric acid will bum clothing and skin. If the acid does contact the skin the area should be washed thoroughly with water and then bi-carbonate of soda applied.
Battery installation-Before installing any battery in an aircraft. be sure you know that the
battery is correct for the aircraft, that the voltage and ampere-pour ratings are as specified and that the battery fits the battery box properly. Some aircraft use two batteries connected in parallel to provide a reserve of current for starting and for extra-heavy electrical loads. Be sure that the batteries installed in this type of arrangement are the batteries specified in the aircraft service manual. Most aircraft use a single-wire electrical system with the negative terminal of
NOTE: When installing the battery, connect the "hot" lead first. If you should short-circuit
between the battery and the aircraft with your wrench, you will not cause a spark if the ground lead has not been connected. When removing a battery, always disconnect the ground lead first for the same reason.
Be sure that the battery box is properly vented, if a vent is required and that the battery box drain extends through the aircraft skin. Some batteries are of the manifold type, which do not require a separate battery box. There is a cover over the cells. and the area above the cells is vented to the outside of the aircraft structure. The fumes emitted from storage batteries as they are charged are highly corrosive to the metals of which aircraft are made and they must be neutralized before they are released into the atmosphere. Many battery installations have vent sump jars containing absorbent pads moistened with a solution of bicarbonate of soda and water. No battery installation is complete until you know that the battery will supply enough current to crank the engine and that the aircraft generating system will keep the battery charged.
The electrolytes used by nickel-cadmium and lead-acid batteries are chemically opposite. and either type of battery can be contaminated by fumes from the charging of the other. For this reason it is extremely important that separate facilities be used for servicing nickel-cadmium batteries, completely away from the area used for lead-acid batteries. The alkaline electrolyte used in nickel-cadmium batteries is corrosive and it can bum your skin or cause severe injury if it gets into your eyes. Be careful when handling this liquid and if any of it is spilled, neutralize it with vinegar or boric acid, and flush the area with clean water.
Most nickel-cadmium batteries will get an accumulation of potassium carbonate on top of the cells. This white powder forms when electrolyte spewed from the battery combines with carbon dioxide. The amount of this deposit is increased by charging the battery too fast or by the electrolyte level being too high. If there is an excessive amount of potassium carbonate, check the voltage regulator and the level of electrolyte in the cells. Scrub all of the deposits off of the top of the cells with a nylon or other type of nonmeta1ic bristle brush. and dry the battery thoroughly with a soft flow of compressed air. Check for electrical leakage between the cells and the steel case by using a milli-ammeter between the positive terminal of the battery and the case. If there is more than about 100 mlll1amps of leakage. the battery should be disassembled and thoroughly cleaned. Check all of the hardware in the cell connectors for their condition and to be sure that there is no trace of corrosion. Dirty contacts or improperly torqued nuts will cause overheating and burned hardware.
The only way of actually determining the condition of a nickel-cadmium battery is to fully charge it and then discharge it at a specified rate and measure its ampere-hour capacity. When charging. use the five-hour rate and charge it until the cell voltage is that specified by the battery manufacturer. When it is fully charged. and immediately after it is taken off of the charger. measure the level of the electrolyte. If it is low. adjust it by adding distilled water. If the level is not checked immediately after the charge is completed. the level will drop. and the correct level is difficult. if not impossible. to ascertain. When water is added. the amount and cell location must be recorded on the battery service record. When the battery is fully charged and the electrolyte adjusted. it must be discharged at a specified rate and its ampere-hour capacity measured. If the capacity is less than it should be it is an indication that some of the cells are un- balanced and they must be equalized by a process known as deep-cycling.
