Electronic Systems Module 3
Acknowledgements General Motors, the IAGMASEP Association Board of Directors, and Raytheon Professional Services, GM's training partner for GM's Service Technical College wish to thank all of the people who contributed to the GM ASEP/BSEP curriculum development project 2002-3. This project would not have been possible without the tireless efforts of many people. We acknowledge: The IAGMASEP Association members for agreeing to tackle this large project to create the curriculum for the GM ASEP/BSEP schools. The IAGMASEP Curriculum team for leading the members to a single vision and implementation. Direct contributors within Raytheon Professional Services for their support of translating a good idea into reality. Specifically, we thank: Chris Mason and Vince Williams, for their leadership, guidance, and support. Media and Graphics department under Mary McClain and in particular, Cheryl Squicciarini, Diana Pajewski, Lesley McCowey, Jeremy Pawelek, & Nancy DeSantis. For his help on the Electrical curriculum volume, Subject Matter Expert, Ken Beish, Jr., for his wealth of knowledge. Finally, we wish to recognize the individual instructors and staffs of the GM ASEP/BSEP Colleges for their contribution for reformatting existing General Motors training material, adding critical technical content and the sharing of their expertise in the GM product. Separate committees worked on each of the eight curriculum areas. For the work on this volume, we thank the members of the Electrical committee: Jack Davis, Community College of Baltimore County - Catonsville Jim Halderman, Sinclair Community College Megan Kuehm, Community College of Allegheny County Frank Longbottom, Camden County College Jeff Rehkopf, Florida Community College at Jacksonville Randy Peters, Des Moines Area Community College David Rodriguez, College of Southern Idaho Ed Schauffler, Longview Community College Vince Williams, Raytheon
Contents Module 3 Electricity Acknowledgements... 2 Objectives... 4 Current... 8 Conventional Current Flow and the Electron Theory... 9 Resistance... 10 Power... 15 Exercise... 16
Objectives At the end of this section, the student will be able to: Define the properties of electricity, including EMF potential (voltage), Current (amperage), Resistance (ohms), and power (watts). Explain the difference between AC and DC current. Use a Compass to indicate current flow in a circuit. Figure 3-1, Detecting Current Flow Experiment objective: Use a compass to indicate current flow in a circuit. Note: Do not turn on the project board power until the instructor has checked your set-up. Set up the circuit shown on the page above. You will find the necessary parts in the project board s storage compartment. Use a tail light socket and bulb (board B) along with the 7.5 amp fuse and switch assembly (board A). When the circuit is properly set up, apply power to the circuit by turning on the project board power supply. When the light glows, you will know current is flowing in the circuit. With the compass sufficiently held away from the board, note which way the needle is pointing. Now hold the compass close to the wire (you may need to experiment with different positions to get best result). Observe what happens to the compass needle as you turn the switch ON and OFF. 3-4
We have said that a number of electrons gathered in one place constitute an electrical charge. We call this charge an electrical potential or voltage. Voltage is measured in volts (V). Since it is used to move electrons an externally applied electrical potential is sometimes called an electromotive force or EMF. Potential, voltage, and EMF all mean the same thing. Voltage is often described as an electrical pressure that drives electron flow or current. This pressure is known as electromotive force, or EMF. A battery and generator are automotive devices used to provide the pressure, or voltage, required to operate electrical components. Figure 3-2, Battery and Generator 3-5
This pressure, or voltage, exists only when there is a higher potential of electrons at one point then at another point. This difference in potential is voltage. Therefore, voltage is pressure available to push electrons from the power supply through the circuit and back to the power supply. There are two types of voltage: direct current, more commonly called DC; and alternating current more commonly called AC. Direct current (DC) is best described as a direct, or continuous, flow of electrons in one direction. Most automotive systems use DC. The advantage of DC is it can be stored electro-chemically in a battery. Figure 3-3, Direct Current 3-6
Alternating current (AC) is best described as an alternating, or back and forth, flow of electrons. Automotive generators produce AC potential. AC is easier to produce in a generator due to the laws of magnetism, but it is extremely difficult to store. Generators incorporate special circuits that convert the AC to DC before it is used in a vehicle s electrical system. Figure 3-4, Alternating Current 3-7
Current The movement of electrons in a circuit is the flow of electricity. Another name for the flow of electricity is current. Current is measured in units known as amperes or amps (A). An amp expresses how many electrons are moving through a circuit at a given time. The time interval we use in electronics is the second. The more electrons moving through a circuit, the higher the amperage. Figure 3-4, Current Flow 3-8
Conventional Current Flow and the Electron Theory Current flow is usually shown as flowing from the positive terminal to the negative terminal. This current flow is the conventional theory. Another way of describing current flow is the electron theory, which states that current flows from the negative terminal to the positive terminal. The conventional theory and the electron theory are two different ways of describing the same current flow. Essentially, both theories are correct. The electron theory follows the logic that electrons move from an area of many electrons (negative charge) to one of few electrons (positive charge). However, in describing the behavior of semiconductors, we often describe current as moving from positive to negative. The important thing to know is which theory is being used by the service literature you happen to be using. Service manual schematics use conventional current flow theory. Conductors Electrons move along a path called a conductor. They move by traveling from atom to atoms. Materials that make it easy for electrons to move through them are called good conductors. Examples of good conductors include aluminum, copper, silver, and gold. A material is a good conductor if it has many free electrons, or electrons that can be easily removed. Other material make it difficult for electrons to move through them. These are called poor conductors or insulators. A material that is a good insulator keeps its electrons tightly bound in orbit. Examples of insulators include rubber, wood, most plastics, and ceramics. No material is a perfect insulator, and some insulators (such as wood) do conduct some current flow when wet. Wires A wire in an automotive harness is made up of a conductor and an insulator. The metal core of the wire, typically made of copper, is the conductor. The plastic, or other material, jacket that coats the core is the insulator. Under normal circumstances, electrons move a few inches per second. Yet, when electrical potential is applied to one end of a wire, the effect is felt almost immediately a the other end of that wire. This is so because the electrons in the conductor effect one another, much like billiard balls in a line. 3-9
