Th é venin and Norton Equivalent Circuits

1. Thévenin and Norton Equivalent Circuits ELEC 308 Elements of Electrical Engineering Dr. Ron Hayne Images Courtesy of Allan Hambley and Prentice-Hall

2. Equivalent Circuits • Two-terminal circuit • Circuit that has any complex interconnection of resistances and sources • Has ONLY two points that can be connected to other circuits • We will learn how to represent complex two-terminal circuits by simple equivalent circuits ELEC 308

3. Thévenin Equivalent Circuits • Consists of voltage source in series with a resistance ELEC 308

4. Circuit Parameters • The Thévenin source voltage is equal to the open-circuit voltage of the original network, or Vt = voc ELEC 308

5. Circuit Parameters • The Thévenin short-circuit current is given by ELEC 308

6. Circuit Parameters • The Thévenin resistance is given by • Thus, we can determine a Thévenin equivalent circuit simply by measuring the OPEN-CIRCUIT VOLTAGE and SHORT-CIRCUIT CURRENT ELEC 308

7. Example 2.16 ELEC 308

8. Exercise • Find the Thévenin equivalent circuit ELEC 308

9. Norton Equivalent Circuits • Consist of independent CURRENT source in PARALLEL with a resistance ELEC 308

10. Circuit Parameters • The Norton source current is equal to the short-circuit current of the original network, or In = isc ELEC 308

11. Circuit Analysis Algorithm • Complete two of the following: • Determine the open-circuit voltage (Vt = voc). • Determine the short-circuit current (In = isc). • Zero the sources and find the Thévenin resistance Rt looking back into the terminals. • Use Ohm’s Law (Vt=InRt) to compute the remaining parameter. • Draw one of the following: • Thévenin equivalent circuit with independent VOLTAGE source Vt in SERIES with Rt • Norton equivalent circuit with independent CURRENT source In in PARALLEL with Rt ELEC 308

12. Exercise 2.29 • Find the Norton equivalent circuit ELEC 308

13. Maximum Power Transfer • Consider the two-terminal circuit and load resistance below. What value of RL would deliver maximum to the load resistance RL? ELEC 308

14. Maximum Power Transfer • The load resistance RL that absorbs the maximum power from a two-terminal circuit is equal to the Thévenin resistance, or RL=Rt • The maximum power transferred to the load resistance is given by ELEC 308

15. Example 2.21 • Find the load resistance for max power transfer • Find the max power ELEC 308

16. Principle of Superposition • Suppose we have a circuit with MULTIPLE independent sources • We wish to determine a specific response (current or voltage) in the circuit • We can EASILY determine the response due to a SINGLE independent source (by zeroing out the other independent sources) • The SUPERPOSITION principle states that the total response is the SUM of the responses to each of the independent sources acting INDIVIDUALLY ELEC 308

17. Superposition Illustration ELEC 308

18. Strain Measurements ELEC 308

19. Wheatstone Bridge • Circuit used to measure UNKNOWN resistances • Used by ME’s and CE’s to measure the resistances of strain gauges in experimental stress studies of machines and buildings ELEC 308

20. Wheatstone Bridge ELEC 308

21. Summary • Thevenin Equivalent Circuit • Norton Equivalent Circuit • Maximum Power Transfer • Superposition • Wheatstone Bridge ELEC 308