To deep-cycle the battery. continue to discharge it at a rate somewhat lower than that used for the capacity test. When the cell voltage is down to around 0.2 volt per cell. short across each cell with a shorting strap. Leave the strap across the cells for three to eight hours to be sure that all of the cells are completely discharged. This process is known as equalization. After equalization the battery is ready to charge. Nickel-cadmium batteries may be charged using either the constant-voltage or constant-current methods. The constant-voltage method will result in a faster charge but the constant-current is most widely used. For either system the battery manufacturer's service instructions must be followed exactly. Monitor the battery during charge. and measure individual cell voltages. The manufacturer will specify a maximum differential between cells during the charging process. If a cell exceeds the specification, it must be replaced.
D.c circuits
Ohm’s law for d.c circuits - The current in an electric circuit is directly proportional to the
potential-difference and is inversely proportional to the resistance, and 1 volt causes 1 ampere to flow through a resistance of 1 ohm.
Limitations of ohm’s law
Ohm’s law is not applicable under the following conditions -
For metals which get heated up due to flow of current through them. For electrolytes where enormous gases are produced on either electrode. For vacuum tube valves.
For gas filled tubes, in which ions are generated as a result of current flow. For arc lamps.
For semiconductors.
For appliances such as metal rectifiers and crystal detectors, in which the operation depends on the direction of current.
Electric power & work- Power is the rate of energy conversion i.e. rate of doing a work. The
unit of power in S.I system is Watt, which is equal to 0.00134 hp. In electrical terms, 1 watt is the power expended when 1 volt at 1 coulomb per second through a conductor i.e. 1 volt at 1 ampere produces 1 watt of power. When power is lost in an electric circuit in the form of heat, it is called the I2R loss because the heat produced is a function of a circuit’s current and resistance, expressed as P = V x I = I2 x R = V2/ R
Energy-Energy is the ability of doing work. The unit of energy or work is Joule and the unit of
electrical energy is Kilowatt hours (kWh) i.e.1 kWh=1 kW x 1 hour=1000 watt hours =1000x60x60 watt sec.
Series circuits
In a series circuit, all the resistances are in a string & the equivalent resistance is the sum of all the individual resistances. The current through all the resistances remains the same.
R1 R2 R3 R4
Total resistance RT = R1 + R2 + R3 + R4 ohms Total voltage VT = I x R1 + I x R2 + I x R3 VT = I x RT Volts Total current IT = I1 = I2 = I3 amps or, IT = VT / RT If the resistances are of equal value R, then, RT = n x R ohms, where, n = number of resistances.
Exercise
In the series circuit shown below, calculate the total resistance, the current, the power absorbed by each resistor, and the total power supplied by the source . The resistances are R1 = 2 Ohm, R2 = 4 Ohm, R3 = 6 Ohm & R4 = 8 Ohm & the d.c source is of 100 Volts.
R1 R2 R3 R4
Solution
As it is a series circuit,
The total resistance = RT = R1+ R2 + R3 + R4 Ohms = 2 + 4 + 6 + 8 = 20 Ohms. The current = VT / RT = 100 / 20 = 5 Amps
Power absorbed by R1 = I2 R1 = 5 2 2 = 50 Watts Power absorbed by R2 = I2 R2 = 5 2 4 = 100 Watts
Power absorbed by R3 = I2 R3 = 5 2 6 = 150 Watts Power absorbed by R4 = I2 R4 = 5 2 8 = 200 Watts
Power absorbed = Power absorbed by R1+ Power absorbed by R2 + Power absorbed by R3 +Power absorbed by R4 = 50+100+150+200=500 W. The power supplied by the source= VtxIt = 100 x 5 = 500 Watts
Parallel circuits-In a parallel circuit, all the resistances are in parallel across the supply and
thus the potential difference across each branch is the same, depending on the magnitude of a branch-resistance, branch-current flows through it. Here, the total current drawn from the supply is the sum of all the branch currents.
Total resistance 1/RT=1/R1 +1/R2+1/R3+…ohms or, RT=1/(1/R1+1/R2+1/R3 +….) ohms
Total voltage VT =V1=V2=V3=…….volts Total current IT = I1 + I2 + I3 + amps If the resistances are of equal value R, then RT=R/n, where, n = number of resistances.
