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Using LabPro for Ohm’s Law
by Jim Sizemore
Contents
Introduction ..................................................................................................................................... 1
Objectives ....................................................................................................................................... 1
Equipment ....................................................................................................................................... 1
Ohm’s Law Measurement ............................................................................................................... 1
Making Current the x-axis ........................................................................................................ 2
Results ............................................................................................................................................. 4
Conclusion ...................................................................................................................................... 4
List of Figures ................................................................................................................................. 4
List of Links .................................................................................................................................... 5
Introduction
Using the LabPro system (LabPro device and LoggerPro software) we will investigate
Ohm’s law. The new information in this topic compared to the Vernier LabPro System Basic
Setup and Operation and RC Time Constant and Energy Stored in Capacitor is making the xaxis another column besides time. We will also observe the same limitations in the current probe
as observed in the RC Time Constant and Energy Stored in Capacitor experiment.
Objectives



Set up LabPro to measure current and voltage of characteristics of a resistor
Making a column besides time the x-axis
Observe linearity of voltage vs. current relationship thus establishing Ohm’s law
Equipment
LabPro system, LabPro voltage probe, LabPro current probe, electrical breadboard, electrical
leads as required, resistors, electrical power supply, SPST switch, DMM.
Ohm’s Law Measurement
1. Connect voltage and current probes to the LabPro (refer to Vernier LabPro System Basic
Setup and Operation document for instructions how to plug in, turn on and calibrate LabPro
system).
2. Use the DMM to measure actual values of resistors.
3. Construct an electrical circuit according to the schematic shown in Figure 1 as follows.
LabPro’s current probe is rated at ±5 V, therefore, do not exceed 5 V on the power supply.
LabPro’s voltage probe is rated for ±5 V and, therefore, is not a concern.
A
<5 V
Variety of
resistors
V
The voltmeter and ammeter are
the probes connected to the
LabPro system.
Figure 1 – Schematic for discharging capacitor.
4. Set up Experiment > Data Collection at an appropriate value. I used 15 seconds, 250
samples per second as described in Vernier LabPro System Basic Setup and Operation.
5. Set power supply to ±5 V, start LabPro data collection and then turn power supply down to
zero volts.
Making Current the x-axis
6. Go to Graph Options dialog discussed in Vernier LabPro System Basic Setup and Operation.
7. Under x-axis select Current as shown in the next figure.
Figure 2 – Select current as x-axis
8. Select range of data to fit curve to.
9. Navigate to Analyze > Curve Fit and obtain the following dialog box.
Using LabPro for Ohm’s Law
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Figure 3 – Select proportional curve fitting.
10. Use the proportional curve fitting to obtain the following:
Figure 4 – Ohm’s law curve fit for 100 resistor
Using LabPro for Ohm’s Law
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11. Several resistors were measured and I won’t display pictures for all of them. However it is
useful to reveal the problems with the current probe for at least one high resistance (low
current) value. Shown next is the data for the 5600 resistor. At this point the data becomes
quite unreliable. Apparently the current probe does well down to about 10mA, but lower
than this can create problems.
Figure 5 – Data for 5600 resistor
Results
Tabulated next are the results for resistance measurements.
Nominal Resistance ()
100
470
5600
8200
LabPro Measured value ()
100.9
470.9
4474
5898
Due to the limitations of the current probe, the LabPro measurements at, I estimate,
1000 and higher resistances are unreliable.
Conclusion
This concludes the Ohm’s law experiment. It’s relatively simple, however has the added
complication of setting the x-axis to something other than time. I also recommend, due to
limitations of the current probe, to keep resistance values below 1000. As pointed out in the
report on the RC time constant, it may be advisable to purchase a differential voltage probe and
insert various shunt resistors to serve as a current probe.
List of Figures
Figure 1 – Schematic for discharging capacitor. ............................................................................ 2
Figure 2 – Select current as x-axis .................................................................................................. 2
Using LabPro for Ohm’s Law
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Figure 3 – Select proportional curve fitting. ................................................................................... 3
Figure 4 – Ohm’s law curve fit for 100 resistor .......................................................................... 3
Figure 5 – Data for 5600 resistor ................................................................................................. 4
List of Links
1.
2.
3.
4.
5.
Vernier Web Site
LabPro User’s Manual (large, 6.5 Mb file)
LabPro Technical Manual (405 kb)
Basic LabPro Setup and Operation
RC Time Constant and Energy Stored in Capacitor
Using LabPro for Ohm’s Law
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