chap33 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 33, the reader should be able to: • Prepare for ASE Electrical/Electronic Systems • • • • (A6) certification test content area “A” (General Electrical/Electronic System Diagnosis) Identify a series circuit State Kirchhoff’s voltage law Calculate voltage drops in a series circuit Explain series and parallel circuit laws 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 OBJECTIVES: After studying Chapter 33, the reader should be able to: • • State Kirchhoff’s current law Identify where faults in a series-parallel circuit can be detected or determined 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: branches • combination circuit • compound circuit Kirchhoff’s current law • Kirchhoff’s voltage law leg • parallel circuit series circuit • series-parallel circuits • shunt total circuit resistance • voltage drop 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 OHM’S LAW AND SERIES CIRCUITS A series circuit is a complete circuit that has more than one  electrical load where all of the current has only one path to flow  through all of the loads Electrical components such as fuses and switches are generally not  considered to be included in the determination of a series circuit.  The circuit must be continuous or have continuity in order for  current to flow through the circuit NOTE: An electrical load needs power and a ground to operate. A break  (open) in a series circuit will cause the current in the circuit to stop Ohm’s law can be used to calculate the value of one unknown  (voltage, resistance, or amperes) if the other values are known 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 Because all current flows through all resistances, total resistance is  the sum (addition) of all resistances.  Total resistance of the circuit  shown here is 6 ohms (1Ω = 2Ω + 3Ω) The formula for total resistance  (RT) for a series circuit is: Using Ohm’s law to find current flow:  Continued Figure 33–1 A series circuit with three bulbs All current flows through all resistances (bulbs) The total resistance of the circuit is the sum of the total resistance of the bulbs, and the bulbs will light dimly because of the increased resistance and the reduction of current flow (amperes) through the circuit 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 With a total resistance of 6 ohms using a 12­volt battery in the series circuit shown, 2 amperes of current will flow through the  entire circuit. If resistance is reduced, more current will flow Here, one resistance has been  eliminated and now the total resistance  is 3 ohms (1Ω + 2Ω) Using Ohm’s law to calculate current  flow yields 4 amperes Figure 33–2 A series circuit with two bulbs Notice current flow was doubled  (4 amps  instead of 2 amps) when the resistance  was cut in half (from 6 ohms to 3 ohms) Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman Continued ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Farsighted Quality of Electricity Electricity almost seems to act as if it “knows” what resistances are ahead on the long trip through a circuit If the trip through the circuit has many high-resistance components, very few electrons (amperes) will choose to attempt to make the trip If a circuit has little or no resistance (for example, a short circuit), then as many electrons (amperes) as possible attempt to flow through the complete circuit If flow exceeds the capacity of the fuse or the circuit breaker, then the circuit is opened and all current flow stops 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 KIRCHOFF’S VOLTAGE LAW Voltage applied through a series circuit drops with each resistor.  The greater the resistance, the greater the drop in voltage German physicist, Gustav Robert Kirchhoff (1824–1887),  developed laws about electrical circuits. His second law,  Kirchhoff’s voltage law, concerns voltage drops It states: The voltage around any closed circuit is equal to the sum  (total) of the voltage drops across the resistances 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 Applying Kirchhoff’s Voltage Law Kirchhoff states in his second law that voltage drops in proportion to resistance and the total of voltage drops will equal the applied voltage.  Figure 33–3 As current flows through a circuit, the voltage drops in proportion to the amount of resistance in the circuit Most, if not all, of the resistance should occur across the load such as the bulb in this circuit All of the other components and wiring should produce little, if any, voltage drop If a wire or connection did cause a voltage drop, less voltage would be available to light the bulb and the bulb would be dimmer than normal.circuit with two bulbs 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 The unknown is the amount of current in the circuit. The Ohm’s law  equation for determining current is: I = E/R R represents total resistance. Because there are two equal  resistances (8Ω), they can be replaced by one resistance of 4Ω (RTotal= Value/Number = 8Ω/2 = 4Ω) Total resistance of circuit  containing two 8­ohm and  one 4­ohm resistor is 2 ohms.  Battery current flow is then  calculated to be 6A I = E/R = 12V/2Ω = 6A  Figure 33–22 Example Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 42 Continued ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SERIES-PARALLEL CIRCUITS Series­parallel circuits are a combination of series and parallel  segments in one complex circuit A series­parallel circuit is called a compound or a combination  circuit. Many automotive circuits include sections that are in  parallel and in series A series­parallel circuit includes both parallel loads or resistances,  plus additional loads or resistances that are electrically connected in  series There are two basic types of series­parallel circuits 43 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 A circuit where the load is in series with other loads is parallel. An  example of this type of circuit is a dash light dimming circuit.  The variable resistor is used to limit current flow to the dash light  bulbs, which are wired in parallel.  Figure 33–23 A series-parallel circuit Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 44 Continued ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 A circuit where a parallel circuit contains resistors or loads is in series in one  or more branches. A headlight and starter circuit is an example of this type.   