Oct 2011 pg. 17

ELG 2133 LAB 2

RESISTOR NETWORKS

OBJECTIVES

1. To verify experimentally the design concept of a voltage divider.

2. To design and build a simple analog ammeter based on the current-divider concept.

3. To verify experimentally the Thevenin equivalent circuit and the maximal power transfer

property.

PREPARATIONS 1. Read sections 3.3, 3.4, 5.4 and 5.6 of your textbook by Dorf and Svoboda.

2. Read and understand the experiment procedure below.

3. Answer the preparation questions.

PARTS AND EQUIPMENT 1 voltage/current source.

1 trim-pot (variable resistor) with a range of 0-500kΩ

2 analog multimeters.

three 1KΩ resistors.

PROCEDURE

A. Voltage Divider Preparation Questions:

By assuming the internal resistance of the voltmeter to be infinite and that of the battery to be

negligible,

P1) design a value for 1R in Fig. 1a such that

2V =0.5V

P2) determine 2V in Fig. 1b for

2R =1KΩ, 3R =10KΩ and the value of

1R designed in Question 1.

Experiment Steps:

1. Set up the circuit in Fig. 1a.

2. Verify by measurement the design of Question P1 above. Discuss and explain any

discrepancies.

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3. Set up the circuit in Fig. 1b, where 2R is loaded by a parallel resistor

3R .

4. Verify by measurement the result of preparation question P2 above.

Questions:

A1) Comment on the effects of 3R on the voltage divider as it ranges from 0 to ∞.

A2) If 3R represents the internal resistance of your voltmeter that is used to measure the voltage

in Fig. 1a, do an adjustment to find out the corrected value of v2.

A3) Determine the range of the impedance (resistance in this case) of the voltmeter so that the

measured value does not vary more than 0.1% due to the loading of the voltmeter.

B. Current Divider and the Ammeter Your TAs will have set up a station demonstrating how a simple analog ammeter can be built

from a d'Arsonval Meter and shunt resistors. You should understand the basic principle based

on the current divider (such as the supplemental reading emailed to you recently). Obtain

enough information from the station (or TA) to allow you to do the following.

1) Determine the range of currents that the setup can measure accurately. Justify your answer.

2) Give a design that will also include measurements whose range is one decade higher?

C. Thevenin's Theorem

Fig. 2

Preparation Questions:

P3) Design a value of 0R in Fig. 2 in order to obtain

0V =0.6V

P4) Apply Thevenin's theorem to determine the internal source resistance thR as seen by the

load 0R .

P5) Design 0R for maximum power dissipated in it. Call this resistance Ropt.

P6) Select 3 or 4 resistance values on both sides of Ropt Compute the corresponding power

dissipation of 0R

and plot the power p as a function of

0R

Some suggested values are 0.05Ropt , 0.1Ropt, 0.5Ropt, 0.9Ropt, 1.1 Ropt , 2Ropt, 5Ropt, 10Ropt,

50Ropt . See if they show the drop-off of power clearly on either side of Ropt. Choose other

0R values if you deem necessary.

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Experiments:

1. Set up the circuit with a voltage source of 5V as shown in Fig. 2. Use a variable resistor to

implement0R .

2. Adjust 0R to obtain

0V =0.6V. Verify your result with your preparation (do any adjustment

for loading effect where required).

3. Select a range of different 0R values. For each

0R setting, measure the corresponding

voltage 0V and current

0I . (do any adjustment for loading effect where required).

4. Plot the adjusted 0V vs adjusted

0I on the graph provided.

5. Determine the equivalent Thevenin voltage (thV ) and resistance (

thR ). Verify these values

against your preparation. Explain and/or justify any discrepancies.

6. Compute the power dissipated in the resistor, and plot it against the resistance0R .

7. Determine the value of 0R that gives the largest power dissipation. Does this value agree

with your design? Explain and justify your observation.

.

Suggestion:

If you finish this lab earlier and the instructor has also given you the instruction of the next lab

on oscilloscope and function generators, please take the opportunity to familiarize yourself with

the oscilloscope and the function generator.