Lab 4:  Thevenin Equivalent

 

Introduction:

 

 

Most electrical and electronic circuits are complex systems.  However, we are often only interested in analyzing the behavior of a part of the system.  A reasonable thing to do in these situations is to replace the parts of the circuit with a simpler equivalent.  This can only be done as long as by doing so we do not affect the behavior of the part of the circuit we wish to analyze.  The Thevenin Theorem provides us with an effective way of doing this.  According to this theorem if we can partition the circuit so that the portion of the circuit of interest communicates with the rest of the circuit through a single port as shown in Figure 4-1 then we can replace the “rest of the circuit” with an equivalent circuit consisting of a voltage source and an impedance as shown in Figure 4-2 without affecting the voltages and currents in the “portion of interest when

 

 dealing with resistive circuits the impedance is simply a resistor.  In order to find this equivalent (known as the Thevenin Equivalent) we don’t even have to know the exact composition of the circuit.  All we need to do is find the open circuit voltage VOC across the terminals, and the short circuit current ISC between the terminals.  The Thevenin voltage VT is simply VOC and the Thevenin resistance RT is given by RT = VOC / ISC .

 

Experiment:

 

(1)    Obtain a black box from the lab instructor and connect the circuit shown in Figure 4-3 where R1 = 1KW, R2 = 2.2KW, R3 = 1KW, R4 = 1KW, V1 = 10V.  Adjust V1 using the multimeter.  Measure and note the voltages Vad, Vbd, and Vcd. 

 

                

(2)    Disconnect the part of the circuit shown in the dotted box.  You will find the Thevenin equivalent of this part of the circuit.  Leave the rest of the circuit alone.  Measure and note the open circuit voltage VOC and the short circuit current ISC.

 

(3)    Construct the Thevenin equivalent by setting the power source voltage to VT and connecting a potentiometer with resistance RT and reconnect it to the rest of the circuit as shown in Figure 4-5.  Measure and record the voltages Vad, Vbd, and Vcd. 

 

Results:

 

(a)    Present the measurements of Part (1) of the experiment.

 

 

(b)    Present the results of Part (2) of the experiment and compute the Thevenin voltage and resistance.

 

(c)    Present the results of Part (3) of the experiment.  Compare these with the results from Part (1).

 

(d)    Discuss how this experiment could be improved.