<ul><li>Prepare for ASE Electrical/Electronic Systems (A6) certification test content area “A” (General Electrical/Electronic Systems). </li></ul><ul><li>Explain capacitance. </li></ul><ul><li>Describe magnetism. </li></ul><ul><li>Describe electromagnetism. </li></ul><ul><li>Explain how electricity can create magnetism and how magnetism can create electricity. </li></ul>OBJECTIVES: After studying Chapter 37, the reader should be able to:
<ul><li>armature capacitance • condenser • counter electromotive force (CEMF) • dielectric </li></ul><ul><li>e coil • electromagnetic induction • electromagnetic interference (EMI) • electromagnetics </li></ul><ul><li>farads • flux density • high energy ignition (HEI) </li></ul><ul><li>ignition coil • ignition control module (ICM) • induced voltage </li></ul>KEY TERMS: Continued
<ul><li>left-hand rule • lenz’s law • leyden jar magnetic flux lines • magnetism • mutual induction permeability • polarity • pole • primary winding radio choke coil • radio-frequency interference (RFI) • relative motion • relay • reluctance • right-hand rule secondary winding • self-induced voltage turns ratio </li></ul>KEY TERMS: Continued
CAPACITANCE <ul><li>The ability of an object or surface to store an electrical charge is called capacitance . Around 1745, Ewald Christian von Kliest and Pieter van Musschenbroek independently discovered capacitance in </li></ul>Figure 37–1 A Leyden jar can be used to store an electrical charge. an electric circuit, using a device, now called a Leyden jar , made from a glass jar filled with water, with a nail piercing the stopper and dipping into the water. They connected the nail to an electrostatic charge. Continued After disconnecting from the charge, they found that a shock could be felt by touching the nail.
<ul><li>In 1747, John Bevis lined both the inside and outside of the jar with foil. This created a capacitor with two conductors (the inside and outside metal foil layers) equally separated by the insulating glass. The Leyden jar was also used by Benjamin Franklin to store the charge from lightning and was also used in other experiments. The natural phenomenon of lightning includes capacitance because huge electric fields develop between cloud layers or between clouds and the earth prior to a lightning strike. </li></ul>Continued
<ul><li>A capacitor consists of two conductive plates with an insulating material between them, called a dielectric . A dielectric is a substance that is a poor conductor of electricity and can include air, mica, ceramic, glass, paper, plastic, or any similar nonconductive material. The dielectric constant is the relative strength of a material against the flow of electrical current. The higher the number, the better the insulating properties. </li></ul>CAPACITOR CONSTRUCTION AND OPERATION Continued
DIELECTRIC CONSTANTS Continued See the chart on Page 382 of your textbook.
Figure 37–2 This simple capacitor, made of two plates separated by an insulating material, is called a dielectric. <ul><li>When a capacitor is placed in a closed circuit, the voltage source (battery) forces electrons around the circuit. Because electrons cannot flow through the dielectric, excess electrons collect on what becomes the negatively charged plate. </li></ul>At the same time, the other plate loses electrons and, becomes positively charged. Continued
<ul><li>Current continues until the voltage charge across the capacitor plates becomes the same as the source voltage. At that time, the negative plate of the capacitor and the negative terminal of the battery are at the same negative potential. See Figure 37–3. </li></ul>NOTE: Capacitors are also called condensers . This term developed be-cause electric charges collect, or condense, on the plates of a capacitor much like water vapor collects and condenses on a cold bottle or glass. The positive plate of the capacitor and the positive terminal of the battery are also at equal positive potentials. There is then a voltage charge across the battery terminals and an equal voltage charge across the capacitor plates. The circuit is in balance, and there is no current. An electrostatic field now exists between the capacitor plates because of their opposite charges. It is this field that stores energy. Figure 37–4
Figure 37–3 As the capacitor charges, the battery forces electrons through the circuit. Figure 37–4 When the capacitor is charged, there is equal voltage across the capacitor and the battery. An electrostatic field exists between the capacitor plates. No current flows in the circuit. Continued
<ul><li>If the circuit is opened, the capacitor will hold its charge until it is connected into an external circuit through which it can discharge. </li></ul>Figure 37–5 The capacitor is charged through one circuit (top) and discharged through another (bottom). When the charged capacitor is connected to an external circuit, it discharges. After discharging, both plates of the capacitor are neutral because all the energy from a circuit stored in a capacitor is returned when it is discharged. Continued
<ul><li>Theoretically, a capacitor holds its charge indefinitely. Actually, the charge slowly leaks off the capacitor through the dielectric. The better the dielectric, the longer the capacitor holds its charge. To avoid an electrical shock, any capacitor should be treated as if it were charged until proven to be discharged. Seen here is the symbol for capacitors as used in electrical schematics. </li></ul>Figure 37–6 Capacitor symbols are shown in electrical diagrams. The negative plate is often shown curved. Continued
FACTORS OF CAPACITANCE <ul><li>Capacitance is governed by three factors: </li></ul>Continued <ul><li>The surface area of the plates </li></ul><ul><li>The distance between the plates </li></ul><ul><li>The dielectric material </li></ul>The larger the surface area and closer the plates are to each other, the greater the capacitance. More electrons collect on a larger plate area than on a small one and a stronger electrostatic field exists between charged bodies that are close together. Insulating qualities of the dielectric material also affect capacitance. The capacitance is higher if the dielectric is a very good insulator.
