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Home » GATE Study Material » Electrical Engineering » Electrical Elements » Thevenin Equivalents

Thevenin Equivalents

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Thevenin Equivalents

Why Use Thevenin Equivalent Circuits?

Whenever you need to predict how something is going to behave you don't need to analyze things down to the lowest possible level. For example, when current flows in a resistor, you don't need to know what happens to every atom in the resistor. That ability to describe what happens to a large number of atoms in the resistor by using a macromodel for the resistor is convenient. Electrical engineers often think at different levels of complexity.

  • When analyzing/describing an amplifier circuit or a digital logic circuit the designer uses a macromodel for the resistors, transistors, capacitors and other components and doesn't worry about what happens inside those components.

  • When analyzing/describing a logic chip the designer uses a macromodel for the gates in the logic circuits, and doesn't worry about the transistors, etc. that comprise the innards of the logic circuits.

  • When analyzing/describing a computer, the designer uses a macromodel for the logic chips and doesn't worry about the gates inside the chips.

Goals Of This Lesson

What do you want to know about Thevenin and Norton equivalent circuits.

  • Given a source - not an ideal source,

    • Be able to determine the Thevenin and Norton equivalent circuits for simple circuits,

    • Be able to predict behavior under load for a non-ideal source.

We can visualize a hierarchy, with

  • Computer networks composed of computers,

  • Computers composed of various kinds of integrated circuit chips, etc.,

  • Integrated circuit chips composed of gates and other logic circuits,

  • Logic circuits composed of transistors, resistors and other electronic components,

  • Transistors composed of atoms of semiconductor material,

  • Atoms composed of protons, electrons, etc.,

  • Protons composed of quarks.

In this hierarchy, if you are an engineer designing a computer network you may want to take into account characteristics of the components one level up or down at most. You probably wouldn't want to worry about every transistor in a computer network - not in a million years, which is probably the time it would take you to do the analysis.

When you want to use different components you often need to use macromodels of the components you use. You do that because you don't always need to have totally detailed knowledge of what goes on inside the component.

Thevenin Equivalent Circuits- or TECs - are macromodels that are used to model electrical sources. Those sources are as diverse as batteries, stereo amplifiers and microwave transmitters. In this lesson we will develop TEC models of sources and learn how to use them in larger circuits.

What Phenomena Does A TEC Explain?

A Thevenin Equivalent Circuit is used to explain some of the things that happen when you use sources. One good example is what happens if you start a car with your headlights on. If you have ever done that you probably noticed that the lights of the car dim when you start the car with them on.

Obviously, something happens that causes the voltage across the headlights to change. The battery is not an ideal voltage source since that dimming occurs whenever the starter motor draws a lot of current from the battery. If the battery were an ideal source, the voltage would never change and the headlights would never dim. In this lesson, we will examine Thevenin Equivalent Circuit - TEC - models for nonideal sources. Those models can be used to predict the sort of voltage decrease that the battery exhibits under heavy load current.

Does a TEC explain this phenomenon? Actually, nothing can explain this phenomenon because it doesn't exist. It violates the laws of physics - i.e. light travels in a straight line. However, although the light beam doesn't droop, the voltage of a nonideal source, like a battery, often droops when current is drawn - the car lights don't droop! They just get dimmer!

What Is A Thevenin Equivalent Circuit?

The Thevenin Equivalent Circuit is an electrical model composed of two components shown below.

  • An ideal voltage source, Vo.

  • A resistor, Ro.


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