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Chapter 3
Electric Circuits
Objectives
• Upon completion of this course, you will
be able to:
– Explain the concepts of a basic electric
circuit
– Explain the characteristics of a series circuit
– Explain the characteristics of a parallel circuit
– Describe how series circuits are used as
control circuits in the air-conditioning industry
Objectives (cont’d.)
– Describe how parallel circuits are used as
power circuits in the air-conditioning industry
– Explain the relationship and characteristics of
the current, resistance, and electromotive
force in a series circuit
– Explain the relationship and characteristics of
the current, resistance, and electromotive
force in a parallel circuit
Objectives (cont’d.)
– Calculate the current, resistance, and
electromotive force in a series circuit
– Calculate the current, resistance, and
electromotive force in a parallel circuit
– Explain the characteristics of the seriesparallel circuit
– Describe how series-parallel circuits are
utilized in the air-conditioning industry
Key Terms
•
•
•
•
•
Closed
Control circuit
Electric circuit
Open
Parallel circuit
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•
•
•
Power circuit
Series circuit
Series-parallel circuit
Voltage drop
Introduction
• Electrical systems
– Composed of various circuits
• Important circuit types
– Parallel circuit: more than one path
– Series circuit: only one path
– Series-parallel circuit: combination of
series and parallel circuits
Basic Concepts of Electric Circuits
• Electric circuit
– Complete path of an electric current and
any necessary elements (e.g., load)
• Closed circuit
– Circuit is complete and current can flow
• Open circuit
– Path of current flow is interrupted
Basic Concepts of Electric Circuits
(cont’d.)
• Electric circuits must have a:
– Complete path for electrons to flow through
– Source of electrons
– Some electric device (load) that requires
electric energy
Figure 3.3 Basic electric circuit with the circuit components labeled.
(Delmar/Cengage Learning)
Series Circuits
• Simplest and easiest electric circuit
– Allows only one path of current flow through
the circuit
• Examples: switches and controls
• Applications
– Control circuit
• Electric circuit that controls some major load
Figure 3.5 Series circuit with three switches controlling an electric motor.
(Delmar/Cengage Learning)
Series Circuits (cont’d.)
• Characteristics and calculations for
current, resistance, and voltage
– Current draw is the same throughout the
entire circuit
• Only one path for the current to flow
• It = I1 = I2 = I3 = I4 . . .
– Total resistance is the sum of all the
resistances in the circuit
• Rt = R1 + R2 + R3 + R4 + ...
Series Circuits (cont’d.)
• Voltage is completely used by all the
loads
– Changes through each load
– Voltage drop: amount used or lost through
any load or conductor when moving current
through that part of the circuit
• Sum of the voltage drops of a series circuit
equals the voltage being applied to the circuit
• Et = E1 + E2 + E3 + E4 + ...
Parallel Circuits
• More than one path for electrons to flow
– Each device is connected to both supply
voltage conductors
• Common in industry
– Most loads operate from line voltage
• Voltage supplied to equipment from main power
source
Figure 3.9 Parallel circuit with four components; each
component is supplied with 120 volts.
(Delmar/Cengage Learning)
Parallel Circuits (cont’d.)
• Applications
– Power circuits
• Supply correct
line voltage to
several circuits
Figure 3.10 Control system with several
circuits (parallel); each circuit is supplied
with line voltage. (Delmar/Cengage Learning)
Parallel Circuits (cont’d.)
• Characteristics and calculations for
current, resistance, and voltage
– Calculations are usually done by
equipment designer
– Current draw: determined for each part of
the circuit
• Depends on resistance of that portion
Parallel Circuits (cont’d.)
• Total current draw: sum of currents in
individual sections
– Calculated by using Ohm’s law when
voltage and resistance are known
• Total ampere draw: It = I1 + I2 + I3 + I4 +...
Parallel Circuits (cont’d.)
• Resistance gets smaller as more
resistances are added
– Total resistance can not be obtained by
taking the sum of all the resistances
• Two resistors: Rt = (R1 x R2)/(R1 + R2)
• More than two resistors: 1/Rt = 1/R1 + 1/R2 +
1/R3 + 1/R4 + …
Parallel Circuits (cont’d.)
• Voltage drop
– Line voltage being supplied to the load
• Each load uses the total voltage being supplied
to the load
– Equation for voltage applied to each of four
components:
• Et = E1 + E2 + E3 + E4
Series-Parallel Circuits
• Combination of series and parallel
circuits
– Used sparingly
• More often seen on full wiring layouts
– Often used to combine control circuits with
circuits that supply power to loads
Figure 3.12 Series-parallel circuit with four loads and controlling
switches (switches in series with loads in parallel).
(Delmar/Cengage Learning)
Series-Parallel Circuits (cont’d.)
• Applications
– Used in most types of heating, cooling, and
refrigeration equipment
• Allows required voltage to be supplied to
electrical loads in the system
• Both series and parallel circuits are used to
supply electrical power to loads
Figure 3.13
Wiring diagram of a
packaged air-conditioning
unit including series-parallel
circuit arrangement where
switches are in series to
loads and loads are in
parallel.
(Delmar/Cengage Learning)