<ul><li>Measurement of Capacitance Capacitance is measured in farads , which is named for Michael Faraday (1791–1867). The symbol for farads is F . If a charge of 1 coulomb is placed on the plates of a capacitor and the potential difference between them is 1 volt, the capacitance is then defined to be 1 farad. One coulomb is equal to the charge of 6.25 × 1018 electrons. One farad is an extremely large quantity of capacitance. Microfarads (0.000001 farad) or µF are more commonly used. </li></ul>Continued
<ul><li>The capacitance of a capacitor is proportional to the quantity of charge that can be stored in it for each volt difference in potential between its plates: </li></ul><ul><li>C = Q/V </li></ul><ul><li>Where C is capacitance in farads, Q is quantity of stored electrical charge in coulombs, and V is the difference in potential in volts. </li></ul><ul><li>Stored electric charge can be calculated using the formula: </li></ul><ul><li>Q = CV The difference in potential or voltage of the capacitor can be calculated using the formula: </li></ul><ul><li>V = Q/C </li></ul>
CAPACITOR USES <ul><li>Because a capacitor stores a voltage charge, it opposes, or slows down, any voltage change in a circuit. </li></ul><ul><li>Capacitors are often used as voltage “shock absorbers.” A capacitor is sometimes attached to one terminal of an ignition coil to absorb and damp changes in ignition voltage that interfere with radio reception. Capacitors are used in many ways in electronic circuits, such as acting as barriers to direct currents, storing memory in a computer chip, or storing a charge for an electronic flash camera. </li></ul>Continued
<ul><li>By connecting a capacitor to the power lead of a radio or sound system amplifier, the AC voltage passes through the capacitor to ground where the other end of the capacitor is connected. The capacitor provides a path for the AC without affecting the DC power circuit. </li></ul>Interference in a sound system or radio is usually due to alternating current (AC) voltage created somewhere in the vehicle, such as in the alternator. A capacitor blocks the flow of DC and allows AC to pass. A Capacitor Makes an Excellent Sound System Noise Filter Figure 37–7 A capacitor blocks direct current (DC) but passes alternating current (AC). A capacitor makes a very good noise suppressor because most of the interference is AC and the capacitor will conduct this AC to ground before it can reach the radio or amplifier.
<ul><li>Computer Memory Main memory of a computer is a high-speed memory, called dynamic random-access memory (DRAM). A single memory chip is made up of several million memory cells and each memory cell consists of a capacitor. When a capacitor is electrically charged, it is said to store the binary digit 1, and when discharged, it represents 0. Condenser Microphones All microphones have a diaphragm that vibrates as sound waves strike. The vibrating diaphragm in turn causes an electrical component to create an output flow of current at a frequency proportional to the sound waves. A condenser microphone uses a capacitor for this purpose. </li></ul>NOTE: Capacitors are often used to reduce radio interference or to improve the performance of a high-power sound system. Additional capacitance can be added by attaching another capacitor in parallel.