A headlight switch is usually  connected in series with a  dimmer switch and in parallel  with the dash light dimmer  resistors.  The headlights are also  connected in parallel along  with the taillights and side  marker lights.  Figure 33–24 This complete headlight circuit with all bulbs and switches is a series- parallel circuit Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 45 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Series­Parallel Circuit Faults  If a conventional parallel circuit,  such as a taillight circuit, had a fault that increased the resistance in  one branch of the circuit, the current flow through that one branch  will be reduced Added resistance, due to corrosion or similar cause, would create a  voltage drop. As a result of this drop, a lower voltage would be  applied and the bulb in the taillight would be dimmer than normal.  Because brightness of the bulb depends on voltage and current  applied, lower voltage and current would cause the bulb to be  dimmer. If added resistance occurred in part of the circuit that fed  both taillights, then both taillights would be dimmer than normal In this case, the added resistance created a series­parallel circuit that  was originally just a simple parallel circuit Continued 46 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 SOLVING SERIES-PARALLEL CIRCUIT PROBLEMS The key to solving series­parallel circuit problems is to combine or  simplify as much as possible.  If there are two loads or  resistances in series in a parallel branch or leg, then the circuit can be made simpler if the two are first added together before attempting to solve the parallel section.  Figure 33–25 Solving a series-parallel circuit problem Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 47 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SERIES-PARALLEL CIRCUIT EXAMPLES Each of the four examples includes solving for the following: Total resistance Current flow (amperes) through each branch, as well as  total current flow Voltage drop across each resistance  48 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 The unknown resistor is in series with the other two resistances,  which are connected in parallel. The Ohm’s law equation to  determine resistance is: R = E/I = 12V/3A = 4Ω The total resistance of the circuit is 4 ohms, and the value of the  unknown can be determined by subtracting the value of the two  resistors that are connected in parallel. The parallel branch  resistance is 2Ω The value of the unknown resistance is 2Ω Total R = 4Ω – 2Ω = 2Ω Figure 33–26 Example Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 49 Continued ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 The unknown unit in this circuit is the voltage of the battery. The  Ohm’s law equation is:  E = I × R  Because each branch contains two 4­ohm resistors in series, values  in each can be added to help simplify the circuit. By adding the  resistors in together, the circuit now consists of two 8­ohm resistors Inserting the value for total  resistance into the Ohm’s law  equation results in a value of 12  volts for the battery voltage E = I × R  E = 3A × 4Ω   E = 12 volts   Figure 33–27 Example Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 50 Continued ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 In this example, the total current through the circuit is unknown.  The Ohm’s law equation to solve for it is: I = E/R To solve for total resistance, the circuit can be simplified by adding  R3 and R4 together because these two resistors are in series in the  same branch of the parallel circuit, similar to Example 2.  With the branches reduced to just one 4­ohm resistor, this can be  added to the 2­ohm (R1) resistor  because it is in series, creating a total circuit resistance of 6 ohms.  Current flow can now be determined from Ohm’s law: I = E/R = 12V/6Ω = 2A  Figure 33–28 Example Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 51 Continued ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 In this example, the value of resistor R1 is unknown. Total resistance  of the circuit is 3 ohms:  R = E/I = 12V/4A = 3Ω Knowing total resistance is not enough to determine value of R1. To  simplify, R2 and R5 combine to create a parallel branch resistance  value of 8 ohms because they are in series.  With the circuit simplified to one resistor in series (R1) with 4 ohm  branches with 4 ohms, the branches can be reduced to the equal of  one 2­ohm resistor The circuit includes one 2­ ohm resistor plus unknown  R1. Total resistance is 3  ohms, so R1 must be 1 ohm 3Ω – 2Ω = 1Ω Figure 33–29 Example Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 52 Continued ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY Series circuits: a In a simple series circuit, the current remains constant  throughout, but the voltage drops as current flows through the resistances of the circuit b The voltage drop across each resistance or load is directly  proportional to the value of the resistance compared to the total resistance in the circuit c The sum (total) of the voltage drops equals the applied voltage (Kirchhoff’s voltage law) d An open or a break anywhere in a series circuit stops all current from flowing 53 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 SUMMARY (cont.) Parallel circuits: a A parallel circuit, such as is used for all automotive lighting,  has the same voltage available to each resistance (bulb) b The total resistance of a parallel circuit is always lower than  the smallest resistance c The separate paths that split and meet at junction points are  called branches, legs, or shunts d Kirchhoff’s current law states: “The current flowing into a  junction of an electrical circuit is equal to current flowing out of that junction.” 54 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 SUMMARY (cont.) Series­Parallel circuits: a A series­parallel circuit is also called a compound circuit or a combination circuit b A series­parallel circuit is a combination of a series and a  parallel circuit, which does not include fuses or switches c A fault in a series portion of the circuit would affect the  operation if the series part was in the power or the ground  side of the parallel portion of the circuit d A fault in one leg of a series­parallel circuit will affect just  the component(s) in that one leg Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman 55 ©©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 56 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458