EGR 2201 Unit 5Linearity, Superposition, & Source Transformation

Read Alexander & Sadiku, Sections 4.1 to 4.4.

Homework #5 and Lab #5 due next week.

Quiz next week.

Techniques That Can Simplify Circuit Analysis

Nodal analysis and mesh analysis are powerful tools for analyzing circuits.

But you should also know other techniques that may simplify the equations that you get when you perform nodal or mesh analysis, or may save you from having to perform nodal or mesh analysis at all.

Example of a Simplifying Technique

One such technique is our method of finding the equivalent resistance of several series-parallel resistors.

Example: In this circuit, if you wantto find i, using this technique will get you the answermuch faster thandoing a full nodalor mesh analysis of the circuit.

Other Useful Techniques

The current-divider rule and voltage-divider rule are two other useful tools that can save you work when analyzing circuits.

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

Linear Circuits

A linear circuit is a circuit whose output is linearly related (or directly proportional) to its input.

In other words, in a linear circuit, if the input voltage or current increases by a factor of k, then the output voltage or current must increase by that same factor.

Typically the input is a voltage source or current source and the output is a resistor’s voltage or current.

Why Linearity is Useful

Linearity is a useful property because it lets us easily answer certain “what-if” questions about linear circuits.

Example: Suppose we know that in the linear circuit shown, i = 2 A when vs = 10 V. Then we know that doubling vs to 20 V will also double i to 4 A.

How to Tell if a Circuit is Linear

Any circuit that contains only linear elements (which include resistors, capacitors, and inductors), independent sources, and linear dependent sources is a linear circuit.

All of the circuits that we will study in this course are linear circuits.

In other courses you will study non-linear circuits.

Caution: Power Is Not Related Linearly to Voltage and Current

Recall that a resistor’s power is non-linearly related to its current and voltage by the equations

p = i 2 R p = v 2 R

Therefore, for example, doubling vs in the circuit shown willnot double R’s power.

Techniques That Can Simplify Circuit Analysis

Chapter 4 presents several new techniques: LinearitySuperposition Source transformation Thevenin’s theorem Norton’s theorem Maximum-power-transfer theorem

Superposition Principle (1 of 2)

The superposition principle says that to find the total effect of two or more independent sources in a linear circuit, you can find the effect of each source acting alone, and then combine those effects.

Superposition Principle (2 of 2)

To find the effect of an independent source acting alone, you must turn off all of the other independent sources.To turn off a voltage source,

replace it by a short circuit.To turn off a current source, replace

it by an open circuit.

Steps in Using the Superposition Principle

1. Select one independent source and turn off the others, as explained above. Find the desired voltage or current due to this source using techniques from earlier chapters.

2. Repeat step 1 for each of the other independent sources.

3. Add the values obtained from the steps above, paying careful attention to the signs (+ or ) of each value.

Example: Using Superposition (1 of 2)

Suppose we wish to find v in the circuit shown.

With the voltagesource acting alone:

With the current source acting alone:

Example: Using Superposition (2 of 2)

In this circuit, usevoltage-divider ruleto find v1 = 2 V.

In this circuit, usecurrent-divider ruleto find i3 = 2 A, Ohm’s law to find v2 = 8 V.

Therefore, in the original circuit, v = 2 V + 8 V = 10 V.

Wouldn’t Nodal Analysis Be Easier?

For this circuit, you might argue that nodal (or mesh) analysis is easier than superposition. And you might be right.

But looking ahead to the second half of this course, if one source were a DC source and the other were an AC source, we would have no choice but to use superposition.

Techniques That Can Simplify Circuit Analysis

Chapter 4 presents several new techniques: Linearity SuperpositionSource transformation Thevenin’s theorem Norton’s theorem Maximum-power-transfer theorem

Combining Series or Parallel Sources

Before looking at source transformation, note the following:Just as we can combine series or parallel resistors into a single equivalent resistor, we combine series or parallel sources into a single equivalent source.

In particular: We can combine series voltage sources

into an equivalent voltage source. We can combine parallel current

sources into an equivalent current source.

Series-Aiding Voltage Sources

If two series-connected voltage sources produce current in the same direction, they are said to be series-aiding.

The net effect on thecircuit is the sameas that of a singlesource whose voltage equals the sum of the two voltage sources. Example: Most flashlights

have two 1.5-V batteriesin series to give 3 V.

Series-Opposing Voltage Sources

If two series-connected sources produce current in opposite directions, they’re said to be series-opposing.

The effect is the sameas that of a single source equal in magnitude to the difference between the source voltages and having the same polarity as the larger of the two.

Parallel Voltage Sources?

Although there are exceptions, in general we don’t connect voltage sources in parallel with each other.

To see why not, think about this: what is the voltage between points a and b in the circuit below?

Parallel Current Sources

Current sources can be connected in parallel. The parallel-connected sources can be replaced by a single equivalent current source that produces a current equal to the algebraic sum of the sources.

Series Current Sources?

In general we don’t connect current sources in series with each other.

To see why not, think about this: how much current flows around the loop in the circuit below?

Source Transformation

Sometimes when you're analyzing a circuit, it can be useful to substitute a voltage source (and a resistor in series with it) by a current source (and a resistor in parallel with it), or vice versa.

Either substitution is called a source transformation.

Two questions:1. Why would this be useful?2. Are we allowed to do this?

Example of Why Source Transformation Can Be Useful

Suppose we wish to find vo in the circuit to the right.This is not difficult,but it requires a bitof work.

We may get the answer more easily ifwe can first replace the current-source-and-parallel-resistor with a voltage-source-and-series-resistor.

Are We Allowed to Do This?

Yes, we’re allowedto make this substitution, as long as we use the right values for the elements.

The resistor R in series with the voltage source must be the same size as the one in parallel with the current source.

The current source’s and voltage source’s values must satisfy the following:

Source Transformation: A More Complicated Example (1 of 2)

In some cases we can use source transformation repeatedly to convert a complicated problem into a much simpler one.

The book’s Example 4.6 shows such a case:

Original circuit:

After transformingboth sources:

Continued on next slide…

Source Transformation: A More Complicated Example (2 of 2)

…From previous slide:

After combining 4- with 2- and transforming 12-V source:

After combining 6- with 3- and combining 2-A with 4-A: