chap32 pps Automotive technology at University of Cambridge

start Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 OBJECTIVES: After studying Chapter 32, the reader should be able to: • • • • • Prepare for ASE Electrical/Electronic Systems (A6) certification test content area “A” (General Electrical/Electronic Systems Diagnosis) State Ohm’s law Identify the parts of a complete circuit State Watt’s law Describe the characteristics of an open circuit, a short-toground, and a short-to-voltage Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 KEY TERMS: circuit • continuity ground (return) path • grounded insulated path kilowatt • load • Ohm’s law • open circuit power source • protection shorted • short-to-ground • short-to-voltage Watt’s law Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 CIRCUITS A circuit is a path that electrons travel from a power source (such as a battery) through a load (such as a light bulb) and back to the power source It is called a circuit because the current must start and finish at the same place (power source) For any circuit to work, it must be continuous from the battery through all wires and components and back to the battery (ground) A circuit that is continuous throughout is said to have continuity Figure 32–1 All complete circuits must have a power source, a power path, protection (fuse), an electrical load (light bulb in this case), and a return path back to the power source Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Parts of a Complete Circuit Every complete circuit contains: A power source, such as a vehicle’s battery Fuses, circuit breakers, and fusible links, which are Protection from harmful overloads (excessive current flow) An insulated path for current flow from the power source to the resistance This path from a power source to the load (a light bulb in this example) is usually an insulated copper wire The electrical load or resistance converts electrical energy into heat, light, or motion A ground (return) path for the electrical current from the load back to the power source so that there is a complete circuit This ground path is usually the metal body, frame, and engine block of the vehicle See Figure 32-2 Switches and controls turn the circuit on and off Figure 32-3 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 32–2 The return path back to the battery can be any electrical conductor, such as the metal frame or body of the vehicle Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 32–3 An electrical switch opens the circuit and no current flows The switch could also be on the return (ground) path wire Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Open Circuits An open circuit is any circuit that is not complete, or that lacks continuity See Figure 32–4 No current at all will flow through an incomplete circuit An open circuit may be created by a break in the circuit or by a switch that opens (turns off) the circuit and prevents the flow of current In any circuit containing a power load and ground, an opening anywhere in the circuit will cause the circuit to stop working A light switch in a home and the headlight switch in a vehicle are examples of devices that open a circuit to control its operation Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 32–4 This figure shows examples of common causes of open circuits Some of these causes are often difficult to find Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Use a Schematic as a Road Map The wiring schematic is the “road map” of a circuit and shows all electrical paths If an open occurs in a circuit, the current stops flowing and the electrical load device does not work Trace the circuit by following the path from the battery through the power side component, load, and on the ground Check for voltage at various points in the circuit to locate where the open is in the circuit Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 10 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Think of a Waterwheel - Part As water drops from a higher level to a lower level, high potential energy (or voltage) is used to turn the waterwheel and results in low potential energy (or lower voltage) The same amount of water (or amperes) reaches the pond under the waterwheel as started the fall above the waterwheel As current (amperes) flows through a conductor, it performs work in the circuit (turns the waterwheel) while its voltage (potential) is dropped Figure 32–8 Electrical flow through a circuit is similar to water flowing over a waterwheel The more the water (amperes in electricity), the greater the amount of work (waterwheel) The amount of water remains constant, yet the pressure (voltage in electricity) drops as the current flows through the circuit Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 19 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 OHM’S LAW German physicist, Georg Ohm, established that electric pressure (EMF) in volts, electrical resistance in ohms, and the amount of current in amperes flowing through any circuit are all related Ohm’s law states: It requires volt to push ampere through ohm of resistance This means that if the voltage is doubled, then the number of amperes of current flowing through a circuit will also double if the resistance of the circuit remains the same Ohm’s law can also be stated as a simple formula used to calculate one value of an electrical circuit if the other two are known See Figure 32–9 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 20 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 32–9 To calculate one unit of electricity when the other two are known, simply use your finger and cover the unit you not know For example, if both voltage (E) and resistance (R) are known, cover the letter I (amperes) Notice that the letter E is above the letter R so divide the resistor’s value into the voltage to determine the current in the circuit Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 21 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 OHM’S LAW STATED I = E/R where: I = Current in amperes (A) E = Electromotive force (EMF) in volts (V) R = Resistance in ohms (Ω) Ohm’s law can determine