21
Electrical Circuits
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
CIRCUITS
• Definition
• A circuit is a complete 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).
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
CIRCUITS
• Parts of a Complete Circuit
• Power source
• Protection
• Power path
• Electrical load
• Return path (ground)
• Switches and controls
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
FIGURE 21.1 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.
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
FIGURE 21.2 The return path back to the battery can be any electrical conductor, such as a copper wire or the
metal frame or body of the vehicle.
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
FIGURE 21.3 An electrical switch opens the circuit and no current flows. The switch could also be on the return
(ground) path wire.
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
CIRCUIT FAULTS
• Open Circuits
• An open circuit is any circuit that is not complete, or
that lacks continuity, such as a broken wire
• Note: A blown fuse opens the circuit to prevent damage
to the components or wiring in the circuit in the:
• Event of an overload caused by a fault in the circuit.
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
FIGURE 21.4 Examples of common causes of open circuits. Some of these causes are often difficult to find.
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
CIRCUIT FAULTS
• Short-to-Voltage
• If a wire (conductor) or component is shorted to
voltage, it is commonly referred to as being shorted.
• When the power side of one circuit is electrically
connected to the power side of another circuit.
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
FIGURE 21.5 A short circuit permits electrical current to bypass some or all of the resistance in the circuit.
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
FIGURE 21.6 A fuse or circuit breaker opens the circuit to prevent possible overheating damage in the event of a
short circuit.
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
CIRCUIT FAULTS
• Short-To-Ground
• A short-to-ground is a type of short circuit that occurs
when the current bypasses part of the normal circuit;
• And flows directly to ground.
• Real-World Fix
• The Short-to-Voltage Story
• High Resistance
• Corroded connections or sockets
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
FIGURE 21.7 A short-to-ground affects the power side of the circuit. Current flows directly to the ground return,
bypassing some or all of the electrical loads in the circuit. There is no current in the circuit past the short. A short-toground will also cause the fuse to blow.
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
CIRCUIT FAULTS
• Loose terminals in a connector
• Loose ground connections
• Slow operation of a motor-driven unit, such as the
windshield wipers or blower motor
• Dim lights
• “Clicking” of relays or solenoids
• No operation of a circuit or electrical component
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
OHMS LAW
• Definition
• It requires 1 volt to push 1 ampere through 1 ohm of
resistance.
• Formulas
• I = Current in amperes (A)
• E = Electromotive force (EMF) in volts (V)
• R = Resistance in ohms (Ω)
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
FIGURE 21.8 To calculate one unit of electricity when the other two are known, simply use your finger and cover
the unit you do not know. For example, if both voltage (E) and resistance (R) are known, cover the letter I
(amperes). Note 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.
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
CHART 21.1 Ohm’s law relationship with the three units of electricity.
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
OHMS LAW
• Ohm’s Law Applied to Simple Circuits
• If two factors are known (volts and ohms in this
example):
• The remaining factor (amperes) can be calculated
using Ohm’s law
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
OHMS LAW
• Note: Before applying Ohm’s law, be sure that each unit
of electricity is converted into base units.
• For example, 10 KΩ should be converted to 10,000
ohms, and 10 mA should be converted into 0.010 A.
• Tech Tip
• Think of a Waterwheel
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458
21
ELECTRICAL CIRCUITS
FIGURE 21.9 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.
Introduction to Automotive Service
James Halderman Darrell Deeter
© 2013 Pearson Higher Education, Inc.
Pearson Prentice Hall - Upper Saddle River, NJ 07458