Figure 37–8 Capacitors in parallel effectively increase the capacitance. <ul><li>Capacitors in Parallel Circuits Capacitance can be increased in a circuit by connecting capacitors in parallel, shown here. </li></ul>To determine total capacitance of several parallel capacitors, add up their individual values. Calculating total capacitance in a circuit containing capacitors in parallel: Capacitance of a capacitor can be increased by increasing the size of its plates. Connecting two or more capacitors in parallel in effect increases plate size. Increasing plate area makes it possible to store more charge and therefore creates greater capacitance.
Figure 37–9 Capacitors in series decrease the capacitance. <ul><li>Capacitors in Series Circuits Capacitance can be decreased in a circuit by capacitors in series, as shown below. </li></ul>Formula for calculating total capacitance in a circuit containing two capacitors in series: Capacitance of a capacitor can be decreased by placing the plates farther apart. Connecting two or more capacitors in series in effect increases the distance between the plates and thickness of the dielectric, decreasing the amount of capacitance.
FUNDAMENTALS OF MAGNETISM <ul><li>Magnetism is a form of energy that is caused by the motion of electrons in some materials. It is recognized by the attraction it exerts on other materials. Like electricity, magnetism cannot be seen. It can be explained in theory because it is possible to see the results of magnetism and recognize the actions that it causes. </li></ul>Figure 37–10 A freely suspended natural magnet will point toward the magnetic north pole. Continued Iron ore exists as a magnet in nature. Other materials can be artificially magnetized, depending upon their atomic structure. Soft iron is very easy to magnetize, while some materials—such as aluminum, glass, wood, and plastic—cannot be magnetized.
Magnets are commonly used in vehicle crankshaft, camshaft, and wheel speed sensors. If a magnet is struck and cracks or breaks, the result is two smaller-strength magnets. Because the strength of the magnetic field is reduced, the sensor output voltage is also reduced. A typical problem occurs when a magnetic crankshaft sensor becomes cracked, resulting in a no-start condition. Sometimes the cracked sensor works well enough to start an engine that is cranking at normal speeds but will not work when the engine is cold. A Cracked Magnet Becomes Two Magnets Figure 37–11 If a magnet breaks or is cracked, it becomes two weaker magnets.
<ul><li>Lines of Force The lines that create a field of force around a magnet are believed to be caused by the way groups of atoms are aligned in magnetic material. </li></ul>In a bar magnet, lines concentrate at both ends of the bar, forming closed, parallel loops in three dimensions around the magnet. Force does not flow along these lines the way electrical current flows, but the lines do have direction. They come out one end, or pole , of the magnet and enter the other end. Figure 37–12 Magnetic lines of force leave the north pole and return to the south pole of a bar magnet. Continued
<ul><li>The opposite ends of a magnet are called its north and south poles. In reality, they should be called the “north seeking” and “south seeking” poles, because they seek the Earth’s North Pole and South Pole, respectively. The stronger the magnet, the more lines of force that are formed. The magnetic lines of force, also called magnetic flux or flux lines , form a magnetic field. The terms “magnetic field,” “lines of force,” “flux,” and “flux lines” are used interchangeably. Flux density refers to the number of flux lines per unit of area. To determine flux density, divide the number of flux lines by the area in which the flux exists. For example, 100 flux lines divided by an area of 10 square centimeters equals a flux density of 10. A magnetic field can be measured using a Gauss gauge, named for German scientist Johann Carl Friedrick Gauss (1777–1855). </li></ul>Continued
Figure 37–13 Iron filings on a compass can be used to observe the magnetic lines of force. <ul><li>Magnetic lines of force can be seen by spreading fine iron filings or dust on a piece of paper laid on top of a magnet. A magnetic field can also be observed by using a compass. </li></ul>A compass is simply a thin magnet or magnetized iron needle balanced on a pivot. The needle will rotate to point toward the opposite pole of a magnet. The needle can be very sensitive to small magnetic fields. Since it is a small magnet, a compass usually has one end marked N, the other marked S. Continued
A piece of steel can be magnetized by rubbing a magnet in one direction along the steel. This causes the atoms to line up in the steel, so it acts like a magnet. Steel often won’t remain magnetized, while a true magnet is permanently magnetized. When soft iron or steel is used, such as a paper clip, it will lose its magnetism quickly. The atoms in a magnetized needle can be disturbed by heating it or by dropping the needle on a hard object, which would cause the needle to lose its magnetism. Soft iron is used for ignition coils because it will not keep its magnetism. Magnetize a Steel Needle
<ul><li>Attracting or Repelling The poles of a magnet are called north (N) and south (S) because, when a magnet is suspended freely, the poles tend to point toward the North and South Poles of the Earth. </li></ul>Figure 37–14 Magnetic poles behave like electrically charged particles— unlike poles attract and like poles repel. An equal number of magnetic flux lines exit from the north pole and bend around to enter the south pole. Magnetic force is equal at both poles of a magnet. Flux lines are concentrated at the poles, magnetic force (flux density) is stronger at the ends. When unlike poles are close together, the magnets are pulled together by flux lines. If like poles are close, flux lines meet head-on, forcing the magnets apart .
