1. THEVENIN EQUIVALENT CIRCUITS INEL3105

2. 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.

3. 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.

4. Thevenin Equivalent Circuits (TEC)  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.

5. 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.

6. What Is A Thevenin Equivalent Circuit?  The Thevenin Equivalent Circuit is an electrical model composed of two components shown below.  An ideal voltage source, V o.  A resistor, R o.

7. Equation or Model  We can write equations that describe the behavior of the TEC when it interacts with other components. First, let us define some variables. If we have a load attached to the terminals some load current will flow. We'll define a load current and a terminal voltage for the TEC as shown below.

8. Finding the Thevenin Voltage  V t = V o – R o I L  Using the expression for the terminal voltage, we can get the plot of terminal voltage against load current.

9. Properties of Thevenin  The first interesting property is that the source, V o, is the voltage that is measured when no load is attached. That's called the open circuit voltage. If you just attach a voltmeter to the output terminals - and didn't attach anything else, the voltmeter woud read the value of V o.

10. Properties of Thevenin  Another interesting property is that the there is a definite limit to how much current this source can supply to a load. If we short the terminals - something you can do in your mind, but not often in practice - the current that will flow is given by:  Short circuit current = V o /R o = I o.

11. Finding A Thevenin Equivalent Circuit  The TEC is a useful way of reducing complexity.  If you have a complex circuit interacting with other circuits you don't want to look at all of the details of what is taking place inside the complex circuit - at least you don't often want that.

12. Finding A Thevenin Equivalent Circuit  The TEC is a useful way of reducing complexity.  If you have a complex circuit interacting with other circuits you don't want to look at all of the details of what is taking place inside the complex circuit - at least you don't often want that.

13. “Things to get”  Open Circuit Voltage  Internal Resistance  Short Circuit Current.

14. Open Circuit Voltage  Open-circuit voltage (abbreviated as OCV or V OC ) -is the difference of electrical potential between two terminals of a device when disconnected from any circuit. There is no external load connected. No external electric current flows between the terminals. It is sometimes given the symbol V oc. In network analysis this voltage is also known as the Thévenin voltage.

15. No Load Voltage and Loaded Voltage  Voltage Divider  The two resistor voltage divider is used often to supply a voltage different from that of an available battery or power supply. In application the output voltage depends upon the resistance of the load it drives.

16. Internal Resistance Original Circuit Circuit with Voltage Source removed R TH =R 1 ||R 2 =(R 1 *R 2 )/(R 1 +R 2 )

17. Thevenin Equivalent Circuit V TH R TH RLRL VLVL