Lab 3: Post-lab
Trey Morris
801006456
ECEN 214-502
(Travis Eubanks)
9/25/2008
Procedure
This lab had three tasks. The first task was done using the HP DMM and variable power supply. We built
a circuit containing one voltage source and several resistors, one of which was considered the load
voltage. The goal was to simulate Thevenin equivalent in the laboratory so we measured the voltage and
current through the load, the voltage with the load open circuit, and the current with the load shorted.
The second task was the first part of verifying the superposition principle in the laboratory. We began by
constructing a circuit containing two voltage sources and several resistors . We then measured the
current through a particular branch with both sources on, and then twice more using only 1 source each
time and having the other shorted. After these measurements were performed the current with both
sources could be seen to equal the addition of the sources measured when each of the sources was
engaged and the other was shorted.
The third task was to prove that the superposition principle only holds for linear devices. In order to
prove it does not hold for nonlinear devices a diode was introduced into the circuit from the second task
and the same measure procedure was followed again. This time it was obvious that the individual source
currents measured would not add up to equal the current with both sources engaged. It is important to
note that the currents induced by each source were contrary through the branch being measured,
otherwise the superposition principle would seem to hold.
Data Tables
Meaningless data table reproduction on page 3.
Sample Calculations
It’s difficult to provide all of the sample calculations required to solve for the Thevenin Equivalent and
the verification of the superposition principle. It is important to note that Ω · for the first
part, and for the second part. The remaining circuits are solved using Kirchoff’s
Current Law and Ohm’s Law, neither of which, I believe, require proof of knowledge at this point in the
course.
Discussion
The results of task one were well with tolerated range. The measured was only 1.06% different from
the calculated . In a perfect world these values would be equivalent. This leads me to believe that
the circuit was constructed properly and the components all worked as expected. The Thevenin and
1
Norton equivalents are shown here.
The superposition principle works in task 2 because only linear devices are involved. A linear device is a
device which works regardless of which direction it is connected to a circuit and continues to work at the
same rate (linearly) throughout its operating range. A resistor, for example, will provide the same
resistance, whether forwards or backwards, from 0 amps up until the heat passing through it starts to
cause a problem and distorts the output. Knowing this it’s obvious that if you apply a voltage on one
side of a resistor, and another voltage on the other side, as in task 2, the current through the resistor
will be the sum of the current those two voltage sources induce through the resistor. This sum can
either be measured with both sources engaged, or by adding the currents with each voltage source on
and the other off because either way, the resistor doesn’t care. It’s linear. Ohm’s law says V=IR
regardless of direction.
In task 3 a nonlinear device was added to the circuit. Diodes, unlike resistors, are not linear. They only
allow current through in one direction. So if the application of the voltage sources in task 2 happens to
induce a current in the proper direction, the diode will allow it to pass. But since the sources induce
contrary currents, when only one is engaged, one will induce current and the other will be blocked by
the diode. This prevents the superposition principle from being true in the 3rd task.
Conclusion
In conclusion, I liked this lab. It was short and sweet. It taught two important circuitry concepts: Norton
and Thevenin equivalents, and the superposition principle. The circuits were not overly complicated as
could have been the case. The pre-lab was also very straight forward and applied directly to the lab. I
was curious why my SPICE simulation allowed a current to flow, albeit a small one, when the diode
should have been blocking it.
2
Task 1 Measured Calculated
IL
1.3mA
1.285mA
Difference
1.15%
VL
2.59 V
2.570V
0.78%
VOC
5.39V
5.372V
0.33%
ISC
Req
2.50mA
2156Ω
2.465mA
2179Ω
1.42%
Task 2 Measured Calculated
I
1.1mA
1.084mA
I1
1.47mA
1.455mA
I2
I1 + I 2
PSPICE
1.084mA
1.455mA
Difference
1.48%
1.03%
PSPICE Difference
1.48%
1.03%
-0.372mA
-0.00037A
-0.0003710A
0.54%
0.26%
1.068A
1.085A
1.084A
1.30%
1.29%
PSPICE
0.92845mA
1.295mA
% Error
1.08%
0.77%
Task 3 Measured Calculated
I
0.94mA
0.93mA
I1
1.31mA
1.3mA
I2
I1 + I 2
1.06%
0mA
0mA
5.140nA
0%
1.31mA
1.3mA
1.29500514mA
0.77%