EGR 2201 Unit 6Theorems: Thevenin’s, Norton’s, Maximum Power Transfer

Read Alexander & Sadiku, Sections 4.5 to 4.11.

Homework #6 and Lab #6 due next week.

Quiz next week.

Techniques That Can Simplify Circuit Analysis

Chapter 4 presents several new techniques: Linearity Superposition Source transformationThevenin’s theorem Norton’s theorem Maximum-power-transfer theorem

Thevenin’s Theorem

Thevenin’s theorem says that any linear two-terminal circuit can be replaced by an equivalent circuit consisting of an independent voltage source VTh in series with a resistor RTh.

There’s a standard procedure for finding the values of VTh and RTh. But first, what exactly does this mean and why is it useful?

Load Often an electrical circuit or system

contains an element (called the “load”) that can be varied while the rest of the circuit or system stays fixed.

Example: When you unplug a lamp at home and plug in a hair-dryer, you’ve varied one small part of a huge electrical system that extends all the way back to the power plant.

What Thevenin’s Theorem Means

Thevenin’s theorem lets us replace everything on one side of a pair of terminals by a very simple equivalent circuit consisting of just a voltage source and a resistor.

Original Circuit Equivalent Circuit

Why It’s Useful

This greatly simplifies computation when you wish to find values of voltage or current for several different possible values of a load resistance.

Original Circuit Equivalent Circuit

Steps in Finding VTH and RTH

1. Open the circuit at the two terminals where you wish to find the Thevenin-equivalent circuit. (In the circuit shown, this means removing the load.)

2. VTH is the voltage across the two open terminals. Pay attention to polarity!

3. RTH is the resistance looking into the open terminals with all independent sources turned off. (Recall that to turn off a voltage source we replace it by a short, and to turn off a current source we replace it by an open.)

More on Step 3 (Finding RTH)

Recall from the previous slide that RTH is the resistance looking into the open terminals with all independent sources turned off.

What about dependent sources? We don’t turn those off.

As shown in the following two examples, how you find RTH depends on whether the circuit contains any dependent sources. If not, then just combine resistors in series and

parallel to find equivalent resistance.Otherwise, it’s a little trickier.

Finding the Thevenin-Equivalent Circuit Example #1: No Dependent Sources

Goal: Find the Thevenin-equivalent circuit for the circuit in thebook’s Figure 4.27:

Finding VTH:

Finding RTH:

Finding the Thevenin-Equivalent Circuit Example #1: No Dependent Sources (Conclusion)

We found that VTH = 30 V and RTH = 4 . So we conclude that, as far as the load is concerned, the original circuit on the left below is equivalent to the much simpler circuit on the right below.

Original Circuit Equivalent Circuit

Finding RTH For a Circuit That Contains Dependent Sources

To understand the steps in finding RTH for a circuit that contains dependent sources, think about this question:Suppose you have lab equipment

including a voltage source and a multimeter, but you don’t know how to use the multimeter to measure resistance. If I give you a resistor of unknown value, how can you find its resistance?

Answer: Connect the voltage source(of value vo) across the resistor and measure how much current io flows through the resistor. Then use Ohm’s law to compute the resistance:

Finding RTH For a Circuit Containing Dependent Sources

If a circuit contains dependent sources, follow these steps to find RTH.1. Turn off all independent sources as

previously described.2. Connect an independent voltage source of

arbitrary value vo to the open terminals.

3. Find the current io flowing into the circuit from the voltage source that you connected in the previous step.

4. To find RTh, divide the voltage vo by the current io. In symbols, RTh = vo ÷ io.

Finding the Thevenin-Equivalent Circuit Example #2: With Dependent Sources

Goal: Find the Thevenin-equivalent circuit for the circuit in thebook’s Figure 4.31:

Finding VTH:

Finding RTH:

Finding the Thevenin-Equivalent Circuit Example #2: With Dependent Sources (Conclusion)

We found that VTH = 20 V and RTH = 6 . So we conclude that, as far as anything connected to terminals a-b is concerned, the original circuit on the left below is equivalent to the much simpler circuit on the right below.

