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**Series-Parallel Circuits**

Topics Covered Finding RT for Series-Parallel Resistances Resistance Strings in Parallel Resistance Banks in Series Resistance Banks and Strings in Series-Parallel © 2007 The McGraw-Hill Companies, Inc. All rights reserved.

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**Topics Covered Analyzing Series-Parallel Circuits with Random Unknowns**

Troubleshooting: Opens and Shorts in Series-Parallel Circuits McGraw-Hill © 2007 The McGraw-Hill Companies, Inc. All rights reserved.

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**Finding RT for Series-Parallel Resistances**

Overview of Series-Parallel Circuits A series-parallel circuit, or combination circuit, combines both series and parallel connections. Most electronic circuits fall into this category. Series-parallel circuits are typically used when different voltage and current values are required from the same voltage source. Series components form a series string. Parallel components form a parallel bank.

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**Finding RT for Series-Parallel Resistances**

Overview of Series-Parallel Circuits There are three branches in this circuit; sections 1 and 2 are series strings. 1 2 3 V

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**Finding RT for Series-Parallel Resistances**

Overview of Series-Parallel Circuits 1 2 3 V There are three series sections in this circuit; sections 1 and 2 are parallel banks.

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**Finding RT for Series-Parallel Resistances**

To find RT for a series-parallel circuit, add the series resistances and combine the parallel resistances. Start reducing the branch farthest from the voltage source and work in. In this diagram, R1 and R2 are in series, and R3 and R4 are in parallel. However, R2 is not in series with the parallel resistances: Resistances in series have the same current, but the current in R2 is equal to the sum of the branch currents I3 and I4. Fig. 6-1b: Schematic diagram of a series-parallel circuit. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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**Finding RT for Series-Parallel Resistances**

For Fig. 6-1b, The series resistances are: 0.5kΩ + 0.5kΩ = 1kΩ The parallel resistances are: 1kΩ / 2 = 0.5kΩ The series and parallel values are then added for the value of RT: 1kΩ + 0.5kΩ = 1.5 kΩ

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**Resistance Strings in Parallel**

In this figure, branch 1 has two resistances in series; branch 2 has only one resistance. Ohm’s Law can be applied to each branch, using the same rules for the series and parallel components that were discussed in the previous unit. Fig. 6-3a: Series string in parallel with another branch (schematic diagram). Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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**Resistance Strings in Parallel**

Series Circuit Current is the same in all components. V across each series R is I × R. VT = V1 + V2 + V etc. Parallel Circuit Voltage is the same across all branches. I in each branch R is V/R. IT = I1 + I2 + I etc.

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**Resistance Strings in Parallel**

I is the same in this section. V V is the same across each parallel branch.

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**Resistance Strings in Parallel**

The current in each branch equals the voltage applied across the branch divided by the branch RT. The total line current equals the sum of the branch currents for all parallel strings. The RT for the entire circuit equals the applied voltage divided by the total line current. For any resistance in a series string, the IR voltage drop across that resistance equals the string’s current multiplied by the resistance. The sum of the voltage drops in the series string equals the voltage across the entire string.

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**Resistance Banks in Series**

In this figure, R2 and R3 are parallel resistances in a bank. The parallel bank is in series with R1. There may be more than two parallel resistances in a bank, and any number of banks in series. Ohm’s Law is applied to the series and parallel components as seen previously. Fig. 6-4a: Parallel bank of R2 and R3 in series with R1 (Original circuit). Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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**Resistance Banks in Series**

To find the total resistance of this type of circuit, combine the parallel resistances in each bank and add the series resistances. V I R = 6Ω 24V 4A 6Ω = 10 Ω (of R2 + R3) 2 branches + 1Ω (R1) 6Ω = 5Ω + 1Ω

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**Resistance Banks and Strings in Series-Parallel**

To solve series-parallel (combination) circuits, it is important to know which components are in series with one another and which components are in parallel. Series components must be in one current path without any branch points. To find particular values for this type of circuit, Reduce and combine the components using the rules for individual series and parallel circuits. Reduce the circuit to its simplest possible form. Then solve for the needed values using Ohm’s Law.

