Download Lab 3 Series and Parallel Circuits

Lab 3
Series and Parallel Circuits
Background
Unlike most of the other labs you will do this semester, you will not have a lab partner for this lab.
You will be on your own. It is critical that you understand what you are going to be doing,
particularly measuring current.
There are many websites that explain how to calculate equivalent resistance for series and parallel
circuits, such as http://www.wikihow.com/Calculate-Series-and-Parallel-Resistance . If you are not
already familiar with the formulas, refer to your textbook, the above website, or some other source to
make sure you understand how to do so before the lab.
Please Note: There are TWO videos for this lab. One covers all of the experimental configurations
that you will measure and the other focuses exclusively on measuring current. Take the time to watch
these videos carefully before coming to class. If you do not, you will probably blow a fuse on the
multimeter.
Measuring current is easy if you follow this procedure (described in detail in the video):
1)
2)
3)
4)
Set the multimeter to measure current (20 A, Unfused)
Connect the black probe to the COM terminal on the multi-meter.
Connect the red probe to the Yellow (20 A, Unfused) terminal on the multi-meter.
Look on the power supply to see how much current is being drawn. It should be less than 0.2
amps (200 milliamps).
5) Remove a wire from your experiment. The current should drop.
6) The wire you removed had two ends. Connect your black probe to where one end of the wire was
connected.
7) Connect the red probe to where the other end of the wire was connected.
8) Verify that your current is below 0.2 amps (200 milliamps). This is a rough measurement and
should not be recorded on your spreadsheet.
9) Move the red wire from the yellow terminal on the multimeter to the white terminal (0.2 amps,
fused).
10) Move the dial on the multimeter to select 200 mA current measurement. This will enable you to
accurately measure the current.
No matter how many times I go over this, some people only learn how to measure current during the
test… where they lose points for doing it wrong.
I.
Series Circuit
Open the Excel Spreadsheet.
In all of the series circuit experiments, you will use two resistors connected in series. There will be
an experimental setup at the front of the lab and you can bring your components up to the front to
duplicate the setup. By convention, the Resistor 1 is the one on the left of the prototype board and
Resistor 2 is the one on the right.
Case 1: 100 Ohm Resistors
1. Setup two 100 ohm resistors in series.
2. Connect the power supply. Typically, the positive end is connected with a red wire to the left
side of the circuit board. Please note that this is simply a convention that makes it easier for
others (such as your lab instructor) to verify your setup.
3. Turn on the power supply and set the current straight up.
4. Set the power supply voltage to approximately 6 to 8 volts. The actual voltage does not matter.
5. Verify that the current drawn (displayed on the power supply) is less than 0.2 amps.
6. Measure the voltages (V1, V2, and Vtot). V1 is the voltage for the resistor on the left.
7. Measure the current through the circuit. If this is your first current measurement, wait until the
lab instructor or lab supervisor is available to verify your setup and the steps that you are taking.
8. Calculate the experimental equivalent resistance (column G) using Ohm’s Law (Vtot/Current).
9. Calculate the theoretical equivalent resistance (column I) using the given resistor values.
Case 2: 100 Ohm and 47 Ohm Resistors
1. Replace the 100 Ohm resistor on the right with a 47 Ohm resistor and repeat the procedure
described in Case 1.
Case 3: 47 Ohm Resistors
1. Replace the remaining 100 Ohm resistor with a 47 Ohm resistor and repeat the procedure
described in Case 1.
2. Turn off the power supply.
II.
Parallel Circuit
In all of the parallel circuit experiments, you will use two resistors connected in parallel. There will
be an experimental setup at the front of the lab and you can bring your components up to the front to
duplicate the setup. By convention, the Resistor 1 is the one at the top of the prototype board and
Resistor 2 is the one at the bottom.
Case 1: 100 Ohm Resistors
1. Setup two 100 ohm resistors in parallel.
2. Connect the power supply. Typically, the positive end is connected with a red wire to the left
side of the circuit board. Please note that this is simply a convention that makes it easier for
others (such as your lab instructor) to verify your setup.
