ECE 291 - Electrical Engineering Laboratory

# LAB 3: 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. 3.
2. Draw a Thevenin equivalent circuit of the circuit in Fig. 3 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: ECE 291 parts kit, a proto-board, a resistance substitution box, leads.

### 1.      THE SUPERPOSITION PRINCIPLE

Assemble the circuit shown in Fig. 3. 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.

Measure VL, between:

a)   Points A and B, with RL infinite (open circuit) and compare it with the calculated value.

b)   Find VL from superposition principle after making 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.

d)    Replace the 6 V dc source with an ac source and, using an oscilloscope, verify that the superposition principle applies also to ac.  You may use the ac of the same amplitude as the replaced dc source for easier comparison with measurements 1. b).

### 2.      THE THEVENIN EQUIVALENT CIRCUIT

a)    Design and assemble the Thevenin equivalent circuit of the circuit in Fig. 3 that you built in part 1 a), without resistor RL.

ADVICE: It is highly unlikely that you will find the right resistor in your parts kit for a Thevenin circuit.  Use a resistance substitution box instead.  The box allows you to obtain any resistance you may need.  Do not trust, however, the numbers on the box.  These boxes were extensively used (and sometimes abused) by your predecessors.  After selecting the desired resistance with switches, measure it with a digital ohmmeter.

b)   Check that the Thevenin circuit gives the same values VL as the original circuit (Fig. 3) for RL = ∝ (open points A and B), and with the same resistor RL that you used in 1 c).

### 3.      CIRCUIT SIMULATION (at home)

Compare results of measurements in 2 b) with Multisim 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.