2. Chapter 7
Parallel Circuits

3. 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.
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4. 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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5. Parallel Circuit Principles
Two or more resistors connected to the source in parallel
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6. Parallel Circuit Voltage
ET = E1 = E2 = E3…= EN
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7. Parallel Circuit Current
Kirchhoff’s current law principles
Branch currents
Mainline current
IT = I1 + I2 + I3…+ IN
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8. 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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9. Product Over the Sum Method
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10. Reciprocal Method © Goodheart-Willcox Co., Inc.
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11. Equal Resistances Method
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12. Graph Method © Goodheart-Willcox Co., Inc.
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13. Power in a Parallel Circuit
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14. Review What is a parallel circuit?
A circuit that provides two or more paths for electron flow through a circuit
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15. 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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16. 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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17. 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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18. Applying Ohm’s Law © Goodheart-Willcox Co., Inc.
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19. Troubleshooting a Parallel Circuit
Compare total resistance reading to the calculated total resistance
Take current readings at each resistor
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20. Using an Ohmmeter to Troubleshoot
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21. Using an Ammeter to Troubleshoot
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22. 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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23. 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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24. Glossary Branch current Kirchhoff’s current law Mainline current
individual current caused by an individual resistance in a parallel circuit.
Kirchhoff’s current law
At any junction of conductors in a circuit, the algebraic sum of the currents equals zero.
Mainline current
Total current in a parallel circuit.
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25. Glossary Parallel circuit Reciprocal
Contains two or more paths for electrons supplied by a common voltage source.
Reciprocal
The reciprocal of a number is one divided by the number.
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