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**Chapter 9 – Network Theorems**

Introductory Circuit Analysis Robert L. Boylestad 1

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9.1 - Introduction This chapter introduces important fundamental theorems of network analysis. They are superposition, Thévenin’s, Norton’s, maximum power transfer, substitution, Millman’s, and reciprocity theorems 2

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**9.2 - Superposition Theorem**

Used to find the solution to networks with two or more sources that are not in series or parallel The current through, or voltage across, an element in a linear bilateral network is equal to the algebraic sum of the currents or voltages produced independently by each source For a two-source network, if the current produced by one source is in one direction, while that produced by the other is in the opposite direction through the same resistor, the resulting current is the difference of the two and has the direction of the larger If the individual currents are in the same direction, the resulting current is the sum of two in the direction of either current 3

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**Superposition Theorem**

The total power delivered to a resistive element must be determined using the total current through or the total voltage across the element and cannot be determined by a simple sum of the power levels established by each source 4

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9.3 - Thévenin’s Theorem Any two-terminal, linear bilateral dc network can be replaced by an equivalent circuit consisting of a voltage source and a series resistor 5

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Thévenin’s Theorem The Thévenin equivalent circuit provides an equivalence at the terminals only – the internal construction and characteristics of the original network and the Thévenin equivalent are usually quite different This theorem achieves two important objectives: Provides a way to find any particular voltage or current in a linear network with one, two, or any other number of sources We can concentration on a specific portion of a network by replacing the remaining network with an equivalent circuit 6

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**Thévenin’s Theorem Sequence to proper value of RTh and ETh Preliminary**

1. Remove that portion of the network across which the Thévenin equation circuit is to be found. In the figure below, this requires that the load resistor RL be temporarily removed from the network. 7

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Thévenin’s Theorem 2. Mark the terminals of the remaining two-terminal network. (The importance of this step will become obvious as we progress through some complex networks) RTh: 3. Calculate RTh by first setting all sources to zero (voltage sources are replaced by short circuits, and current sources by open circuits) and then finding the resultant resistance between the two marked terminals. (If the internal resistance of the voltage and/or current sources is included in the original network, it must remain when the sources are set to zero) 8

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**Thévenin’s Theorem ETh:**

4. Calculate ETh by first returning all sources to their original position and finding the open-circuit voltage between the marked terminals. (This step is invariably the one that will lead to the most confusion and errors. In all cases, keep in mind that it is the open-circuit potential between the two terminals marked in step 2) 9

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**Thévenin’s Theorem Conclusion:**

5. Draw the Thévenin equivalent circuit with the portion of the circuit previously removed replaced between the terminals of the equivalent circuit. This step is indicated by the placement of the resistor RL between the terminals of the Thévenin equivalent circuit Insert Figure 9.26(b) 10

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**Thévenin’s Theorem Experimental Procedures**

Two popular experimental procedures for determining the parameters of the Thévenin equivalent network: Direct Measurement of ETh and RTh For any physical network, the value of ETh can be determined experimentally by measuring the open-circuit voltage across the load terminals The value of RTh can then be determined by completing the network with a variable resistance RL 11

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**Thévenin’s Theorem RTh = Voc / Isc Measuring VOC and ISC**

The Thévenin voltage is again determined by measuring the open-circuit voltage across the terminals of interest; that is, ETh = VOC. To determine RTh, a short-circuit condition is established across the terminals of interest and the current through the short circuit Isc is measured with an ammeter Using Ohm’s law: RTh = Voc / Isc 12

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**9.4 - Norton’s Theorem Norton’s theorem states the following:**

Any two linear bilateral dc network can be replaced by an equivalent circuit consisting of a current and a parallel resistor. The steps leading to the proper values of IN and RN Preliminary 1. Remove that portion of the network across which the Norton equivalent circuit is found 2. Mark the terminals of the remaining two-terminal network 13

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Norton’s Theorem RN: 3. Calculate RN by first setting all sources to zero (voltage sources are replaced with short circuits, and current sources with open circuits) and then finding the resultant resistance between the two marked terminals. (If the internal resistance of the voltage and/or current sources is included in the original network, it must remain when the sources are set to zero.) Since RN = RTh the procedure and value obtained using the approach described for Thévenin’s theorem will determine the proper value of RN 14

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Norton’s Theorem IN : 4. Calculate IN by first returning all the sources to their original position and then finding the short-circuit current between the marked terminals. It is the same current that would be measured by an ammeter placed between the marked terminals. Conclusion: 5. Draw the Norton equivalent circuit with the portion of the circuit previously removed replaced between the terminals of the equivalent circuit 15

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**9.5 - Maximum Power Transfer Theorem**

The maximum power transfer theorem states the following: A load will receive maximum power from a linear bilateral dc network when its total resistive value is exactly equal to the Thévenin resistance of the network as “seen” by the load RL = RTh 16

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**Maximum Power Transfer Theorem**

For loads connected directly to a dc voltage supply, maximum power will be delivered to the load when the load resistance is equal to the internal resistance of the source; that is, when: RL = Rint 17

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9.6 - Millman’s Theorem Any number of parallel voltage sources can be reduced to one This permits finding the current through or voltage across RL without having to apply a method such as mesh analysis, nodal analysis, superposition and so on. 1. Convert all voltage sources to current sources 2. Combine parallel current sources 3. Convert the resulting current source to a voltage source, and the desired single-source network is obtained 18

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**9.7 - Substitution Theorem**

The substitution theorem states: If the voltage across and the current through any branch of a dc bilateral network is known, this branch can be replaced by any combination of elements that will maintain the same voltage across and current through the chosen branch Simply, for a branch equivalence, the terminal voltage and current must be the same 19

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9.8 - Reciprocity Theorem The reciprocity theorem is applicable only to single-source networks and states the following: The current I in any branch of a network, due to a single voltage source E anywhere in the network, will equal the current through the branch in which the source was originally located if the source is placed in the branch in which the current I was originally measured The location of the voltage source and the resulting current may be interchanged without a change in current 20

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9.9 - Application Speaker system Insert Fig 9.111 21

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