<ul><li>Permeability Magnetic flux lines cannot be insulated. There is no known material through which magnetic force does not pass, if the force is strong enough. Some materials allow the force to pass though more easily than others. This degree of passage is called permeability . Iron allows magnetic flux lines to pass through much more easily than air, so iron is very permeable. Reluctance There is no absolute insulation for magnetism, but certain materials resist passage of magnetic force. This can be compared to resistance without an electrical circuit. Air does not allow easy passage, so air has a high reluctance . Magnetic flux lines tend to concentrate in permeable materials and avoid reluctant materials. </li></ul>Continued
ELECTROMAGNETISM <ul><li>Scientists discover in 1820 that current-carrying conductors also are surrounded by a magnetic field. These fields may be made many times stronger than those surrounding conventional magnets. Magnetic field strength around a conductor may be controlled by changing the current. As current increases, more flux lines are created and the magnetic field expands. As current decreases, the magnetic field contracts, or collapses. This takes place when the current is shut off. These discoveries greatly broadened practical uses of magnetism and opened an area of study known as electromagnetics . </li></ul>Continued
Electricity and magnetism are closely related because whenever an electrical current is flowing through a conductor, a magnetic field is created. When a conductor is moved through a magnetic field electrical current is created. This relationship can be summarized as : Electricity and Magnetism From a service tech’s point of view, this is important because wires carrying current should always be routed as the factory intended to avoid interference with another circuit or electronic component. This is especially important when installing or servicing spark plug wires, which carry high voltages and can cause a lot of electromagnetic interference. <ul><li>Electricity creates magnetism. </li></ul><ul><li>Magnetism creates electricity. </li></ul>
<ul><li>Creating an Electromagnet Magnetizing a piece of iron or steel or by using electricity to makes an electromagnet. </li></ul>Figure 37–15 A magnetic field surrounds a straight, current-carrying conductor. Continued To create an electromagnet: wrap a nail with 20 turns of insulated wire and connect the ends to a 1.5-volt dry cell battery. When energized, the nail becomes a magnet and picks up tacks or other small steel objects. Straight Conductor The magnetic field surrounding a straight, current-carrying conductor consists of several concentric cylinders of flux the length of the wire. The current flow (amperes) determines how many flux lines (cylinders) there will be and how far out they extend from the surface of the wire.
Figure 37–16 The left-hand rule for magnetic field direction is used with the electron flow theory. <ul><li>Left- and Right-Hand Rules Flux cylinders have direction, just as the flux lines surrounding a bar magnet have direction. The left-hand rule is a simple way to determine this direction. </li></ul>When you grasp a conductor with your left hand so that your thumb points in the direction of electron flow ( to +) through the conductor, your fingers curl around the wire in the direction of the magnetic flux lines. Continued
Figure 37–17 The right-hand rule for magnetic field direction is used with the conventional theory of electron flow. <ul><li>Most automotive circuits use the conventional theory of current (+ to ), therefore, the right-hand rule is used to determine the direction of the magnetic flux lines. </li></ul>Continued
Figure 37–18 Conductors with opposing magnetic fields will move apart into weaker fields. <ul><li>Field Interaction The cylinders of flux surrounding conductors interact with other magnetic fields. </li></ul>The cross symbol (+) indicates current moving inward, or away from you. It represents the tail of an arrow. The dot symbol (·) represents an arrowhead and indicates current moving outward. If two conductors carry current in opposite directions, their magnetic fields also carry current in opposite directions (according to the left-hand rule). If placed side-by-side, the opposing flux lines between the conductors create a strong magnetic field. Current-carrying conductors tend to move out of a strong field into a weak field, so the conductors move away from each other.