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**Resistance Banks and Strings in Series-Parallel**

Example: Find all currents and voltages in Fig. 6-5. Step 1: Find RT. Step 2: Calculate main line current as IT = VT / RT Fig. 6-5: Reducing a series-parallel circuit to an equivalent series circuit to find the RT. (a) Actual circuit. (b) R3 and R4 in parallel combined for the equivalent RT. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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**Resistance Banks and Strings in Series-Parallel**

Fig. 6-5, cont. (c) RT and R6 in series added for R13. (d) R13 and R5 in parallel combined for R18. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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**Resistance Banks and Strings in Series-Parallel**

Fig. 6-5e: The R18, R1, and R2 in series are added for the total resistance of 50Ω for RT. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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**Analyzing Series-Parallel Circuits with Random Unknowns**

In solving such circuits, apply the same principles as before: Reduce the circuit to its simplest possible form. Apply Ohm’s Law.

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**Analyzing Series-Parallel Circuits with Random Unknowns**

Example: In Fig. 6-6, we can find branch currents I1 and I2-3, and IT, and voltage drops V1, V2, and V3, without knowing the value of RT. Fig. 6-6: Finding all the currents and voltages by calculating the branch currents first. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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**Analyzing Series-Parallel Circuits with Random Unknowns**

Find I1, I2-3, and IT. V R I1 = I1 = 90V 30Ω (parallel branches have the same voltage) I1= 3A

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**Analyzing Series-Parallel Circuits with Random Unknowns**

= V R I2-3 = 2A 90V 20Ω + 25Ω 45Ω IT = I1 + I2-3 IT = 3A + 2A IT = 5A

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**Analyzing Series-Parallel Circuits with Random Unknowns**

Find voltage drops V1, V2, and V3: Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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**Analyzing Series-Parallel Circuits with Random Unknowns**

V1 = VA (parallel branches have the same voltage) V1 = 90V or V1 = I1R1 V2 = I2-3R2 V3 = I2-3R3 V1 = 3A × 30Ω V2 = 2A(20 Ω) V3 = 2A(25 Ω) V1 = 90V V2 = 40V V3 = 50V Note: V2 + V3 = VA 40V + 50V = 90V

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**Analyzing Series-Parallel Circuits with Random Unknowns**

= RT IT VA 5A 90A 18Ω

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**Troubleshooting: Opens and Shorts in Series-Parallel Circuits**

In series-parallel circuits, an open or short in one part of the circuit changes the values in the entire circuit. When troubleshooting series-parallel circuits, combine the techniques used when troubleshooting individual series and parallel circuits.

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**Troubleshooting: Opens and Shorts in Series-Parallel Circuits**

Effect of a Short in a Series-Parallel Circuit The total current and total power increase. . Fig. 6-13: Effect of a short circuit with series-parallel connections. (a) Normal circuit with S1 open. (b) Circuit with short between points A and B when S1 is closed; now R2 and R3 are short-circuited. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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**Troubleshooting: Opens and Shorts in Series-Parallel Circuits**

Effect of a Short in a Series-Parallel Circuit The total current increases from 2A with S1 open to 10A with S1 closed. With S1 closed, R2 and R3 are shorted out. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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**Troubleshooting: Opens and Shorts in Series-Parallel Circuits**

Effect of an Open in a Series-Parallel Circuit Fig. 6-14: Effect of an open path in a series-parallel circuit. (a) Normal circuit with S2 closed. (b) Series circuit with R1 and R2 when S2 is open. Now R3 in the open path has no current and zero IR voltage drop. With S2 open, R3 is effectively removed from the circuit. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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**Troubleshooting: Opens and Shorts in Series-Parallel Circuits**

Effect of an Open in a Series-Parallel Circuit With S2 open the voltage across points C and D equals the voltage across R2,which is 89V. The voltage across R3 is zero. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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