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40
ELECTRICAL CIRCUITS
AND OHM’S LAW
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
All Rights Reserved
40
ELECTRICAL CIRCUITS AND OHM’S LAW
Objectives
• The student should be able to:
– Prepare for ASE Electrical/Electronic
Systems (A6) certification test content area
“A” (General Electrical/Electronic Systems
Diagnosis).
– Explain Ohm’s law.
– Identify the parts of a complete circuit.
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Objectives
• The student should be able to:
– Explain Watt’s law.
– Describe the characteristics of an open, a
short-to-ground, and a short-to-voltage.
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
CIRCUITS
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Circuits
• Definition
– Complete path electrons travel from power
source (battery) through load (light bulb)
and back
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Circuits
• Definition
– Current must start and finish at same place
(power source)
– Circuit must be continuous (said to have
continuity)
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James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Circuits
• Parts of a Complete Circuit
– Power source (vehicle’s battery)
– Protection from harmful overloads
• Fuses, circuit breakers, fusible links
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James Halderman
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ELECTRICAL CIRCUITS AND OHM’S LAW
Circuits
• Parts of a Complete Circuit
– Power path for the current to flow through
• Power source to load (usually insulated
copper wire)
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Circuits
• Parts of a Complete Circuit
– Electrical load or resistance
• Converts electrical energy into heat, light, or
motion
Automotive Technology, Fourth Edition
James Halderman
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ELECTRICAL CIRCUITS AND OHM’S LAW
Circuits
• Parts of a Complete Circuit
– Return path (ground) for the electrical
current
• Metal body, frame, ground wires, engine
block
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ELECTRICAL CIRCUITS AND OHM’S LAW
Circuits
• Parts of a Complete Circuit
– Switches and controls
• Turn circuit on and off
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ELECTRICAL CIRCUITS AND OHM’S LAW
Figure 40-1 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.
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James Halderman
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ELECTRICAL CIRCUITS AND OHM’S LAW
Figure 40-2 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.
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James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Figure 40-3 An electrical switch opens the circuit and no current flows. The switch could also be
on the return (ground) path wire.
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
CIRCUIT FAULT TYPES
Automotive Technology, Fourth Edition
James Halderman
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ELECTRICAL CIRCUITS AND OHM’S LAW
Circuit Fault Types
• Open Circuits
– Any circuit that is not complete, or that
lacks continuity
– No current at all will flow through an open
circuit
– May be created by break or switch that
opens circuit and prevents flow
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Circuit Fault Types
• Open Circuits
– Opening can be anywhere in circuit
– Fuse blows (opens) when current in circuit
exceeds fuse rating
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ELECTRICAL CIRCUITS AND OHM’S LAW
(TECH TIP Page 429, column 2)
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Figure 40-4 Examples of common causes of open circuits. Some of these causes are often
difficult to find.
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Circuit Fault Types
• Short-to-Voltage
– Power side of one circuit connected to
power side of another circuit
– Complete circuit in which current usually
bypasses resistance
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James Halderman
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ELECTRICAL CIRCUITS AND OHM’S LAW
Circuit Fault Types
• Short-to-Voltage
– Involves copper-to-copper connection (two
power-side wires touching)
– Usually affects more than one circuit
– May or may not blow a fuse
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James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
(REAL WORLD FIX Page 430, column
2)
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Figure 40-5
the circuit.
A short circuit permits electrical current to bypass some or all of the resistance in
Automotive Technology, Fourth Edition
James Halderman
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ELECTRICAL CIRCUITS AND OHM’S LAW
Figure 40-6 A fuse or circuit breaker opens the circuit to prevent possible overheating damage in
the event of a short circuit.
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Circuit Fault Types
• Short-to-Ground
– Current bypasses part of normal circuit and
flows directly to ground
– Because ground return is metal, often
identified as current flowing from copper to
steel
Automotive Technology, Fourth Edition
James Halderman
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ELECTRICAL CIRCUITS AND OHM’S LAW
Circuit Fault Types
• Short-to-Ground
– Occurs where power path wire touches
return path wire or conductor
– Component or circuit shorted to ground
commonly called grounded
– Almost always results in blown fuse,
damaged connectors, melted wires
Automotive Technology, Fourth Edition
James Halderman
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ELECTRICAL CIRCUITS AND OHM’S LAW
Figure 40-7 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-to ground will also cause the fuse to blow.
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Circuit Fault Types
• High Resistance
– Causes
• Corroded connections or sockets
• Loose terminals in a connector
• Loose ground connections
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ELECTRICAL CIRCUITS AND OHM’S LAW
Circuit Fault Types
• High Resistance
– Resulting problems
• Slow operation of a motor-driven unit
• Dim lights
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ELECTRICAL CIRCUITS AND OHM’S LAW
Circuit Fault Types
• High Resistance
– Resulting problems
• “Clicking” of relays or solenoids
• No operation of a circuit or electrical
component
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James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
(TECH TIP Page 431, column 1)
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
OHM’S LAW
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James Halderman
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ELECTRICAL CIRCUITS AND OHM’S LAW
Ohm’s Law
• Definition
– Electric pressure, resistance, and current in
any circuit are related
– It requires 1 volt to push 1 ampere through
1 ohm of resistance
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Ohm’s Law
• Definition
– If voltage doubles, number of amperes
flowing through circuit also doubles if
resistance remains same
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Ohm’s Law
• Formulas
– Can determine resistance if volts and
amperes known: R = E/I
– Can determine voltage if resistance (ohms)
and amperes known: E = I × R
– Can determine amperes if resistance and
voltage known: I = E/R
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Figure 40-9 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). 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.
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Chart 40-1
Ohm’s law relationship with the three units of electricity.
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Ohm’s Law
• Ohm’s Law Applied to Simple Circuits
– Can calculate number of amperes that flow
through wires and resistor
– Can determine wire diameter needed based
on amperage
– Can select correct fuse rating to protect
circuit
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Figure 40-10 This closed circuit includes a power source, power-side wire, circuit protection
(fuse), resistance (bulb), and return path wire. In this circuit, if the battery has 12 volts and the
electrical load has 4 ohms, then the current through the circuit is 4 amperes.
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
WATT’S LAW
Automotive Technology, Fourth Edition
James Halderman
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ELECTRICAL CIRCUITS AND OHM’S LAW
Watt’s Law
• Background
– Watt is unit of electrical power represented
by current of 1 ampere through a circuit
with potential difference of 1 volt
– W is symbol for watt
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James Halderman
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ELECTRICAL CIRCUITS AND OHM’S LAW
Watt’s Law
• Formulas
–W=I×E
– Can also be written as P (power) = I × E
Automotive Technology, Fourth Edition
James Halderman
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ELECTRICAL CIRCUITS AND OHM’S LAW
Watt’s Law
• Formulas
– Engine power commonly rated in watts or
kilowatts
• 1 horsepower equal to 746 watts
Automotive Technology, Fourth Edition
James Halderman
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ELECTRICAL CIRCUITS AND OHM’S LAW
Watt’s Law
• Formulas
– To calculate watts, amperes and voltage in
circuit must be known
– Watt’s circle
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
(TECH TIP Page 432, column 2)
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
All Rights Reserved
40
ELECTRICAL CIRCUITS AND OHM’S LAW
Figure 40-11 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.
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Watt’s Law
• Magic Circle
– Formulas for calculating any combination of
electrical units
Automotive Technology, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
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ELECTRICAL CIRCUITS AND OHM’S LAW
Figure 40-12 “Magic circle” of most 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, Fourth Edition
James Halderman
© 2011 Pearson Education, Inc.
All Rights Reserved