<ul><li>Motor Principle Electric motors, such as automobile starter motors, use this field interaction to convert electrical energy into mechanical energy. If two conductors carrying current in opposite directions are placed between strong north and south poles, the magnetic field of the conductor interacts with the magnetic fields of the poles. The clockwise field of the top conductor adds to the fields of the poles and creates a strong field beneath the conductor. The conductor then tries to move up to get out of this strong field. The counterclockwise field of the lower conductor adds to the field of the poles and creates a strong field above the conductor. The conductor tries to move down to get out of this strong field. </li></ul>Continued
Figure 37–19 Electric motors use the interaction of magnetic fields to produce mechanical energy. <ul><li>These forces cause the center of the motor, where the conductors are mounted, to turn clockwise, as shown here. </li></ul>Continued
Figure 37–20 The magnetic lines of flux surrounding a coil look similar to those surrounding a bar magnet. <ul><li>Coil Conductor If several loops of wire are made into a coil, the magnetic flux density is strengthened. Flux lines around a coil are the same as the flux lines around a bar magnet. </li></ul>They exit from the north pole, enter the south. Use the left-hand thread rule to determine the north pole of a coil. Figure 37–21 The left-hand rule for coils is shown. The magnetic field of a coil can be strengthened by increasing the number of turns in the wire, by increasing the current through the coil, or both. Continued
Figure 37–22 An iron core concentrates the magnetic lines of force surrounding a coil. <ul><li>Electromagnets The magnetic field surrounding a current-carrying conductor can be strengthened using a soft-iron core. </li></ul>Because soft iron is very permeable, magnetic flux lines pass through it easily. If a piece of soft iron is placed inside a coiled conductor, the flux lines concentrate in the iron core, rather than pass through the air, which is less permeable. The concentration of force greatly increases the strength of the magnetic field inside the coil. Coils with an iron core are called electromagnets . Continued
Figure 37–23 This figure shows an electromagnetic relay. <ul><li>Relays One common automotive use of electromagnets is in a device called a relay . A relay is a control device which allows a small amount of current to control a large amount of current in another circuit. </li></ul>Continued A simple relay contains an electromagnetic coil in series with a battery and a switch. Near the electromagnet is a movable flat blade, or armature , of some material which is attracted by a magnetic field.
<ul><li>The armature pivots at one end and is held a small distance away from the electromagnet by a spring (or the armature itself). A contact point, made of a good conductor, is attached to the free end of the armature. Another contact point is fixed a small distance away. The two contact points are wired in series with an electrical load and the battery. </li></ul>Continued When the switch is closed: Current travels from the battery through the electromagnet. The magnetic field created by the current attracts the armature, pulling it down until the contact points meet. Closing contacts allows current in the second circuit from battery to load. When the switch is open: The electromagnet loses its current and its magnetic field. Spring pressure brings the armature back. The second circuit is broken by the opening of the contact points.
<ul><li>Relays also may be designed with normally closed contacts that open when current passes through the electromagnet. A solenoid is an example of an electromagnetic switch. A solenoid uses a movable core rather than a movable arm and is generally used in higher-amperage applications, such as a starter solenoid. </li></ul>Figure 37–24 In this electromagnetic switch, light current (low amp) produces an electromagnet and causes the contact points to close. The contact points then conduct a heavy current (high amperes) to an electrical unit.
ELECTROMAGNETIC INDUCTION <ul><li>Magnetic flux lines create an electromotive force, or voltage, in a conductor if either the flux lines or the conductor is moving. This movement is called relative motion . This process is called induction, and the resulting electromotive force is called induced voltage . This creation of a voltage in a conductor by a moving magnetic field is called electromagnetic induction . If the conductor is in a complete circuit, current flows. </li></ul>Continued Voltage is induced when magnetic flux lines are broken by a conductor.
