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Network Theorems

SUPERPOSITION THEOREM

THÉVENIN’S THEOREM

NORTON’S THEOREM

MAXIMUM POWER TRANSFER THEOREM

MILLMAN’S THEOREM

SUBSTITUTION THEOREM

RECIPROCITY THEOREM

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SUPERPOSITION THEOREM

In general, the theorem can be used to do the following:

• Analyze networks that have two or more sources that are not in series or

parallel.

• Reveal the effect of each source on a particular quantity of interest.

• For sources of different types (such as dc and ac which affect the parameters

of the network

in a different manner), apply a separate analysis for each type, with the total

result simply the

algebraic sum of the results.

The superposition theorem states the following:

The current through, or voltage across, any element of a network is equal to

the algebraic sum of the currents or voltages produced independently by each

source.

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Only one source at a time can be used to find voltage or current.

Once we have the solution for each source, we can combine the results to obtain the total

solution.

When removing a voltage source from a network schematic,

replace it

with a direct connection (short circuit) of zero ohms. Any

internal

resistance associated with the source must remain in the

network.

When removing a current source from a network

schematic, replace it

by an open circuit of infinite ohms. Any internal resistance

associated

with the source must remain in the network.

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The effect of each source is determined independently, the number of networks to be

analyzed will equal the number of sources.

If a particular current of a network is to be determined, the contribution to that current

must be determined for each source. When the effect of each source has been

determined, those currents in the same direction are added, and those having the opposite

direction are subtracted. The total result is the direction of the larger sum and the magnitude

of the difference.Similarly, if a particular voltage of a network is to be determined, the contribution to that

voltage must be determined for each source. When the effect of each source has been

determined, those voltages with the same polarity are added, and those with the opposite

polarity are subtracted. The total result has the polarity of the larger sum and the magnitude

of the difference.

Superposition cannot be applied to power effects

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EXAMPLE Using the superposition theorem, determine current I1

for the network.

Since two sources are present, there are two networks to be

analyzed.

First determine the effects of the voltage source by setting

the current source to zero amperes.

The resulting current is defined as I1 because it is the current

through resistor R1 due to the voltage source only.

Due to the open circuit, resistor R1 is in series (and, in fact, in

parallel)

with the voltage source E. The voltage across the resistor is

the applied voltage, and current I1 is determined by

Solution:

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The contribution due to the current source: Setting the voltage source to zero volts results in the

network in Fig. The current source has been replaced with a short-circuit equivalent that is directly

across the current source and resistor R1. Since the source current takes the path of least resistance, it

chooses the zero ohm path of the inserted short-circuit equivalent, and the current through R1 is zero

amperes. This is clearly demonstrated by an application of the current divider rule as follows:

Since have the same defined direction in Fig, the

total current is defined by

The voltage source is in parallel with the current source and load resistor R1, so the voltage

across each must be 30 V. The result is that I1 must be determined solely by

EXAMPLE Using the principle of superposition, find the current l2 through the 12 k resistor in Fig.

Solution: Considering the effect of the 6 mA current source

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Current divider rule:

Considering the effect of the 9 V voltage source

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Since have the same direction through R2, the desired current is the sum of the two:

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EXAMPLE Find the current through the 2 Ω resistor of the network in Fig. The presence of three

sources results in three different networks to be analyzedSolution: Considering the effect of the 12 V source

Considering the effect of the 6 V source

Fig. A

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Considering the effect of the 3 A source

Applying the current divider rule

The total current through the 2 resistor appears

Fig. A Fig. A

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THÉVENIN’S THEOREM

In general, the theorem is used to do the following:

• Analyze networks with sources that are not in series or parallel.

• Reduce the number of components required to establish the same characteristics at the

output terminals.

• Investigate the effect of changing a particular component on the behavior of a network

without having to

analyze the entire network after each change.Thévenin’s theorem states the following:

Any two-terminal dc network can be replaced by an

equivalent circuit

consisting solely of a voltage source and a series resistor

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Thévenin’s Theorem Procedure

Preliminary:

1. Remove that portion of the network where the Thévenin equivalent circuit is found.

2. Mark the terminals of the remaining two-terminal network.

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.)ETh:

4. Calculate ETh by first returning all sources to their original position and finding the open-

circuit voltage

between the marked terminals.

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Conclusion:

5. Draw the Thévenin equivalent circuit with the portion of the circuit previously removed

replaced between the terminals of the equivalent circuit.

In Fig. , it requires that the load resistor RL be temporarily removed from the

network.Place the resistor RL between the terminals of the Thévenin equivalent circuit

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The theorem states that the entire network inside the blue shaded area can be replaced by one voltage

source and one resistor

If the replacement is done properly, the voltage across, and the current through, the resistor RL will be

the same for each network. The value of RL can be changed to any value, and the voltage, current, or

power to the load

resistor is the same for each configuration.