Resistance Resistance is the opposition to the movement of electrons, or current flow. Resistance is measured in units called ohms. Most resistance sources are designed into the circuit and are known as loads, such as light bulbs or motors. As a matter of fact, all electrical devices, including wires, have some resistance. As a resistance works to oppose current flow, it changes electrical energy into some form of energy, such as heat, light, or motion. Resistance Factors The resistance of a conductor is determined by a combination of four factors: Atomic structure (how many free electrons). The more free electrons a material has, the less resistance it offers to current flow. Figure 3-5, Atom of Copper 3-10
Length. The longer the conductor, the higher the resistance. Width (cross-sectional area). The larger the cross-sectional area of a conductor, the lower the resistance (a bigger pipe flows more water). Temperature. For most materials, the higher the temperature, the higher the resistance. There are a few materials whose resistance goes down as temperature goes up. Wire Chart Below is a wire size conversion chart from Metric to American wire gauge. Figure 3-6, Conversion Chart Metric to American Wire Gauge Wanted/Unwanted Resistance Resistance is useful in electrical circuits. We use it to produce heat, make light, limit current, and regulate voltage. However, resistance in the wrong place cause circuit. Unwanted resistance can cause component failure, diminished component operation, etc. 3-11
Predicting Resistance Sometimes you can predict that high (unwanted) resistance is present by just looking at an electrical connection. Expect resistance to be high if the: Material is discolored Wires are loose Connection is loose Wires are to small Corrosion Resistance can also be affected by the physical condition of a conductor. For example, battery terminals are made of lead, ordinarily an excellent conductor. However, when a battery terminal is covered with corrosion, resistance is substantially increased. This makes the terminal a less effective conductor. OHMS The basic unit of resistance measurement is the ohm. The symbol for ohms is the Greek letter Omega (Ω). If the resistance of a material is high (close to infinite ohms), it is an insulator. If the resistance of a material is low (close to zero ohms), it is a conductor Resistor Ratings Resistors are rated by how many ohms of resistance they create and by how many watts they can handle. Common ratings for carboncomposition resistors are ¼ watt, ½ watt, 1 watt and 2 watts. A resistor converts electrical energy to heat. As the resistor works, it always generates some heat. If a resistor is forced to handle more watts than it was designed for, it will generate excessive heat. When substantially overloaded, it may fail prematurely. 3-12
Figure 3-7, Resistor Ratings Figure 3-8, Resistor Color Codes 3-13
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Power The final property of electricity is power. Power is the rate at which work is being done in a circuit. The output of automotive engines is usually expressed in horsepower, as is the output of electric motors. Many electrical devices are rated by how much electrical power they consume, rather than by how much power they produce. Power consumption is expressed in watts. 746 watts = 1 horsepower The relationship among power, voltage and current is determined by the Power Formula. The basic equation or the Power Formula is P=I X E, or Watts=Amps X volts. Power is the product of current multiplied times the voltage. In a circuit, if voltage or current increases, power increases. In a circuit, if current decreases, power decreases. Definition of a Watt The unit of measurement for power is the watt. One watt is equal to oneampere times one volt. The most common application of a rating in watts is probably the light bulb. The number of watts they consume classes light bulbs. So are resistors. Common examples of additional items with wattage ratings are audio speakers, some motors, and most home appliances. In an electrical circuit, resistance is the thing that uses electrical power. Recall, however, that many kinds of devices can have resistance. Devices that offer electrical resistance include conductors, insulators, resistors, coils, and motors. You can multiply the voltage times the current in any circuit and find how much power is consumed. For example, a typical hair dryer can draw almost 10 amps of current. You know that the voltage in your home is about 120. Multiply these two values and you get 1200 watts 3-15
Exercise Read each questions carefully and answer by filling in the blanks. 1. The movement of electrons in a conductor is called. a. DC voltage b. Current c. AC voltage d. Resistance 2. Electromotive Force (EMF) and potential mean the same thing as. a. current b. work c. voltage d. wattage 3. Materials that are easy for electricity to flow through are called. a. conductors b. insulators c. resistors d. isotopes 4. Materials that are hard for electricity to flow through are called. a. conductors b. insulators c. resistors d. isotopes 3-16
5. Electricity flows through a conductor when there is a difference in: a. potential b. current c. both a and b d. neither a and b 6. When current flows in a single direction, it is called a. Direct current b. Alternating current c. Fluctuating current d. Oscillating current 7. The kind of current used in most automotive electrical circuits is: a. Direct current b. Alternating current c. Fluctuating current d. Oscillating current 8. Service manual schematics use a. Conventional flow theory b. Electron flow theory c. Ohm s Law d. Electromotive force theory 9. The basic unit of measurement for power is the a. Ampere b. Volt c. Watt d. Ohm 3-17