Electric Circuits
Electricity for Refrigeration,
Heating and Air Conditioning 7th
Edition
Chapter 3 Electric Circuits
Electric Circuits
Upon completion of this chapter the student will be
able to:
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Explain the concepts of a basic electric circuits.
Explain the characteristics of a series circuit.
Explain the characteristics of a parallel circuit.
Describe how series circuits are utilized as control circuits in the air conditioning
industry.
Describe how parallel circuits are utilized as power circuits in the airconditioning industry.
Explain the relationship and characteristics of the current, resistance and
electromotive force in a parallel circuit.
Calculate the current resistance, and electromotive force in a series circuit.
Calculate the current, resistance and electromotive force in a parallel circuit.
Electrical Safety
Upon completion of this chapter the student
will be able to:
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Explain the characteristics of the series-parallel circuit.
Describe how series-parallel circuits are utilized in the airconditioning industry.
Key Terms
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Closed
Control Circuit
Electric Circuit
Open
Parallel Circuit
Power Circuit
Series Circuit
Series-Parallel Circuit
Voltage Drop
Basic Concepts of Electric Circuits
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An electric circuit is the complete path of an electric current, along
with any necessary elements, such as power source and a load.
When the circuit is complete so that the current can flow, it is
termed closed.
When the path of the current flow is interrupted, the circuit is
termed open.
All electric circuits must have a complete path for electrons to flow
through, a source of electrons, and some electric device (load)
that requires electric energy for its operation.
Basic Concepts of Electric Circuits
Series Circuits
• Switches and controls are commonly wired in series with
each other to control one or more loads.
• The simplest and easiest electric circuits to understand
is the series circuits.
• The series circuit allows only one path of current flow
through the circuit.
• An example of a series circuit is Christmas Lights
Series Circuits
Characteristics of a Series Circuit
and Calculations for Current,
Resistance and Voltage
• The current drawn in a series circuit is the same
throughout the entire circuit because there is only one
path for the current to flow.
• The total resistance in a series circuit is the sum of all
the resistances in the circuit
• Rt = R1 + R2 + R3 + R4 + R….
Characteristics of a Series Circuit and Calculations
for Current, Resistance and Voltage
• The voltage in a series circuit is completely used by all the loads in
the circuits.
• The voltage of a series circuit changes through each load.
• This change is called voltage drop.
• The voltage drop is the amount of voltage (electrical pressure) used
or lost through any load or conductor in the process of moving the
current (electron flow) through that part od the circuit.
• The sum of the voltage drop of any part of a series circuit is equal to
the voltage being applied to the circuit.
Series Circuit Example
In the circuit below find the total resistance, the voltage
drop for each element, and the current for the circuit.
Series Circuit Example
1.
Find the total resistance of the circuit.
The total resistance of the circuit can be found by using the
following formula.
Rt = R1 + R2 + R3 + R4 + R….
Rt = 5 ohms + 20 ohms + 35 ohms + 50 ohms
Rt = 110 ohms
Series Circuit Example
2.
Find the amperage draw of the circuit.
The amperage draw of the circuit can be determined using
Ohms law.
I = E/R
I = 120 Volts / 110 Ohms
I = 1.09 Amps
Series Circuit Example
3.
Find the voltage drop across each element in the
circuit.
The voltage drop across each element in the circuit can be determined
by rewriting ohms law.
E = IR
For the 5 ohm resistor the voltage drop would be:
E = IR
E = 1.09 amps * 5 ohms
E = 5.45 volts
Series Circuit Example
For the 20 ohm resistor the voltage drop would be:
E = IR
E = 1.09 amps * 20 ohms
E = 21.8 volts
Series Circuit Example
For the 50 ohm resistor the voltage drop would be:
E = IR
E = 1.09 amps * 50 ohms
E = 54.5 volts
Series Circuit Example
For the 50 ohm resistor the voltage drop would be:
E = IR
E = 1.09 amps * 35 ohms
E = 38.15 volts
Parallel Circuits
• The Parallel circuit has more than one path for the
electrons to flow.
• Electric devices are arranged in the circuit so that each
is connected the supply voltage conductors.
• Parallel circuits are the most common in industry
because almost all devices operate at the line voltage
(120 volts to 240 volts).
• Example of a parallel circuit is standard wiring in a
residence.
Parallel Circuits
Characteristics of a Parallel Circuit and
Calculations for Current, Resistance, and
Voltage
• The current draw in a parallel circuit is determined for each part of
the circuit, depending on the resistance of that portion of the circuit.
• The total current draw of the entire parallel circuit is the sum of the
currents in the individual sections of the parallel circuit.
• The resistance of a parallel circuit gets smaller as more resistances
are added to the circuit.
• The total resistance of a parallel circuit can not be obtained by
taking the sum of all the resistances.
Characteristics of a Parallel Circuit and
Calculations for Current, Resistance, and
Voltage
• The resistance for two resistors in a parallel circuit can be obtained
using the following formula.
• Rt = (R1 * R2)/(R1 + R2)
• The resistance for three of more resistor in a parallel circuit can be
obtained using the following formula.
• 1/Rt = 1/R1 + 1/R2 + 1/R3 + 1/R4 + …..
• The voltage drop in a parallel circuit is the line voltage being
supplied to the load.
Parallel Circuit Example
In the following parallel circuit calculate the total resistance
of the circuit, the total current draw of the circuit and the
current draw for each resistor.
Parallel Circuit Example
1.
Calculate the current draw for each individual load.
The current for each individual resistor can be determined using Ohms
law.
I = E/R
For the 35.7 Ohm resistor the current draw would be:
I = 120 volts / 35.7 ohms
I = 3.36 Amps
Parallel Circuit Example
For the 15.5 Ohm resistor the current draw would be:
I = 120 volts / 15.5 ohms
I = 7.74 Amps
Parallel Circuit Example
2.
Determine the total resistance for the circuit.
The total resistance of the parallel circuit can be obtained
using the formula:
Rt = (R1 * R2)/(R1 + R2)
Rt = (15.5 * 35.7)/(15.5 + 35.7)
Rt = (553.35)/(51.2)
Rt = 10.80 Ohms
Parallel Circuit Example
3.
Determine the total current draw for the circuit.
The total current draw for the circuit can be determined
using Ohms Law
I = E/R
I = 120 volts / 10.80 Ohms
I = 11.11 Amps
Complex Circuits
• Complex circuits are also know as Series-Parallel Circuits.
• A Series-Parallel circuit is a combination circuit containing loads in
both series and parallel.
• Complex circuits are the most widely used circuits in industry today.
Solving a Complex Circuit
• Start with a small simple chunk of the circuit that you
know how to work with that’s either all series or all
parallel.
• Then keep redrawing circuit at each step as it simplifies.