ELEN 3441 – Fundamentals of Power Engineering Lab # 2 Spring 2008 Lab 2: Serial, parallel, and serial-parallel connection of resistors
and the Ohm’s law.
Objective: to review serial and parallel connections of resistors; to further familiarize with the
lab equipment and the acquisition interface.
Equipment: Power Supply, DAI, Variable resistance (8311)
Theory:
The following expressions are used to find equivalent resistance of N resistors connected in
a) Series:
N
Req = ∑ Ri
i =1
b) Parallel:
N
1 Req = ∑ 1 Ri
i =1
Ohm’s law:
E = IR
Experiment:
1) Construct the following circuit using the “Variable resistance” module:
7 A I1 E1 V N 2) Turn ON the DAI and the Power Supply (make sure the voltage adjustment knob is in the
“zero” position. Start the Metering window and set the channels for E1 and I1 to the DC
mode.
3) Determine the leakage current for your circuit: apply a DC voltage of approximately 100
V to your circuit while all three switches are in the OFF (down) position. Note: the load
Page | 1 ELEN 3441 – Fundamentals of Power Engineering Lab # 2 Spring 2008 resistance would be assumed as infinite in an ideal case. However, a small leakage
current may flow through the closed switches. Observe and record this leakage current
(you will need this value later). Decrease the input voltage to approximately 50 V. Did
the leakage current change?
4) Set the load resistance to 300 Ω by the corresponding switch. Set the voltage to
approximately 40 V. Record the values of voltage and current to a Data table. Repeat
measurements (while recording the values of voltage and current to a Data table) for four
other voltages from 50 V to 80 V with the steps of approximately 10 V.
5) Repeat the same measurements as in 4) for the load resistance of 600 Ω while recording
values for voltages and currents in your Data table (for five input voltages in the range
from 40 V to 80 V with steps of 10 V).
6) Repeat the same measurements as in 4) for the load of two resistors 600 Ω and 300 Ω
connected in parallel. Record values for voltages and currents in your Data table.
7) Repeat the same measurements as in 4) for the load of two identical resistors of 300 Ω
connected in series (you need to re-wire your circuit). Record values for voltages and
currents in your Data table.
8) Repeat the same measurements as in 4) for the following load:
300 Ω 600 Ω 600 Ω while recording the values for voltages and currents in your Data table.
9) Set the load resistance to 300 Ω and apply the DC voltage of 50 V. Start the
Oscilloscope.
Page | 2 ELEN 3441 – Fundamentals of Power Engineering Lab # 2 Spring 2008 Turn ON the channels for E1 I1, set both channels in DC mode with 20 V/div and 0.1
A/div respectively.
Notice that the current in your circuit was considered as DC (i.e., a straight line). Observe also
the RMS and average values for voltage and current (red circles).
One very useful feature of the Oscilloscope is the opportunity to export the data to a file. To do
this, select File\Export in the menu. Then specify the file name and save your data. You will
need this data file for your report.
Page | 3 ELEN 3441 – Fundamentals of Power Engineering Lab # 2 Spring 2008 In your report:
1. Using Matlab and the Data table you recorded, plot five graphs (each on separate axes)
voltage E vs. current I (i.e. current will be along the x-axis and voltage – along the y-axis)
for the experiments 4 to 8 (5 graphs total). When doing this, you need to decrease all
currents measured by the leakage current that you have determined in the experiment 3.
2. For each graph, determine the average slope. The values of slopes must approximately be
equal to the values of the load resistance. Compare the values for resistance of loads you
determined from slopes to the values you computed theoretically for each experiment.
Discuss possible sources of discrepancy.
3. Compare results you obtained for the experiments 5, 7, and 8. Discuss similarities and
dissimilarities.
4. Using Matlab and the data exported from the Oscilloscope, plot the voltage and current as
functions of time (index). Notes:
a) You may need to modify the data file (in Notepad, for instance) before importing it in
Matlab: i.e., you may need to create a new ASCII file that contains only the numerical
values for time indexes, voltages, and currents. Cut off the file header and channels’
labels. Use Matlab command load to import data.
b) You may find it easier to plot voltage and current on different axes.
Determine in Matlab and report their average values (mean). Are these values similar to
the corresponding values you measured in Part 4?
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