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AUT 242 – Automotive Electricity II

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  1. AUT 242 – Automotive Electricity II Series Circuits

  2. SERIES CIRCUITS • A series circuit is a complete circuit that has only one path for current to flow through all of the electrical loads. • Electrical components such as fuses and switches are generally not considered to be included in the determination of a series circuit.

  3. OHM’S LAW AND SERIES CIRCUITS • Ohm’s law can be used to calculate the value of one unknown (voltage, resistance, or amperes) if the other two values are known. • 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.

  4. SERIES CIRCUITSCONTINUITY • The circuit must be continuous without any breaks. • This is called continuity. • Every circuit must have continuity in order for current to flow through the circuit. • Because there is only one path for current to flow, the current is the same everywhere in a complete series circuit.

  5. OHM’S LAW AND SERIES CIRCUITSSERIES CIRCUIT TOTAL RESISTANCE • A series circuit is a circuit containing more than one resistance in which all current must flow through all resistances in the circuit. • Ohm’s law can be used to calculate the value of one unknown (voltage, resistance, or amperes) if the other two values are known. • Because all current flows through all resistances, the total resistance is the sum (addition) of all resistances.

  6. OHM’S LAW AND SERIES CIRCUITSSERIES CIRCUIT TOTAL RESISTANCE • A series circuit with three bulbs. • All current flows through all resistances (bulbs). • The total resistance of the circuit is the sum of the individual resistances of each bulb • The bulbs will light dimly because of the increased resistance and the reduction of current flow (amperes) through the circuit.

  7. 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 another load, such as a light bulb, were added in series, the current flow would decrease and the bulbs would be dimmer than before the other bulb was added. If the flow exceeds the capacity of the fuse or the circuit breaker, then the circuit is opened and all current flow stops.

  8. KIRCHHOFF’S VOLTAGE LAWDEFINITION • A 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.

  9. 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. KIRCHHOFF’S VOLTAGE LAWApplying Kirchhoff’s Voltage Law

  10. In a series circuit, the voltage is dropped or lowered by each resistance in the circuit. The higher the resistance, the greater the drop in voltage. KIRCHHOFF’S VOLTAGE LAWApplying Kirchhoff’s Voltage Law

  11. A voltmeter reads the differences of voltage between the test leads. The voltage read across a resistance is the voltage drop that occurs when current flows through a resistance. A voltage drop is also called an “IR” drop because it is calculated by multiplying the current (l) through the resistance (electrical load) by the value of the resistance (R). KIRCHHOFF’S VOLTAGE LAWApplying Kirchhoff’s Voltage Law

  12. KIRCHHOFF’S VOLTAGE LAWUse of Voltage Drops • Voltage drops, due to built-in resistance, are used in automotive electrical systems to drop the voltage in the following examples. • Dash lights • Blower motor

  13. KIRCHHOFF’S VOLTAGE LAWVoltage Drops as a Testing Method • Any resistance in a circuit causes the voltage to drop in proportion to the amount of the resistance. • The formula for voltage drop is: • E = I xR where E is the voltage drop and I is the current in the circuit. • Notice that as the value of the resistance (R ) increases, the voltage drop increases.

  14. SERIES CIRCUIT LAWS • Electrical loads or resistance connected in series behave following series circuit laws. • Law 1 The total resistance in a series circuit is the sum total of the individual resistances. • Law 2 The current is constant throughout the entire circuit. • Law 3 Although the current (in amperes) is constant, the voltage drops across each resistance in the circuit.

  15. In this series circuit with a 2 ohm resistor and a 4 ohm resistor, the current (2 amperes) is the same throughout, even though the voltage drops across each resistor are different. SERIES CIRCUIT LAWS

  16. Why Check the Voltage Drop Instead of Measuring the Resistance? • Imagine a wire with all strands cut except for one. An ohmmeter can be used to check the resistance of this wire and the resistance would be low, indicating that the wire was okay, but this one small strand cannot properly carry the current (amperes) in the circuit.

  17. Why Check the Voltage Drop Instead of Measuring the Resistance? • A voltage drop test is therefore a better test to determine the resistance in components for two reasons: • An ohmmeter can only test a wire or component that has been disconnected from the circuit and is not carrying current. The resistance can, and does, change when current flows. • A voltage drop test is a dynamic test because as the current flows through a component, the conductor increases in temperature, which in turn increases resistance. This means that a voltage drop test is testing the circuit during normal operation and is therefore the most accurate way of determining circuit conditions.

