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Tufts University
School of Engineering
Department of Electrical and Computer Engineering
ES3  Introduction to Electrical Circuits
Fall 2007
Lab Section: Friday
2:304:30pm
Experiment 3
Thevenin Equivalent Circuits and Maximum Power Transfer
Name:
Teddy Portney
Edward
.
Portney@ tufts
.
edu
Submitted to:
Bertan Hallacoglu
Experiment Performed:
October 25, 2007
Experiment Due:
November 2, 2007
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PURPOSE
The purpose of this lab was to learn how to create Thevenin equivalent circuits for
complex laboratory devices, such as a function generator, a 9volt battery, and even a
microphone. As creating theoretical Thevenin equivalent circuits was discussed in
lecture, it is useful to also know how to measure the properties needed to find the
equivalent circuit experimentally. This lab also dealt with the concept of maximum
power transfer, an idea, which goes hand in hand with the Thevenin equivalent circuit.
II.
INTRODUCTION
A Thevenin equivalent circuit is a model of a real circuit. Its purpose is to simplify
extremely complicated circuits that would otherwise be difficult or even impossible to
solve into a circuit that only contains two elements. Specifically, it is possible to model
the function generator as a Thevenin equivalent circuit. The function generator in all
likelihood consists of many different components, at least several of which have not even
been mentioned in lecture. However, through experimental measurements using a
voltmeter and an ohmmeter, it is possible to reduce the function generator to a circuit that
consists of solely a voltage source and a Thevenin resistance. The basic design of a
Thevenin equivalent circuit is shown in the appendix of this lab report. This concept of
reducing complex circuits into circuits that behave in exactly the same manner is very
useful. For example, because the Thevenin equivalent circuit for a function generator is
so easy to find, it is completely unnecessary to know the actual schematic for the function
generator in order to perform basic calculations. Thevenin equivalent circuits also allow
for modeling of devices that are not actually complex circuits, or even circuits at all. The
example of this used in this lab was using a 9volt battery and creating a Thevenin
equivalent circuit based on certain measurements taken.
As mentioned above, this lab also brought in the idea of maximum power transfer.
This is very useful, like the Thevenin equivalent circuits, because for one set Thevenin
equivalent circuit you can calculate what load must be connected to it in order to deliver
the maximum power. As can be shown by Ohm’s Law, power is equal to voltage times
current, and is therefore equal to current squared times the resistance. P = V
2
/ R, and
P = I
2
* R. As one can see in these formulas, neither a short circuit, which provides
maximum current, nor an open circuit, which provides maximum voltage drop, will
provide the maximum power. In fact, both of these cases will provide a power of 0 Watts.
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This note was uploaded on 03/27/2008 for the course ES 3 taught by Professor A during the Fall '08 term at Tufts.
 Fall '08
 A

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