Figure 37–25 Voltage can be induced by the relative motion between a conductor and magnetic lines of force. Voltage Strength Voltage is induced when a conductor cuts across magnetic flux lines. The amount of the voltage depends upon the rate at which flux lines are broken. The more flux lines broken per unit of time, the greater the induced voltage. If a single conductor breaks one million flux lines per second, one volt is induced. The motion can be a conductor moving across a magnetic field or a field moving across a stationary conductor (as in AC generators and ignition coils). In both cases, induced voltage is generated by relative motion between the conductor and the magnetic flux lines. Continued
<ul><li>Increase the strength of the magnetic field, so there are more flux lines. </li></ul><ul><li>Increase the number of conductors breaking the flux lines. </li></ul><ul><li>Increase speed of the relative motion between the conductor and the flux lines so that more lines are broken per time unit. </li></ul><ul><li>Increase the angle between the flux lines and the conductor to a maximum of 90 degrees. </li></ul>Continued There is no voltage induced if the conductors move parallel to, and do not break any, flux lines, as shown in Figure 37–26. Maximum voltage is induced if the conductors break flux lines at 90 degrees. Induced voltage varies proportionately at angles between 0 and 90 degrees. See Figure 37–27. There are four ways to increase induced voltage.
Figure 37–26 No voltage is induced if the conductor is moved in the same direction as the magnetic lines of force (flux lines). Figure 37–27 Maximum voltage is induced when conductors cut across the magnetic lines of force (flux lines) at a 90-degree angle. Continued
<ul><li>Voltage can be electromagnetically induced and can be measured. Induced voltage creates current. The direction of induced voltage (and the direction in which current moves) is called polarity and depends upon the direction of the flux lines, as well as the direction of relative motion. An induced current moves so that its magnetic field opposes the motion which induced the current. This principle is called lenz’s law . The relative motion of a conductor and a magnetic field is opposed by the magnetic field of the current it has induced. </li></ul>Continued
<ul><li>Self-Induction When current begins to flow in a coil, the flux lines expand as the magnetic field forms and strengthens. As current increases, flux lines continue to expand, cutting across the wires of the coil and actually inducing another voltage within the same coil. Following Lenz’s law, this self - induced voltage tends to oppose the current that produces it. If the current continues to increase, the second voltage opposes the increase. When current to the coil is shut off, the collapsing magnetic flux lines self-induce a voltage in the coil that tries to maintain the original current. The self-induced voltage opposes and slows down the decrease in the original current. The self-induced voltage that opposes changes in current flow is an inductor called counter electromotive force ( CEMF ). </li></ul>Continued
<ul><li>Mutual Induction When two coils are close together, energy may be transferred from one to the other by magnetic coupling called mutual induction. Mutual induction means that the expansion or collapse of the magnetic field around one coil induces a voltage in the second coil. Usually, the two coils are wound on the same iron core. One coil winding is connected to a battery through a switch and is called the primary winding . The other coil winding is connected to an external circuit and is called the secondary winding . When the switch is open, there is no current in the primary winding. There is no magnetic field, no voltage in the secondary winding. When the switch is closed, current is introduced and a magnetic field builds up around both windings. </li></ul>Continued
<ul><li>The primary winding changes electrical energy from the battery into magnetic energy of the expanding field. As the field expands, it cuts across the secondary winding and induces a voltage in it. See Figure 37–28. When the magnetic field has expanded to its full strength, it remains steady as long as the same amount of current exists. The flux lines have stopped their cutting action. There is no relative motion and no voltage in the secondary winding, as shown on the meter. When the switch is opened, primary current stops, and the field collapses. As it does, flux lines cut across the secondary winding but in the opposite direction. This induces a secondary voltage with current in the opposite direction, as shown on the meter. </li></ul>Continued
