# Lab 11: Thevenin's Theorem

## Objectives

• Become aware of an experimental procedure to determine VTh and RTh.
• Validate Thévenin’s theorem through experimental measurements using two methods.
• Validate the maximum power transfer relation for a Thévenin circuit.

## Background

Through the use of Thévenin’s theorem, a complex two-terminal, linear, multisource DC circuit can be replaced by one having a single source and resistor. The Thévenin equivalent circuit consists of a single DC source referred to as the Thévenin voltage and a single fixed resistor called the Thévenin resistance. The Thévenin voltage is the open-circuit voltage across the terminal points of the load. The Thévenin resistance can be calculated using two methods:

• Method-1: Determine the Thévenin resistance when all sources deactivated.
• Method-2: Determine the Thévenin resistance using the ratio of the Thévenin voltage to the short circuit current of the load resistance – vOC / iSC.

## Questions

1. Explain why Rload is needed to be replace with a 5 GΩ resistor in the PSpice simulations. Why not just take Rload out of the circuit and simulate the resulting circuit?
2. Suppose you have two boxes in front of you. One box contains a Thevenin Equivalent (voltage source in series with a resistor) and the other box contains a Norton Equivalent (current source in parallel with a resistor). Each box has a pair of terminals available for measurement. You cannot open the boxes. You may make any electrical measurements at the terminals. You also have access to the outside surface of the boxes. Can you determine which box contains the Thevenin Equivalent and which box contains the Norton Equivalent? Or is it impossible to determine which circuit is in which box? Justify your answer in detail.