  18. Why Check the Voltage Drop Instead of Measuring the Resistance? • A voltage drop test is also easier to perform because the resistance does not have to be known, only that the loss of voltage in a circuit should be less than 3%, or less than about 0.36 volt for any 12 volt circuit.

  19. Light Bulbs and Ohm’s Law • If the resistance of a typical automotive light bulb is measured at room temperature, the resistance will often be around 1 ohm. If 12 volts were to be applied to this bulb, a calculated current of 12 amperes would be expected. However, as current flows through the filament of the bulb, it increases in temperature and becomes incandescent, thereby giving off light.

  20. Light Bulbs and Ohm’s Law • When the bulb is first connected to a power sourceand current starts to flow, a high amount of current, called surge current, flows through the filament. Then within a few thousandths of a second, the current flow is reduced to about 10% of the surge current due to the increasing resistance of the filament, resulting in an actual current flow of about 1.2 A or about 100 ohms of resistance when the bulb is working.

  21. FIGURE 5–7 Example 1. SERIES CIRCUIT EXAMPLES

  22. FIGURE 5–8 Example 2. SERIES CIRCUIT EXAMPLES

  23. FIGURE 5–9 Example 3. SERIES CIRCUIT EXAMPLES

  24. FIGURE 5–10 Example 4. SERIES CIRCUIT EXAMPLES

  25. SUMMARY • In a simple series circuit, the current remains constant throughout, but the voltage drops as current flows through the resistances of the circuit. • 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. • The sum (total) of the voltage drops equals the applied voltage (Kirchhoff’s voltage law). • An open or a break anywhere in a series circuit stops all current from flowing.

  26. REVIEW QUESTIONS • What is Kirchhoff’s voltage law? • What would current (amperes) do if the voltage were doubled in a circuit? • What would current (amperes) do if the resistance in the circuit were doubled? • What is the formula for voltage drop?

  27. CHAPTER QUIZ • The amperage in a series circuit is _____. • The same anywhere in the circuit • Variable in the circuit due to the different resistances • High at the beginning of the circuit and decreases as the current flows through the resistance • Always less returning to the battery than leaving the battery

  28. CHAPTER QUIZ 2. The sum of the voltage drops in a series circuit equals the _____. • Amperage • Resistance • Source voltage • Wattage

  29. CHAPTER QUIZ 3. If the resistance and the voltage are known, what is the formula for finding the current (amperes)? • E = I xR • I = E xR • R = E xI • I = E/R

  30. CHAPTER QUIZ 4. A series circuit has three resistors of 6 ohms each. The voltage drop across each resistor is 4 volts. Technician A says that the source voltage is 12 volts. Technician B says that the total resistance is 18 ohms. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

  31. CHAPTER QUIZ 5. If a 12 volt battery is connected to a series circuit with three resistors of 2 ohms, 4 ohms, and 6 ohms, how much current will flow through the circuit? • 1 ampere • 2 amperes • 3 amperes • 4 amperes

  32. CHAPTER QUIZ 6. A series circuit has two 10 ohm bulbs. A third 10 ohm bulb is added in series. Technician A says that the three bulbs will be dimmer than when only two bulbs were in the circuit. Technician B says that the current in the circuit will increase. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

  33. CHAPTER QUIZ 7. Technician A says that the sum of the voltage drops in a series circuit should equal the source voltage. Technician B says that the current (amperes) varies depending on the total value of the resistance in a series circuit. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

  34. CHAPTER QUIZ 8. Two light bulbs are wired in series and one bulb burns out (opens). Technician A says that the other bulb will work. Technician B says that the current will increase in the circuit because one electrical load (resistance) is no longer operating. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

  35. CHAPTER QUIZ 9. Four resistors are connected to a 12 volt battery in series. The values of the resistors are 10 ohms, 100 ohms, 330 ohms, and 470 ohms. Technician A says that the greatest voltage drop will occur across the 10 ohm resistor. Technician B says that the greatest voltage drop will occur across the 470 ohm resistor. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B

  36. CHAPTER QUIZ 10. Three light bulbs are wired in series. A fourth bulb is connected to the circuit in series. Technician A says that the total voltage drop will increase. Technician B says that the current (amperes) will decrease. Which technician is correct? • Technician A only • Technician B only • Both Technicians A and B • Neither Technician A nor B