ELEC 101, Spring 2005
Prof. Rich Kozick
Laboratory 5 - Optional
Thevenin Equivalent Circuit Models and Maximum Power to a Load
This lab is not required, but students that finish the D/A converter
Lab 5 will be asked to
work on these exercises on February 24.
In this lab, we will
determine the Thevenin equivalent model for a mystery circuit
and investigate maximum power transfer to a light bulb.
We will use a "grain of wheat" bulb as a load in this lab.
Begin by determining the resistance of your bulb,
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.)
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.
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.
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.
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.
Modify the resistor in the equivalent circuit that you built in step 4
in order to maximize the power delivered to your actual light
Verify that the bulb shines brighter using this modified circuit, and
calculate the power dissipated by the bulb.