EXAMPLE Find the Thévenin equivalent circuit for the network in the shaded area of the network in

Fig. Then find the current through RL for values of 2Ω , 10 Ω , and 100 Ω .

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Solution:Steps 1 and 2: The load resistor RL has been removed and the

two “holding” terminals have been defined as a and b.

They are the two terminals across which the Thévenin resistance is measured. It is no longer the total

resistance as seen by the source..

Steps 3: Replacing the voltage source E1 with a short-circuit

equivalent

yields the network in Fig. , where

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Step 4: Replace the voltage source. The open circuit voltage ETh is

the same as the voltage drop across the 6 resistor.

Applying the voltage divider rule,

Step 5

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Solution: Steps 1 and 2

EXAMPLE Find the Thévenin equivalent circuit for the network in the shaded area of the network.

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Step 3: All the remaining elements are in parallel, and the network can be redrawn as shown.

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Step 4: The network can be redrawn as shown in Fig. Since the voltage is the same across parallel

elements, the voltage across the series resistors R1 and R2 is E1, or 8 V.

Applying the voltage divider rule Step 5:

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NORTON’S THEOREM

The theorem states the following:

Any two-terminal linear bilateral dc network can be replaced by an

equivalent circuit consisting of a current source and a parallel

resistor

Norton’s Theorem Procedure

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.

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IN:

4. Calculate IN by first returning all 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.

5. Draw the Norton equivalent circuit with the portion of the circuit previously removed

replaced between the terminals of the equivalent circuit.

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.

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The Norton and Thévenin equivalent circuits can also be found from each other by using the source transformation

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Steps 1 and 2

Step 3:

EXAMPLE Find the Norton equivalent circuit for the network in the shaded area

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Step 5:

Step 4: The short-circuit connection between terminals a and b is in parallel with R2 and eliminates its effect. IN is therefore the same as through R1, and the full battery voltage appears across R1

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Converting the Norton equivalent circuit to a Thévenin equivalent circuit.

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EXAMPLE (Two sources) Find the Norton equivalent circuit for the portion of the network to the left of a-b

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Solution: Steps 1 and 2: Step 3:

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Step 4: (Using superposition) For the 7 V battery For the 8 A source, both R1 and R2 have been“short circuited” by the direct connection between a and b, and

Step 5:

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MAXIMUM POWER TRANSFER THEOREM

A load will receive maximum power from a network when its resistance is exactly

equal to the Thévenin resistance of the network applied to the load. That is,

For the Thévenin equivalent circuit, when the

load is set equal to the Thévenin resistance,

the load will receive maximum power from the

network.

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The maximum power delivered to the load can be determined by first finding the current

Then substitute into the power equation:

and

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MAXIMUM POWER TRANSFER occurs when the load voltage and current are one-half of their maximum possible values.

The current through the load is determined by

The voltage is determined by

The power by

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MAKE A TABLE FOR THE FOLLOWING PARAMETERS

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MILLMAN’S THEOREM Any number of parallel voltage sources can be reduced to one.

The three 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.

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Three steps are included in its application.

Step 1: Convert all voltage sources to current sources.

Step 2: Combine parallel current sources.

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Step 3: Convert the resulting current source to a voltage source,

and the

desired single-source network is obtained. In general, Millman’s

theorem states that for any number of parallel voltage sources,

The plus-and-minus signs include those cases where the sources may not be supplying energy in the same direction.

The equivalent resistance is

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In terms of the resistance values,

The dual of Millman’s theorem

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EXAMPLE Using Millman’s theorem, find the current through and voltage across the resistor RL.

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SUBSTITUTION THEOREM

If the voltage across and the current through any branch of a dc bilateral network

are known, this branch can be replaced by any combination of elements that will

maintain the same voltage across and current through the chosen branch.

The theorem states that for branch equivalence, the terminal voltage and current must be the same

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A known potential difference and current in a network can be replaced by an

ideal voltage source and current source, respectively.

This theorem cannot be used to solve networks with two or more sources that are not in series

or parallel. For it to be applied, a potential difference or current value must be known.

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RECIPROCITY THEOREM Applicable only to single-source networks

The current I in any branch of a network, due to a single voltage source E

anywhere else 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. The theorem requires that the polarity of the voltage source have the same correspondence

with the direction of the branch current in each position.

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The current I due to the voltage source E was determined. If the position of each is interchanged, the

current I will be the same value as indicated.

The total resistance is

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For the network in Fig. 9.108, which corresponds to that in Fig.9.106(b),