Figure 37–28 Mutual induction occurs when the expansion or collapse of a magnetic field around one coil induces a voltage in a second coil. <ul><li>Mutual induction is used in ignition coils, where low-voltage primary current induces a very high secondary voltage because of the different number of turns in primary and secondary windings. </li></ul>
IGNITION COILS <ul><li>The heart of any ignition system is the ignition coil , which creates a high-voltage spark by electromagnetic induction. Many ignition coils contain two separate but electrically connected windings of copper wire. Other coils are true transformers in which the primary and secondary windings are not electrically connected. See Figure 37–29. </li></ul>Continued
Figure 37–29 Internal construction of an oil-cooled ignition coil. Notice that the primary winding is electrically connected to the secondary winding. The polarity (positive or negative) of a coil is determined by the direction in which the coil is wound. <ul><li>The center of an ignition coil contains a core of laminated soft iron. </li></ul>Surrounding the core are 20,000+ turns of fine wire (42 ga). Surrounding those secondary windings are approximately 150-200 turns of heavy wire. Continued
<ul><li>The secondary winding has about 100 times the number of turns of the primary winding, referred to as the turns ratio (approximately 100:1). </li></ul>Continued In many coils, these windings are surrounded with a thin metal shield and insulating paper and placed into a metal container. This helps retain the magnetic field produced in the coil windings. Windings produce heat because of electrical resistance. Many coils contain oil for cooling. Other designs, such as GM’s high energy ignition ( HEI ) systems, use an air-cooled, epoxy-sealed E coil , so named because the laminated, soft iron core is E shaped. The coil wire turns wrap around the center “finger” of the E , the primary winding wrapped inside the secondary winding. See Figures 37–30 and 37–31.
Figure 37–30 Typical air-cooled epoxy-filled E coil.
Figure 37–31 Cutaway of a General Motors Type II distributorless ignition coil. Note that the primary windings are inside of the secondary windings. <ul><li>The primary windings of the coil extend through the case of the coil and are labeled as positive and negative. </li></ul>Continued The positive terminal of the coil attaches to the ignition switch, which supplies current from the positive battery terminal.
<ul><li>The negative terminal is attached to an ignition control module ( ICM or igniter ), which opens and closes the primary ignition circuit by opening or closing the ground return path of the circuit. When the ignition switch is on, voltage should be available at both the positive terminal and the negative terminal of the coil if the primary windings of the coil have continuity. The labeling of positive (+) and negative ( ) of the coil indicates that the positive terminal is more positive (closer to the positive terminal of the battery) than the negative terminal of the coil. This condition is called the coil polarity. </li></ul>The polarity of an ignition coil is determined by the direction of rotation of the coil windings .
<ul><li>Married Also called a tapped transformer design. The primary winding is electrically connected to the secondary winding. Common in older distributor-type ignition system coils, and many coil-on-plug designs. The inductive kick, called flyback voltage, created when the primary field collapses is used by the PCM to monitor secondary ignition performance. </li></ul>What is a “Married” and “Divorced” Coil Design? An ignition coil contains a primary winding and a secondary winding. These windings can be connected together at one end or kept separate. Divorced Called a true transformer design, used by most waste spark ignition coils to keep both primary and secondary winding separated. Figure 37–32 Tapped- (married) type of ignition coil where the primary winding is tapped (connected) to the secondary winding.
<ul><li>Until the advent of the onboard computer, electromagnetic interference ( EMI ) was not a source of real concern to automotive engineers. The problem was mainly one of radio-frequency interference ( RFI ) , caused primarily by the use of secondary ignition cables containing a high-resistance, nonmetallic core made of carbon, linen, or fiberglass strands impregnated with graphite. </li></ul>ELECTROMAGNETIC INTERFERENCE (EMI) SUPPRESSION Continued NOTE: Radio-frequency interference (RFI) is a part of electromagnetic interference (EMI), which affects radio reception.
