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ECE 291 - LABORATORY III

THE SUPERPOSITION PRINCIPLE

AND THE THEVENIN EQUIVALENT CIRCUIT

 

OBJECTIVES

1.   Verifying experimentally the superposition principle for a circuit with two voltage sources.

2.   Designing and testing the Thevenin equivalent circuit.

 

PRELAB

  1. Make a brief plan of measurements to demonstrate the superposition principle for a circuit shown in Fig. 1.

     

  2. Draw a Thevenin equivalent circuit of the circuit in Fig. 1 without RL, which is an external "load" resistor.          Express the voltage of the source and the resistance in the Thevenin circuit in terms of R1 and R2.

 

LABORATORY

Equipment needed from the stockroom: ECE 291 parts kit, a proto-board, a resistance substitution box, leads.

 

1.    THE SUPERPOSITION PRINCIPLE

Assemble the circuit shown in Fig. 1. Choose different values for R1 and R2 from resistors in 1k to 30k range such that their ratio is no more than 5. At first, do not connect the load resistor RL.

Fig.1 A network of three resistors with two voltage sources

 

Measure the voltage between points A and B, with RL infinite (open circuit):

a)   With both sources in the circuit

b)   With only one of the sources in the circuit while the other is replaced with a wire ("short circuit"). Read the warning below before making these two voltage measurements.

Warning: do not short the power supplies when replacing it with a short circuit! Disconnect them first from the circuit.

c)   Repeat a) and b) after connecting a resistor RL (in 3k to 20k range) between points A and B. After the measurements put aside this resistor; you will need it again in part 2.

Check that the measurements a) and b), without RL, and measurements c), with RL, agree with the superposition principle. Calculate any discrepancy in % and comment if it is reasonable, based on the precision of your instruments. Compare also the measured value of VL with calculations for a circuit with two sources.

 

 

2.   THE THEVENIN EQUIVALENT CIRCUIT

a)    Design and assemble the Thevenin equivalent circuit of the circuit in Fig. 1, without resistor RL, that you built in part 1 a). You have already measured Vth in 1 a) (no load condition). Now measure also Thevenin resistance Rth. It is highly unlikely that you will find the resistor with the value Rth in your parts kit. Use a resistance substitution box instead. The box allows you to obtain any resistance you may need. After selecting the desired resistance with switches, make sure that it is what you want by measuring it with a digital ohmmeter.

b)   Check that the Thevenin circuit gives the same values VL as the original circuit (Fig. 3) with the same resistor RL that you used in 1 c).

 

3.   CIRCUIT SIMULATION (at home)

Compare results of measurements in 2 b) with PSPICE simulation.

 

REPORT

  • Include all schematics with the measured values of the components. Present the data, indicating clearly which are measured and which calculated values. How well the measurements agree with the superposition principle? (give differences in %). Does the Thevenin equivalent circuit provide the same voltage across a load as the circuit it substitutes? If not explain possible reasons for discrepancies.

  • Do not forget to number figures and tables and to give them captions (titles).

  • Number all pages of the report.

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