Laboratory 5
Electronics Engineering 1270
DC Thevenin and Norton Equivalent Circuits
Purpose:
To demonstrate equivalent circuits using computer simulations and actual circuits.
Students should gain additional experience using computer simulations tools.
Equipment and Components:
•
•
•
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Prototyping board
Multimeter
Power supply
Resistors: 5.1 kΩ, 13 kΩ, 20 kΩ, 24 kΩ, 43 kΩ, 56 kΩ, 120 kΩ, and others
depending upon your design
Preliminary:
Using the circuit shown in Figure 5.1, prepare the following
1. Calculate the Thevenin and Norton equivalent circuit by:
a.
b.
c.
d.
Find the open circuit voltage (Voc) using node analysis.
Find the short circuit current (Isc) using mesh analysis.
Calculate the source resistance using Voc and Isc.
Suppress the source and use resister combinations to find Rs. Does this
match part c.
2. Use PSpice to determine the Thevenin and Norton equivalent circuits using the two
different methods described below. Include output from your PSpice program in
you lab book.
a. Using PSpice, find the open circuit voltage (Voc) and short circuit current
(Isc). From those two values, calculate the source resistant (Rs = Voc/Isc).
Hint: You may want to layout two similar circuits and simulating both at
the same time.
In addition, PSpice cannot handle floating nodes and only returns
branch/element voltages and currents. You must insert a “large” and/or
“small” resistor as the loads in order to gain load type information.
1. The open circuit voltage can be determined approximately by adding a
resistor between points A and B (called Rload) and setting Rload=
1.0x1012Ω or 1E+12 in Spice format (the approximate resistance of
air) and examining the voltage across the load.
2. The short circuit current can be determined approximately by setting
Rload=1.0x10-12Ω or 1E-12 in Spice format and examining the current
through the load.
Warning: When dealing with the extreme values keep in mind that the
resulting matrixes may be ill-conditioned. Verify that the final answers do
not violate KCL / KVL rules. If this is a problem try relocating the
ground.
b. Suppress the power supply (replace it with a short – 0V) and apply a test
voltage source to the output. Using a 1 V source determine the current
flowing into the circuit. The source resistance can be found by Rs = Vtest /
Iforced.
Include in your lab book schematics of the Thevenin and Norton equivalent circuits.
3. Design a simple parallel and/or series network of two or three 10% or better resistors
(of standard values) that produce an equivalent resistance to the source resistance
calculated in part 1.c.
Hint: They do not produce a 8.6 kΩ resistor but two 4.3 kΩ resistors could be
connected in series to yield an equivalent 8.6 kΩ resistor.
Procedure:
1. Create the circuit shown in Figure 5.1 on a prototyping board.
2. Measure the open circuit voltage (Voc) using a voltmeter and calculate the Thenvenin
equivalent voltage source.
3. Measure and calculate the equivalent source resistance using the following three
methods.
a. Measure the short circuit current (Isc) using an ammeter and calculate Rs
from the open circuit voltages measured in part 2.
Warning: In practice it is a bad idea to short the output terminals of any
random circuit. Before shorting any terminals, users must verify that the
internal circuitry of the system can dissipate the resulting power. There is
no need to do these calculations with the circuit provided in this lab
assuming 1/8 W resisters are used.
b. Suppress the voltage source in the circuit (replace it with a short circuit)
and measure the input resistance (between terminals A and B) directly
with an ohmmeter.
Remember: When using a multimeter to measure resistances, all other
source must be removed or the resulting values will be erroneous. This
can complicate any circuit that requires a bias voltage, like transistors
and op-amps. What reading do you get on the multimeter if the voltage
source is NOT suppressed.
c. Suppress the voltage source in the circuit and attach a test voltage source
to the output. Measure the forced current that follows into the circuit with
an ammeter. Calculate the effective resistance of the circuit.
4. Attach the following load resistors to the circuit in Figure 5.1 one at a time.
Measure the voltage drop and current through the resistor. Calculate the power
dissipated by the resistors. Display all results in a table.
a. A match load (Rl = Rs)
b. A resistor ½ the value of Rs
c. A resistor 2 times the value of Rs
5. Create the Thevenin equivalent circuit of preliminary part 3. Attach the same load
resistors from procedure part 4 and measure the voltage, current, and power
associated with each resistor. Display all results in a table.
Conclusion
Compare the calculated value, simulated values, and all measured values of the Thevenin
equivalent source, include percent error between the different methods. How do the
various methods for measuring Rs compare, and note which method was the simplest and
most accurate for this example. Also identify how your equivalent circuit and the
original circuit compared in regards to the power delivered to the load.
5.1 kW
24 kW
120 kW
A
13 kW
56 kW
6V
Figure 5.1
20 kW
43 kW
B