Filename PWAMod 04Prob 04 ppt Find the Thvenin

Filename: PWA_Mod 04_Prob 04. ppt Find the Thévenin Equivalent at terminals A and B. Problems With Assistance Module 4 – Problem 4 Go straight to the First Step Go straight to the Problem Statement Next slide

Overview of this Problem In this problem, we will use the following concepts: • Equivalent Circuits • Thévenin’s Theorem • Equivalent Resistance with Dependent Sources Go straight to the First Step Go straight to the Problem Statement Next slide

Textbook Coverage The material for this problem is covered in your textbook in the following sections: • Circuits by Carlson: Sections #. # • Electric Circuits 6 th Ed. by Nilsson and Riedel: Sections #. # • Basic Engineering Circuit Analysis 6 th Ed. by Irwin and Wu: Section #. # • Fundamentals of Electric Circuits by Alexander and Sadiku: Sections #. # • Introduction to Electric Circuits 2 nd Ed. by Dorf: Sections #-# Next slide

Coverage in this Module The material for this problem is covered in this module in the following presentation: • DPKC_Mod 04_Part 04 Next slide

Problem Statement Find the Thévenin Equivalent at terminals A and B. Next slide

Solution – First Step – Where to Start? Find the Thévenin Equivalent at terminals A and B. How should we start this problem? What is the first step? Next slide

Problem Solution – First Step Find the Thévenin Equivalent at terminals A and B. How should we start this problem? What is the first step? a) Define the opencircuit voltage. b) Label the terminals of resistor R 4 and remove it. c) Define the shortcircuit current. d) Combine resistors R 2 and R 3 in parallel. e) Combine resistors R 1 and R 2 in series.

Your choice for First Step – Define the open-circuit voltage Find the Thévenin Equivalent at terminals A and B. This is a good choice for the first step. We could find the open-circuit voltage, and that will be the Thévenin voltage. We could also have solved for the short-circuit current first. Either is a reasonable first choice. Let’s define the open-circuit voltage, and then solve for it.

Your choice for First Step – Label the terminals of resistor R 4 and remove it Find the Thévenin Equivalent at terminals A and B. This is not a good choice for the first step. The resistor R 4 is in parallel with the two terminals at which we are finding the equivalent. However, we are not finding the equivalent seen by R 4, so this resistor will be a part of the equivalent. Thus, it needs to be left in place. Please go back and try again.

Your choice for First Step – Define the short-circuit current Find the Thévenin Equivalent at terminals A and B. This is a good choice for the first step. Since we are finding the Thevenin equivalent, we will eventually have to find the open-circuit voltage, or solve for it after finding the equivalent resistance. However, this is a very reasonable first choice. So, let’s label the shortcircuit current and solve for it.

Your choice for First Step was – Combine resistors R 2 and R 3 in parallel Find the Thévenin Equivalent at terminals A and B. This is a good first step, but is not the choice that we would like to follow here, simply due to the approach we are going to take with this problem. Note that the resistors R 2 and R 3 are in parallel, and that combining them would not change the voltage v. X that we need for the v. S 2 dependent source. This was a fine choice, but we would like for you to go back and try again.

Your choice for First Step was – Combine resistors R 1 and R 2 in series This is not a good choice. Find the Thévenin Equivalent at terminals A and B. The problem here is that resistors R 1 and R 2 are not in series, since they do not have the same current going through them. Please go back and try again.

Defining the Open-Circuit Voltage and Solving for It Find the Thévenin Equivalent at terminals A and B. We have defined the opencircuit voltage, and called it v. OC. In preparation for solving for this voltage, we have also chosen a reference node for the node-voltage method, and defined the other node voltage, v. D. Note that there are only three essential nodes, so we will have two equations, plus two more for the dependent source variables. Thus we will need to write four equations. Let’s go to the next slide and write these equations. Next slide

Writing the Node. Voltage Equations We have written the four equations needed here. Let’s go to the next slide and simplify these equations. Find the Thévenin Equivalent at terminals A and B. Next slide

Simplifying the Node. Voltage Equations We canceled the last two terms of the second equation, and plugged the last two equations into the first two. Let’s go to the next slide and solve these equations. Find the Thévenin Equivalent at terminals A and B. Next slide

Solving the Node-Voltage Equations Find the Thévenin Equivalent at terminals A and B. When we take these two equations, and solve, we find that v. OC = 0. Thus, the open-circuit voltage is zero. If you had chosen to find the short-circuit current, you would have found that i. SC is also zero. Now, we have no choice but to find the equivalent resistance, REQ.

Defining the Short-Circuit Current and Solving for It Find the Thévenin Equivalent at terminals A and B. We have defined the shortcircuit current, and called it i. SC. In preparation for solving for this current, we have also chosen a reference node for the node-voltage method, and defined the other node voltage, v. E. Note that there are only two essential nodes, so we will have one equations, plus two more for the dependent source variables, and once for i. SC. Thus we will need to write four equations. Let’s go to the next slide and write these equations. Next slide

Writing the Node. Voltage Equations We have written the four equations needed here. Let’s go to the next slide and simplify these equations. Find the Thévenin Equivalent at terminals A and B. Next slide

Simplifying the Node. Voltage Equations We canceled the last two terms of the first equation, and plugged the last two equations into the first two. Let’s go to the next slide and solve these equations. Find the Thévenin Equivalent at terminals A and B. Next slide

Solving the Node-Voltage Equations Find the Thévenin Equivalent at terminals A and B. When we take these two equations, and solve, we find that i. SC = 0. Thus, the short-circuit current is zero. If you had chosen to find the open-circuit voltage, you would have found that v. OC is also zero. Now, we have no choice but to find the equivalent resistance, REQ.

