Chapter 4-7 •Thevenin and Norton Equivalents Thevenin and Norton Equivalents:: Useful when interested at what is taking place at a specific pair of terminals Thevenin equivalent circuit : is a voltage source VTh in series with a resistance RTh . The combination is equivalent to the original circuit at the terminals of interest To find the Thevenin voltage compute the open circuit voltage of the original circuit at the terminals of interest. To find the Thevenin resistance we compute the short circuit current at the same terminals isc then use the relationship: 𝑅𝑇ℎ = 𝑉𝑇ℎ 𝑖𝑠𝑐 Example VTh Example RTh The Value? if apply a load across a and b, the voltage across the load, and current will be the same as in the original circuit. You Do The Norton equivalent circuit consist of a current source in parallel with a Norton resistance. The Norton current is equal to the short circuit current at the terminals of interest, and the Norton resistance is equal to the Thevenin resistance. So we can derive the Norton equivalent, once we know the Thevenin values, from 𝑉 𝑖𝑁 = 𝑅𝑇ℎ and 𝑅𝑁 = 𝑅𝑇ℎ 𝑇ℎ Alternative method for finding the Thevenin resistance for a network that contains only independent sources: 1. Deactivate all independent sources A. Short circuit Voltage sources B. Open circuit Current sources 2. Find the equivalent resistance looking into the terminals of interest You Do Alternative method for finding the Thevenin resistance for a network that contains dependent sources: 1. Deactivate all independent sources A. Short circuit Voltage sources B. Open circuit Current sources 2. Leave dependent sources in circuit 3. Apply a small test voltage (vT), or small test current (iT), at the terminals of interest 4. Then the Thevenin resistance is the ration of the voltage across the test source to the current delivered by the test source Why did I choose vT ? Write your equations and solve for 𝑣𝑇 𝑖𝑇 What is the Thevenin voltage? What you should know 1. Know how to find simple Thevenin and Norton equivalents.