ELEC 105, Spring 2004
Prof. Rich Kozick

## Laboratory 4 Thevenin Equivalent Circuit Models and Maximum Power to a Load

### Objective:

In this lab, we will determine the Thevenin equivalent model for a mystery circuit and investigate maximum power transfer to a light bulb.

### Lab Activities:

1. We will use a "grain of wheat" bulb as a load in this lab. Begin by determining the resistance of your bulb, RL. If you do not have an ammeter, how can you measure the resistance using a voltmeter? Perhaps place the bulb in a voltage divider with a known resistance?

(Does the ohmmeter provide an accurate measurement of the bulb's resistance? The bulb's resistance may be different when it lights up because it gets warmer.)

2. Determine a Thevenin equivalent model for your mystery circuit. That is, make measurements and determine Vt and Rt.

In your lab notebook, clearly explain your approach, present your data, and show your computations.

3. Connect the light bulb to your mystery circuit.
• Observe the brightness of the bulb.
• Determine the power that is dissipated by the bulb. Clearly explain your method of calculating the power.

4. Use the power supply and one or more resistors to construct a circuit that behaves equivalently to your mystery circuit. That is, the circuit you construct must behave in the same way as your mystery circuit with respect to loads that are connected to the terminals.

Attach the light bulb to the equivalent circuit that you constructed, and verify that the light bulb dissipates the same power as it did when connected to the mystery circuit.

5. Suppose you were asked to design a new light bulb that will extract maximum power from your mystery circuit. What resistance should this new bulb have? How much power would the mystery circuit deliver to this new bulb?

In the process of answering this question, explain why matching RL = Rt maximizes the power delivered to the load.

6. Modify the resistor in the equivalent circuit that you built in step 4 in order to maximize the power delivered to your actual light bulb. Verify that the bulb shines brighter using this modified circuit, and calculate the power dissipated by the bulb.