Exercise
For the circuit shown below, find the total resistance, branch-currents, total current, power consumed by each resistor.
Solution
Total resistance 1/ RT = 1/R1 + 1/R2 + 1/R3 = 1/2 + 1/4 + 1/8 = (4+2+1) / 8 = 7/8 So, Total resistance RT = 8/7 ohm
Branch currents–as all the resistors are across the same supply so the potential difference across them remains the same i.e. 24 Volts.
I1 = Vt / R1 = 24 / 2 = 12 amps I2 = Vt / R2 = 24 / 4 = 6 amps
I3 = Vt / R3 = 24 / 8 = 3 amps Total current=Vt/Rt = 24 /(8/7)=24x7/8 = 21amps
Power consumed by – R1 = I12 x R1 = 12 2 x 2=288W Power consumed by – R2 = I22 x R2 = 6 2 x 4=144W
Power consumed by – R3 = I32 x R3 = 3 2 x 8 = 72W
Series – parallel circuits
In a series-parallel there are sections with resistors are in series with a bank of resistors in parallel. RT, VT and IT are found by first reducing the parallel circuit to a single resistance, and then solving the whole as a simple series circuit.
R1
R2
R3 V1
e.g. Req. parallel = (R2.R3 / R2 + R3) Ohm RT = R1 + Req. parallel = R1 +( R2.R3/R2+R3) = (R1R2 + R1R3 + R2R3 ) / R2+R3 Ohm i.e. IT = VT/RT = VT.(R2+R3)/(R1R2+R1R3+R2R3) Amps + Vt = 24 V -R1 = 2 Ohm R2 = 4 Ohm R3 = 8 Ohm R1 R2 R3
Exercise
For the circuit shown, find The total resistance, Total current, Branch-currents, Power consumed by each resistor. V1 30V R2 10ohm R3 10ohm R1 10ohm Solution
For the parallel resistors, the equivalent resistor
Rp=R2x R3/R2+R3=10 x10/(10+10) = 5 ohms Rp is in series with R1 so, total resistance Rt=10+5= 15ohms
Total current It = Vt / Rt = 30 / 15 = 2 amps. As the parallel resistors are equal, so the current of 2 A will be divided equally, passing through each of the parallel resistor. So, the branch current is of 1 A. Power consumed by R1 = It2 x R1 = 2 2 x 10=40 W Power consumed by R2 = I22 x R2 = 1 2 x 10 = 10 W Power consumed by R3 = I32 x R3 = 1 2 x 10 = 10 W
Total power consumed = 40 + 10+ 10=60 W
Kirchoff’s current law-The algebraic sum of the currents entering and leaving any point in a
circuit must equal zero. Or stated another way, the algebraic sum of the currents into any point of the circuit must equal the algebraic sum of the currents out of that point. Otherwise, charge would accumulate at the point, instead of having a conducting path. An algebraic sum means combining positive and negative values.
Kirchoff’s voltage law-The algebraic sum of the voltages around any closed path is zero. In
determining the algebraic signs for voltage terms, in a KVL equation, first mark the polarity of each voltage. A convenient system then is go around any closed path and consider any voltage whose negative terminal is reached first as a negative term and any voltage whose positive terminal is reached first as a positive term. The direction can be clockwise or anti-clockwise. If you do not come back to the start, then the algebraic sum is the voltage between the start and finish points.
Exercise
Calculate all the branch currents in the circuit shown below.
R1 12ohm R2 3.0ohm V1 84V V2 21V R3 6ohm
Writing the loop equation
84 – VR1 – VR3 = 0 21 – VR2 – VR3 = 0
VR1 = I1R1 = I1 X 12 = 12I1 VR3 = I2R2 = I2 X 3 = 3I2 VR3 = (I1 + I2) X R3 = 6 X (I1 + I2) The above equations reduce to
3 I1 + I2 = 14 2 I1 + 3 I2 = 7
On solving, I1 = 5 A & I2 = -1 A, the negative sign for I2 means that this current is opposite to the assumed direction.