the resistance if the volts and amperes are known: R = E/I Ohm’s law can determine the voltage if the resistance (ohms) and amperes are known: E = I × R Ohm’s law can determine the amperes if the resistance and voltage are known: I = E/R Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 22 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 OHM’S LAW RELATIONSHIPS See the chart on Page 328 of your textbook Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 23 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Ohm’s Law Applied to Simple Circuits If a battery with 12 volts is connected to a resistor of ohms, as shown here, how many amperes will flow through the circuit? Using Ohm’s law, we can calculate the number of amperes that will flow through the wires and the resistor Figure 32–10 This closed circuit includes a power source, power-side wire, circuit protection (fuse), resistance (bulb), and return path wire If two factors are known (volts and ohms in this example), the remaining factor (amperes) can be calculated using Ohm’s law Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 24 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 I = E/R = 12 V/4Ω The values for voltage (12) and resistance (4) were substituted for the variables E and R, making I = amperes (12/4 = 3) If we want to connect a resistor to a 12-volt battery, we now know that this simple circuit requires amperes to operate This may help us for two reasons We can now determine the wire diameter that we will need based on the number of amperes flowing through the circuit The correct fuse rating can be selected to protect the circuit Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 25 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 WATT’S LAW James Watt (1736–1819), a Scottish inventor, first determined the power of a typical horse while measuring the amount of coal being lifted out of a mine The power of one horse was determined to be 33,000 foot-pounds per minute Electricity can also be expressed in a unit of power called a watt and the relationship is known as Watt’s law, which states: A watt is a unit of electrical power represented by a current of ampere through a circuit with a potential difference of volt Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 26 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 The symbol for a watt is the capital letter W The formula for watts is: W=I× E Another way to express this formula uses the letter P to represent the unit of power The formula then becomes: P=I× E HINT: An easy way to remember this equation is that it spells “pie.” Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 27 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 To calculate watts, both the current in amperes and the voltage in the circuit must be known If two of these factors are known, the other remaining factor can be determined by the following equation: P = I × E (watts equal amperes times voltage) I = P/E (amperes equal watts divided by voltage) E = P/I (voltage equals watts divided by amperes) A Watt’s circle can be drawn and used like the Ohm’s law circle diagram See Figure 32–11 Magic Circle The formulas for calculating any combination of electrical units are shown in Figure 32–12 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 28 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 32–11 To calculate one unit when the other two are known, simply cover the unknown unit to see what unit needs to be divided or multiplied to arrive at the solution Figure 32–12 “Magic circle” of most of the formulas for problems involving Ohm’s law Each quarter of the “pie” has formulas used to solve for a particular unknown value: current (amperes), in the upper right segment; resistance (ohms), in the lower right; voltage (E), in the lower left; and power (watts), in the upper left Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 29 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Wattage Increases by the Square of the Voltage The brightness of a light bulb, such as an automotive headlight or courtesy light, depends on the number of watts available The watt is the unit by which electrical power is measured If the battery voltage drops, even slightly, the light becomes noticeably dimmer The formula for calculating power (P) in watts is P × I = E, also be expressed Watts = Amps × Volts According to Ohm’s law, I = E/R Therefore, E/R can be substituted for I in the previous formula resulting in P = E/R × E or P = E2 / R E2 means E multiplied by itself A small change in the voltage (E) has a big effect on the total brightness of the bulb (Remember, household light bulbs are sold according to wattage.) Thus, if voltage to an automotive bulb is reduced, such as by a poor electrical connection, brightness of the bulb is greatly affected A poor electrical ground causes a voltage drop The voltage at the bulb is reduced and the bulb’s brightness is reduced Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 30 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Why are Vehicle Engines for Europe Rated in Kilowatts? Engine power is commonly rated in watts or kilowatts (1,000 watts equal kilowatt) because horsepower is equal to 746 watts For example, a 200-horsepower engine can be rated as having the power equal to 149,200 watts or 149.2 kilowatts (kW) Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 31 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY All complete electrical circuits have a power source (such as a battery), a circuit protection device (such as a fuse), a powerside wire or path, an electrical load, a ground return path, and a switch or a control device A short-to-voltage involves a copper-to-copper connection and often affects more than one circuit A short-to-ground involves a copper-to-steel connection and usually causes the fuse to blow An open is a break in the circuit resulting in absolutely no current flow through the circuit Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 32 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 end Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 33 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458