<ul><li>As the use of electronic and computerized components and systems increased, the problem of electromagnetic interference increased. Whenever there is current in a conductor, an electromagnetic field is created. When current stops and starts, as in a spark plug cable or a switch that opens and closes, field strength changes. Each time this happens, it creates an electromagnetic signal wave. If rapid enough, the high-frequency signal waves, or EMI, interfere with radio and television or other electronic systems, such as those under the hood; an undesirable side effect of electromagnetism. Static electric charges caused by friction of the tires with the road, or the friction of engine drive belts contacting their pulleys, also produce EMI. Drive axles, driveshafts, and clutch or brake lining surfaces are other sources of static electric charges. </li></ul>Continued
<ul><li>EMI Transmission There are four ways of transmitting EMI, all of which can be found in a vehicle. </li></ul>Continued <ul><li>Conductive coupling is actual physical contact through circuit conductors. </li></ul><ul><li>Capacitive coupling is the transfer of energy from one circuit to another through an electrostatic field between two conductors. </li></ul><ul><li>Inductive coupling is the transfer of energy from one circuit to another as the magnetic fields between two conductors form and collapse. </li></ul><ul><li>Electromagnetic radiation is the transfer of energy by the use of radio waves from one circuit or component to another. </li></ul>
<ul><li>EMI Suppression Devices There are four general ways in which EMI is reduced. </li></ul>Continued <ul><li>By the addition of resistance to conductors that suppresses conductive transmission and radiation. </li></ul><ul><li>By the use of capacitors and radio choke coil combinations to reduce capacitive and inductive coupling. </li></ul><ul><li>By the use of metal or metalized plastic shielding, which reduces EMI radiation in addition to capacitive and inductive coupling. </li></ul><ul><li>By an increased use of ground straps to reduce conductive transmission. </li></ul>
Continued <ul><li>Across the primary circuit of some electronic ignition modules. </li></ul><ul><li>Across the output terminal of most alternators. </li></ul><ul><li>Across the armature circuit of electric motors. </li></ul>Radio choke coils reduce fluctuations resulting from self-induction, and are often combined with capacitors to act as EMI filter circuits. Filters also may be incorporated in wiring connectors. Suppression Capacitors and Coils Capacitors are installed across many circuits and switching points to absorb voltage fluctuations. Among other applications, they are used:
<ul><li>Shielding Metal shields, such as the ones used in breaker point distributors, block the waves from components that create RFI signals. The circuits of onboard computers are protected to some degree from external electromagnetic waves by their metal housings. </li></ul>Continued Resistance Suppression Adding resistance to a circuit to suppress RFI works only for high-voltage systems. This has been done by the use of resistance spark plug cables, resistor spark plugs, and the silicone grease used on the distributor cap and rotor of some electronic ignitions.
Figure 37–33 To help prevent underhood electromagnetic devices from interfering with the antenna input, it is important that the hood be grounded to the body to form one continuous metal covering around the engine compartment. This is particularly important if the vehicle has a front fender-mounted antenna. If necessary, add a braided ground strap as this Ford Mustang owner did to eliminate radio interference. <ul><li>Ground Straps Ground or bonding straps between engine and chassis help suppress EMI conduction and radiation by providing a low-resistance circuit ground path. </li></ul>Suppression ground straps are often installed between rubber-mounted components and body. The strap has no other job than to suppress EMI.
A cell (cellular) phone emits a weak signal if it is turned on, even though it is not being used. This signal is picked up and tracked by cell phone towers. When the cell phone is called, the cell phone emits a stronger signal to notify the tower that it is on and capable of receiving a phone call. It is this “handshake” signal that can cause interference in the vehicle. Often this signal simply causes some static in the radio speakers even though the radio is off, but it can also cause a false antilock brake (ABS) trouble code to set. These signals from the cell phone create a voltage that is induced in the wires of the vehicle. Because the cell phone usually leaves with the customer, the service technician is often unable to verify the customer concern. Remember, the interference occurs right before the cell phone rings. To fix the problem, connect an external antenna to the cell phone. This step will prevent the induction of a voltage in the wiring of the vehicle. Cell Phone Interference
SUMMARY <ul><li>Capacitors (condensers) are used in numerous automotive applications. Because of their ability to block direct current and pass alternating current, they are used to control radio-frequency interference and are in-stalled in various electronic circuits to protect and control changing current. </li></ul><ul><li>Most automotive electrical components use magnetism, and the strength of the magnetism depends on both the amount of current (amperes) and the number of turns of wire of each electromagnet. </li></ul><ul><li>The strength of electromagnets is increased by using a soft-iron core. </li></ul>Continued
SUMMARY <ul><li>Voltage can be induced from one circuit to another. </li></ul><ul><li>Electricity creates magnetism and magnetism creates electricity. </li></ul><ul><li>Radio-frequency interference (RFI) is a part of electromagnetic interference (EMI). </li></ul>( cont. )