3.2 THEVENIN AND NORTON EQUIVALENTS

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• A sometimes useful transformation is based on the equivalence of certain circuit elements,

R1

First, consider the short circuit case where,

this gives the basic relationship,

Next, consider the open circuit case,

we find the simple relationships,

• We can use this to test an unknown circuit for open circuit voltage, and short circuit current, and then replace it with an equivalent circuit.

1.Measure open circuit voltage Vs

2.Measure short circuit current Is

3.a) If using a Thevenin equivalent calculate, Rs = Vs/Is 3.b) If using a Norton equivalent calculate, Rs = Vs/Is 4. Draw the appropriate circuit.

* note the resistor values are the same for both circuits.

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• We can also use the Thevenin/Norton transformation to simplify circuits. Consider example 4.13 from [Nilsson].

Find V

V

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We can replace the first loop (120V source) with a Norton equivalent,

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Next we can convert the remaining voltage source to a current source,

The current sources and resistors may now be combined to simplify the circuit,

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