EE1620 Thevenin online simulations PDF

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THEVENIN’S THEOREM Aims: • to provide experimental understanding of Thévenin’s theorem, • to further generate interest in the behaviour of circuits and systems. Learning Outcomes – students should be able to: • apply Thévenin’s theorem for simple circuits, • use simulations to verify Thévenin’s theorem,

Introduction This experiment is about understanding and applying theory taught in the EE1620 module. Firstly, you will use Thévenin’s theorem to predict the equivalent voltage source and source resistance of some simple circuits. You then create PSPice simulations to verify these predictions. So before you start the experimental work, you need to do some examples to obtain the predictions. These examples are given after a brief statement of the theory. You MUST record all your work in an electronic laboratory notebook (lab book). The experiment consists of several tasks that must be completed. You need to work steadily to finish the whole experiment and some of you may not reach the final tasks. Do not rush your work as it is more important to understand what you are doing than it is to complete all the tasks. You MUST submit your lab report via Wiseflow before the deadline (the day after the lab).

Theory Thévenin’s theorem states that any linear circuit (i.e. one just containing resistors, capacitors and inductors) can be represented by an equivalent voltage source and a resistance (which can be thought of as a source resistance) in series.

Examples: I 1. 100 kΩ

10V

100 kΩ

R

V

What is the value of the voltage V, current I, and power P dissipated in the resistor R for values of R of: a) zero Ω b) 50 kΩ c) infinity?

Figure 1

I 2.

What graph do you expect if V is plotted against I as R varies?

RTh +

R

VTh

3.

V

How can this graph give values for VTh and RTh ?

Figure 2 Determine a Thévenin equivalent circuit for the circuit shown in Figure 3. A

180Ω

740Ω

10V 1k Ω

B Figure 3

Equipment – record the details in your lab book. The Experiment 1)

Verification of Thévenin’s Theorem

Use the Orcad PSpice simulator to show that Thévenin's theorem is valid with the aid of the circuit from Figure 4. As in the previous labs, use the Horizon VMWare to access Orcad PSpice.

Figure 4 Notes: • • • •

Find Vth by Open Circuit Voltage Apply “Short Circuit Method” for Rth Always add GND at one of the terminals DC, so time domain analysis will give you constant values.

PSpice procedure for Thevenin Circuit (Pspice can NOT directly calculate the Thevenin equivalent circuit with respect to two nodes, but it can help you calculate it quite simply). Open Circuit ▪ Insert a resistor of big value, like 1 or 10 MEG or TERA between terminals A and B. This will allow PSpice to simulate the circuit as open circuit.

▪ Find the node voltage at the terminal (Vth) Short Circuit ▪ Copy the open circuit and replace the MEG resistor with one of very little value, like 1 u [micro] or femto. This will simulate the short circuit. ▪ Find the current through the short circuit resistor ▪ Rth =Vth / Isc. You can modify the trace to show this vaule directly. From your simulated results, determine the Thévenin equivalent for the circuit. Determine manually via calculations the equivalent Thevenin circuit and compare the PSpice results with the theoretical prediction. 2)

Thévenin Modelling

Thévenin’s theorem states that any linear circuit can be replaced by an equivalent circuit containing just a voltage source and a source resistance. This equivalent circuit will then behave identically to

the original circuit so you should not be able to tell them apart. In practice, there may be slight differences however – see if you can spot them and try to explain them. Set up a simulation in Orcad PSpice to verify the Thévenin equivalent for circuit 3 from Examples (figure 3), i.e. use the power supply to give a voltage source equal to VTh in series with a resistance equal to RTh . Follow the procedure from the simulations performed in section 1). Further task: Generate the Thevenin equivalent impedance for the circuit in Figure 5. Draw an impedance phasor to help you and identify the Thevenin impedance on the phasor diagram. Using the simulator measure the open circuit voltage. Using this Voltage and the Thevenin Impendence you calculated, simulate the Thevenin equivalent circuit to verify that both the circuit in Figure 5 and your Thevenin equivalent circuit are the same. C1

R2 40Ω

V1 240Vpk 50Hz 0°

20µF

L1 0.2H

R1 40Ω

Figure 5

Conclusions Comment on your findings with those derived from theory using the network values. Is Thévenin's theorem valid and is it useful? Under which conditions?

March 2021...