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Chapter 7
Parallel Circuits
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Objectives
• Determine the total resistance of a parallel circuit.
• Determine the voltage drops in a parallel circuit.
• Determine the current values of a parallel circuit.
• Determine the wattage values of a parallel circuit.
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Objectives
• Apply Ohm’s law to solve for unknown voltage, current, and resistance in a parallel circuit.
• Apply parallel circuit theory to assist in troubleshooting a parallel circuit.
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Parallel Circuit Principles
• Two or more resistors connected to the source in parallel
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Parallel Circuit Voltage
• ET = E1 = E2 = E3…= EN
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Parallel Circuit Current
• Kirchhoff’s current law principles– Branch currents– Mainline current
• IT = I1 + I2 + I3…+ IN
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Parallel Circuit Resistance
• Total resistance is always less than the smallest resistance
• Methods to find total resistance– Product over sum method– Reciprocal method– Equal resistances method– Graph method
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Product Over the Sum Method
21
21T RR
RR R
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Reciprocal Method
N321T RRRRR
1
1
1
1
1
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Equal Resistances Method
N
R RT
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Graph Method
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Power in a Parallel Circuit
N321T PPPPP
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Review
What is a parallel circuit?
A circuit that provides two or more paths for electron flow through a circuit
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Review
What does the source voltage in a parallel circuit equal?
The same voltage value as any point in the circuit
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Review
What does Kirchhoff’s current law state?
The algebraic sum of all currents entering any point will equal the sum of all currents leaving that point
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Review
How does the total resistance compare to the individual resistances in a parallel circuit?
Total resistance is less than the smallest resistance
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Applying Ohm’s Law
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Troubleshooting a Parallel Circuit
• Compare total resistance reading to the calculated total resistance
• Take current readings at each resistor
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Using an Ohmmeter to Troubleshoot
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Using an Ammeter to Troubleshoot
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Review
Why is checking voltage drops not a practical method for finding a bad component in a parallel circuit?
Voltage is the same everywhere in a parallel circuit
© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only.
Review
How is an ammeter used to troubleshoot?
Current readings across an open switch are taken and then compared to calculated total current