Original Circuit Equivalent Circuit

Summary: Thevenin’s Theorem

As we’ve seen, Thevenin’s theorem says that any linear two-terminal circuit can be replaced by an equivalent circuit consisting of an independent voltage source VTh in series with a resistor RTh.

Original Circuit Thevenin-Equivalent Circuit

Techniques That Can Simplify Circuit Analysis

Chapter 4 presents several new techniques: Linearity Superposition Source transformation Thevenin’s theoremNorton’s theorem Maximum-power-transfer theorem

Norton’s Theorem

Norton’s theorem says that any linear two-terminal circuit can be replaced by an equivalent circuit consisting of an independent current source IN in parallel with a resistor RN.

Original Circuit Norton-Equivalent Circuit

Not Surprising, Is It?

Norton’s theorem follows from Thevenin’s theorem, since we can substitute a voltage-source-plus-series-resistor by a current- source-plus-parallel-resistor, or vice versa. (Remember source transformation?)

Original Circuit

Norton-Equivalent Circuit

Thevenin-Equivalent Circuit

Finding IN and RN

The book describes how to find IN and RN. The procedure is similar to how we find VTh and RTh.

But you don’t need to learn this new procedure. Instead, just find VTh and RTh, and then apply a source transformation:

Norton-Equivalent CircuitThevenin-Equivalent Circuit

Techniques That Can Simplify Circuit Analysis

Chapter 4 presents several new techniques: Linearity Superposition Source transformation Thevenin’s theorem Norton’s theoremMaximum-power-transfer theorem

Source and Load We can think of many electrical systems

as being composed of a source of power and a load connected to that source.

Examples of sources: amplifier, generator, power supply.

Examples of loads: loudspeaker, electric motor, antenna.

Maximizing the Load Power Replacing the source with its Thevenin-

equivalent circuit, we have the following situation:

In many applications, we wish to maximize the power transferred from the source to the load.

Thevenin-equivalent of source

Variable loadresistance

The Load’s Power Depends on the Load Resistance

For this circuit, the load resistor’s power is given by:

Question: For fixed values of VTh and RTh, what value of RL will result in maximum load power? The answer is not obvious, since RL appears

in both the numerator and the denominator.

Variation in the Load’s Power as RL

Varies

A graph of the equation

looks like this:

Note that power approaches 0 as RL approaches 0. Also, power approaches 0 as RL approaches .

What is the value of RL where the graph peaks?....

2

ThL

Th L

Vp R

R R

Maximum Power Transfer Theorem

The maximum power transfer theorem says that maximum power is transferred to a load when the load resistance equals the source’s Thevenin resistance (RL = RTh).

To see this, find , set it equal to 0, and solve for RL.

Matching Source and Load

When source and load have the same resistance, they are said to be matched.

In many practical applications, a major part of a circuit designer’s effort is to ensure that components are matched.

We’ve been using ideal voltage sources. These are theoretical models that maintain the same output voltage no matter what you attach to their terminals. Such sources do not exist in real life.

What about real voltage sources, such as batteries or power supplies? To a first approximation, we treat them

as ideal voltage sources.

An Important Application of Thevenin’s Theorem: Source Modeling

Symbols forideal voltage

sources:

For a better approximation, we treat a real voltage source (also called a “practical” voltage source) as an ideal voltage source in series with a resistor.

This resistor is called the source’s internal resistance: the lower it is, the better.

Source Modeling

Suppose we connect a load resistor across a real voltage source rated at 10 V.

If we treat the real source as an ideal source, we’ll conclude that the voltage across the load resistor is 10 V.

But if we take into account the source’s internal resistance, we’ll find that the voltage across the load resistor is less than 10 V.

Using Source Modeling to Analyze a Circuit

This model does not perfectly predict all aspects of a real voltage source’s behavior, but it gives more accurate results than treating the real source as an ideal source.

It’s Still Just an Approximation