Chapter 7 Parallel Circuits © Goodheart-Willcox Co., Inc.Permission granted to reproduce for educational use only. Objectives Determine the total resistance.

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

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Parallel Circuit Voltage

• ET = E1 = E2 = E3…= EN

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Parallel Circuit Current

• Kirchhoff’s current law principles– Branch currents– Mainline current

• IT = I1 + I2 + I3…+ IN

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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

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Product Over the Sum Method

21

21T RR

RR R

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Reciprocal Method

N321T RRRRR

1

1

1

1

1

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Equal Resistances Method

N

R RT

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Graph Method

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Power in a Parallel Circuit

N321T PPPPP

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Review

What is a parallel circuit?

A circuit that provides two or more paths for electron flow through a circuit

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Review

What does the source voltage in a parallel circuit equal?

The same voltage value as any point in the circuit

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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

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Review

How does the total resistance compare to the individual resistances in a parallel circuit?

Total resistance is less than the smallest resistance

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Applying Ohm’s Law

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Troubleshooting a Parallel Circuit

• Compare total resistance reading to the calculated total resistance

• Take current readings at each resistor

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Using an Ohmmeter to Troubleshoot

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Using an Ammeter to Troubleshoot

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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

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Review

How is an ammeter used to troubleshoot?

Current readings across an open switch are taken and then compared to calculated total current