Ohm’s Law, Alternating current, and Oscilloscopes
In this lab, we will try to verify whether the voltage across a resistor is directly proportional to the
current passing through it. Components that obey this relationship are called Ohmic resistors.
We’ll do this using AC current in the resistor. The RMS current will be measured using a digital
meter. The RMS voltage across the resistor will be measured using an oscilloscope.
We’ll also learn how to measure frequencies with an oscilloscope.
1. Looking at a simple AC circuit with an oscilloscope.
Draw the circuit in the space below.
Your instructor will explain the details of ...
 Resistors and the resistor color code. Black = 0, Brown = 1, Red = 2, Orange = 3.
Tolerances are 5% (gold) or 10% (silver).
 The protoboards. It’s important to understand how the holes in the board are electrically
connected. A good habit is to use the bottom ‘rail’ as the ground (ie. zero voltage).
 The AC signal generator: how to adjust the amplitude; how to adjust the frequency;
how to pick a regular AC signal, or other wave shapes
 Connecting the AC generator into the circuit using the coaxial cable. The woven
metallic sleeve of the cable is the ‘ground’ and should be electrically connected to the
side of the double banana connector with the little tab.
 The oscilloscope. It’s basically a voltmeter, with voltages represented by the up/down
direction on the screen. It shows voltage changes over time, by representing time
increasing to the right across the screen. The knobs and dials have three sections:
Vertical, Horizontal, and Trigger.
Your instructor will have you try various settings on the signal generator and look at the
voltage across the resistor with the oscilloscope. Since the oscilloscope is only a measuring
instrument, it never changes the frequency or amplitude of any voltages in the circuit.
Adjusting the oscilloscope dials only rescales or repositions the trace on the screen.
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2. Measuring rms currents through, and voltages across, resistors
In this part, we pick various settings for the amplitude of the AC signal. For each, we measure
the current using the ammeter and we measure the voltage across the resistor using the
oscilloscope. We then plot the voltage versus the current. Ohmic resistors always produce a
straight line for this, and the slope is the resistance R of the resistor.
Notes:
 The instructor will show you how to connect the digital ammeter into the circuit. The
current needs to enter at the
socket, and leave from the COM socket. You need to
set the dial to the mA position, and to press the leftmost button below the screen until it
reads ‘AC.’ The reading is the RMS current, which is
√
where is the maximum positive value of the current in the AC cycle.
 We will measure the peak-to-peak voltage
using the oscilloscope. Then we obtain the RMS voltage from
√
 An important general principle with oscilloscopes is to always adjust the scale setting
until the measured quantity is as large as possible on the screen. This gives the best
measurement precision.
 The independently-varied quantity is the amplitude of the signal from the function
generator. Pick five evenly-spaced settings of the amplitude dial, ranging from about 12
o’clock to the maximum. Make sure this amplitude stays fixed while you measure the
RMS current (with the multimeter) and the RMS voltage (with the oscilloscope).
Plots: Do two plots on the same axes, using Excel. Do the first with
=1000 , and the second
using the two resistors in series. For each plot, you will have five data points. Show the slope of
each graph using the trendline options.
Table: Record your original data by hand on the cover-sheet table. Duplicate this table with Excel
and use it to produce the graph. Remember that points are allocated for observing the presentation
guidelines for tables and graphs. Note about units: 1 Ω = 1 V/A.
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3. Voltage measurements with the oscilloscope:
Voltage differences are measured vertically on the
oscilloscope screen. The voltage scale is set by the ‘Volts/Div’
dial for each channel. Make sure the inner button labeled
‘CAL’ is fully turned to the right so that it is calibrated.
Before making a measurement, click the outer dial as far to
the right as possible so that the measured quantity is as large
as possible on the oscilloscope screen.
The measurement involves reading the number of divisions on
the screen and multiplying by the volts/division scale for the
channel.
In the diagram, the peak-to-peak vertical extent of the CH1 signal is 6.2 divisions on the
screen. To get the peak-to-peak voltage, you need to read off the CH1 Volts/Div setting.
Suppose it’s
. Then,
(
)
(
)
4. Period measurements with the oscilloscope
Elapsed time is measured in the horizontal direction on an
oscilloscope screen. The horizontal time scale is set by the
timebase dial, labeled ‘Sec/Div.’ Make sure the inner dial is in the
rightmost calibrated ‘CAL’ position. Before making a
measurement, click the timebase dial as far to the right as possible
to spread the measured signal over as much of the screen as
possible. This improves the precision of the measurement.
The measurement is made by reading the number of divisions in
the horizontal direction and multiplying by the Sec/Div scale.
In the diagram, the period
the timebase dial is set at
of one full cycle of the AC signal covers 5.5 Divisions. Suppose
. Then the period of the signal is:
(
)
(
)
5. Frequency measurements
You can get an approximate frequency setting by using the dial on the function generator, but
to get it accurately, you need something better – like the oscilloscope. The instructor will
discuss the details of how to set up the timebase scale to optimize the measurement precision
of the period of the oscillation. Then, you can find the frequency in Hz units using
Use the table provided on the cover sheet to record your original data by hand. Find the
frequency accurately for the four approximate settings shown in the table.
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6. A basic description of how the oscilloscope works
The inner surface of the oscilloscope screen gives off a bright greenish glow at any point where the
thin beam of electrons hits it. The electron beam starts from the heated filament, from which
negatively charged electrons are ejected. They are accelerated by the electric field between the two
circular plates of the electron gun, and pass through the small hole in the positive plate. On their
way to the screen, their direction is altered by the deflection plates. The sweeping left-to right
motion of the trace is accomplished by creating an increasing potential difference across the
plates. The timebase knob controls this process. The input signal being measured is connected to
channel 1 or 2, and goes to the electrodes. The larger the input voltage, the further the vertical
deflection of the trace.
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Ohm’s Law, AC current and Oscilloscopes
partner:
Voltage versus current plot
Color codes:
__________________
___________________
p-p divisions
volts/div
V p-p (V)
V rms (V)
R1+R2
R1
Irms (mA)
Name:
Slopes found on your graphs: :
__________________
___________________
Period and Frequency: In the last two columns, include the correct units in each entry.
Approx. generator
frequency
Divisions
Sec / Div
Period T
Frequency f
~ 580 Hz
~ 4.5 kHz
~ 13 kHz
~ 55 kHz
Hand-in list:
i) This cover sheet, with the tables completed by hand
ii) Your Excel table, duplicating the table at the top of this sheet
ii) Your Excel graph with the two plots on the same axes. Show the trendline equation for each plot.
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