3. Turn on the power supply and set the current straight up.
4. Set the power supply voltage to approximately 6 to 8 volts. The actual voltage does not matter.
5. Verify that the current drawn (displayed on the power supply) is less than 0.2 amps. Note: This is
important for the parallel setups. If your current is too high, simply reduce it until the current is
about 0.16 amps.
6. Measure the voltages (V1, V2, and Vtot). V1 is the voltage for the uppermost resistor.
7. Measure the current through the circuit. If this is your first current measurement, wait until the
lab instructor or lab supervisor is available to verify your setup and the steps that you are taking.
8. Calculate the experimental equivalent resistance (column G) using Ohm’s Law (Vtot/Current).
9. Calculate the theoretical equivalent resistance (column I) using the given resistor values.
Case 2: 100 Ohm and 47 Ohm Resistors
1. Replace the 100 Ohm resistor on the right with a 47 Ohm resistor and repeat the procedure
described in Case 1. Remember to check the current!
Case 3: 47 Ohm Resistors
1. Replace the remaining 100 Ohm resistor with a 47 Ohm resistor and repeat the procedure
described in Case 1. Remember to check the current!
2. Turn off the power supply.
III.
Currents
In both of these experiments, you will use two resistors connected in parallel. The setup is the same
as in Part II.
Case 1: 100 Ohm/47 Ohm Resistors
1. Setup the 100 ohm and 47 ohm resistors in parallel.
2. Connect the power supply. Typically, the positive end is connected with a red wire to the left
side of the circuit board.
3. Turn on the power supply and set the current straight up.
4. Set the power supply voltage to approximately 6 to 8 volts. The actual voltage does not matter.
5. Verify that the current drawn (displayed on the power supply) is less than 0.2 amps. If your
current is too high, simply reduce it until the current is about 0.16 amps.
6. Measure the current through each resistor. This will mean removing a different wire than you did
in Part II for measuring current. The wire that you remove should only pass current through the
resistor you are measuring.
7. Note: You should get different current values. If your values are close to the same, you have
done something wrong.
Case 2: 470 Ohm/47 Ohm Resistors
1. Setup the 47 ohm and 470 ohm resistors in parallel.
2. Repeat the measurements described in Case 1.
3. Note: You should get VERY different current values. If your values are close to the same, you
have done something wrong.
4. Turn off the power supply
IV.
Conclusion
1. Answer the questions at the bottom. The questions are very straightforward and your data should
support your answers.
2. Save your data file in dropbox (either in the morning or afternoon folder). The file name should
be S[Section]S[Station] Lab 1 EField.xls, where [Section] is 1 for the morning and 2 for the
afternoon and [Station] is your station number. Make sure that the names of all of the members
of your group are listed in case 1.
3. Please note that dropbox does not always work. Verify that the instructor has your lab report
before returning your equipment.
4. If you are in the morning class, log off your computer and turn off your monitor. If you are in the
afternoon class, shut down your computer and turn off your monitor.
5. Leave.
V.
Troubleshooting
1. No current measurement: If you get a current on the unfused input but *not* on the fused input,
check to make sure you have the dial set to 200 mA. If it’s still 0, you probably blew a fuse.
2. Current measurement always reads 1: You probably set the dial to 20 mA or 2 mA. It doesn’t
harm the multimeter, but it’s over the limit.
3. Current doesn’t change: Check the HOLD button.
4. No readings at all: The battery on the multimeter is probably dead. Bring it to the lab instructor
or lab supervisor and get a new one. Do not put it back with the rest.
5. No voltage: If you do not read any voltage (or it seems very different from the power supply
voltage):
A) Check that the HOLD button is not set on the multimeter. This will also show up on the
multimeter display.
B) Verify that there is a voltage across both resistors (Vtot).
a. If there is a voltage, think about if the voltage makes sense. Vtot should be about the
same as the power supply voltage.
b. If there is no voltage, measure the voltage across the power supply terminals. If there
is no voltage (and it is plugged in, powered on, and set to 6 to 8 Volts), contact the
instructor. If there is a voltage across the power supply terminals but not the
resistors, you probably have not wired it up properly (usually the case) or one of your
cables is broken (very rare).