Chapter 7
Parallel Circuits
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.
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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.
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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 RT
R1 R2 R1 R2
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Reciprocal Method
1 1 1 1 1 RT R1 R2 R3 RN © Goodheart-Willcox Co., Inc.
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Equal Resistances Method R RT N
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Graph Method
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Power in a Parallel Circuit PT P1 P2 P3 PN
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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
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