Finding the Equivalent Resistance Find the Thévenin Equivalent at terminals A and B. We wish to find the equivalent resistance, REQ. What should be our first step? 1) Combine resistors R 2 and R 3 in parallel. 2) Apply a test source. 3) Set independent sources equal to zero. 4) Set dependent sources equal to zero. 5) Set all sources equal to zero.

You Chose: Combine resistors R 2 and R 3 in parallel Find the Thévenin Equivalent at terminals A and B. You said that the first thing to combine the parallel resistors. These resistors are in parallel, and they can be combined. Thus, this is a reasonable step, but taking this as the first step does not emphasize the very important step that we must always apply when finding the equivalent resistance. So, please go back and try again.

You Chose: Apply a test source Find the Thévenin Equivalent at terminals A and B. You said that the first thing would be to apply a test source. This is not correct. Applying a test source will be important here, but for this to work, we need to get rid of the independent sources. This is very important. Many students neglect this step, and get the wrong answer. Therefore we emphasize it by always performing it first. Let’s go back and try again.

You Chose: Set independent sources equal to zero Find the Thévenin Equivalent at terminals A and B. You said that the first thing to do was to set the independent sources equal to zero. This is the best first step. Always do this first, and you will not forget to do it. Let’s set the independent source, v. S 1, equal to zero.

You Chose: Set dependent sources equal to zero Find the Thévenin Equivalent at terminals A and B. You said that the first thing to do was to set the dependent sources equal to zero. This is incorrect. We do not set dependent sources equal to zero. We do set the independent sources equal to zero. Let’s do that. Go back and try again.

You Chose: Set all sources equal to zero Find the Thévenin Equivalent at terminals A and B. You said that the first thing to do was to set all of the sources equal to zero. This is incorrect. We do not set dependent sources equal to zero. We do set the independent sources equal to zero. Let’s do that. Go back and try again.

Setting Independent Sources Equal to Zero Find the Thévenin Equivalent at terminals A and B. We have set the independent source, v. S 1, equal to zero. Before going further, let’s simplify this circuit. The resistor R 1 is in parallel with a short circuit, and will have no effect. It can be removed. The resistors R 2 and R 3 are in parallel, as many of you have already noted. Let’s go ahead and combine them in parallel, with a resistor R 6. Note that the voltage v. X is across this resistor. We make these simplifications in the next slide. Next slide

What is the Next Step? We have simplified the circuit. What is the next step to take? 1) Combine resistors R 6 and R 5 in series. 2) Apply a test source. 3) Simplify by setting i. S 1 equal to i. X. Find the Thévenin Equivalent at terminals A and B.

You Chose: Combine resistors R 6 and R 5 in series You said that the first thing would be to combine resistors R 6 and R 5 in series. This is not a good thing to do. The resistors R 6 and R 5 are in series. However, if we did this, the voltage v. X would no longer be present, and the i. S 1 dependent current source depends on v. X. Let’s go back and try again. Find the Thévenin Equivalent at terminals A and B.

You Chose: Apply a test source You said that the first thing would be to apply a test source. This is the best choice for next step. Let’s apply a test source, between terminals A and B. Find the Thévenin Equivalent at terminals A and B.

You Chose: Simplify by setting i. S 1 equal to i. X You said that the first thing would be to simplify by setting i. S 1 equal to i. X. This is not a good thing to do. The current i. X is not equal to i. S 1, as we will see when we begin writing equations. Let’s go back and try again. Find the Thévenin Equivalent at terminals A and B.

Applying a Test Source We have applied a test source, between terminals A and B. We chose a voltage source, because we thought that this would make the solution a little easier. A current source would have been nearly as good. We chose to give it a value, 1[V], just to make it easier to solve this with a typical calculator. The key is to find the ratio of v. T/i. T, since we have used the active sign convention to define i. T. This ratio will give us REQ. Let’s solve. Find the Thévenin Equivalent at terminals A and B. Next slide

Writing Equations for i. T Let’s solve. We apply KCL to the top essential node, and get Find the Thévenin Equivalent at terminals A and B. Next slide

Solving for i. T To solve these equations for i. T, we plug the last two equations into the first, and get Find the Thévenin Equivalent at terminals A and B. Next slide

Finding i. T We plug in values and get Find the Thévenin Equivalent at terminals A and B. Next slide

Finding REQ Now we can solve for REQ and get Find the Thévenin Equivalent at terminals A and B. Note that this value could be negative. For example, if we had defined v. X with the opposite polarity, this answer would have been negative, with the value – 2. 74[W]. You could try it yourself. Next slide

The Thevenin Equivalent The Norton equivalent is a zero valued voltage source, in series with the equivalent resistance. This is, of course, just the resistance. Thus, we have the circuit drawn below. Find the Thévenin Equivalent at terminals A and B. Go back to Problem Statement Go to Comments Slide

What Happened Here? • It seemed like a strange problem, with the open-circuit voltage and short-circuit current ending up as zero. Actually, if either is zero, the other will be zero. When there are no independent sources in the circuit, this will always happen. • Here in this problem, there was an independent source. However, if you look carefully you will note that part of the circuit was shorted out, and had no effect on the rest of the circuit. This was clear from the node-voltage equations that we wrote to solve for i. SC and for v. OC. Note that in both cases the terms that included the independent source cancelled out. This is why the independent source seemed to not be there. Go back to Overview slide.