Download Title: Basic Electronic Instruments

University of Puget Sound Introductory Physics Laboratory
3. Using basic electronic instruments
Name:____________________
Date:___________________
Objectives
1. To become familiar with the concepts of voltage, current, and resistance.
2. To gain a working knowledge of digital multimeters, power supplies, and
function generators.
Equipment
Digital multimeter, function generator, power supply, and a decade
resistance box.
Helpful reading
Hecht Chapters 16 an 17 or Rex/Jackson Chapter 21 provide an introduction
to potential difference, current, and resistance.
Background: Voltage
Everyone has pedaled their bike up a hill and coasted back down. In
Physics 121 you developed skills for analyzing motion in a gravitational field
based on energy concepts. The gravitational system provides a useful analogue
for electrical circuits, where the situation might not be so familiar.
If you carry an object of mass m from a height h1 up to a height h2, you
must add energy equal to the gravitational potential energy difference of mg(h2 h1). If you carried up a mass M instead, the potential energy difference would of
course just be Mg(h2-h1). The potential energy difference per unit mass is just g(h2h1). The quantity gh is the gravitational equivalent of a voltage. In an electrical
circuit it is charge that matters, not mass. The voltage difference between two
places in an electrical circuit is just the potential energy difference per unit charge.
So if a charge q moves from where the voltage is V1 to a place where the voltage
is V2, it costs an energy equal to the potential energy difference q(V2-V1). Voltage
is measured in Volts, defined as a Joule per Coulomb.
Note that only differences in voltage really matter, just like only
differences in gravitational potential matter. There is no absolute reference
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voltage, just like there is no absolute reference height on the surface of the earth.
You may decide to set up a convenient reference point like sea level. Then to
measure the height of a mountain you could measure the height of the base of
the mountain above sea level and the height of the top of the mountain above sea
level, and take the difference. Or you could forget about sea level and measure
the height of the mountain above its base level. In either case the measurements
you make are between two points. In this respect a voltmeter works like a ruler:
one end of the instrument goes at one point and the other end goes at another
point. Sea level in an electrical circuit is called ground.
Current
Imagine a stream flowing down a hill. How would you describe how
much water is flowing? You might say how many gallons per second were
passing by any particular point on the bank. Or you could say how many
kilograms pass by per second. In an electrical circuit we call the amount of
charge per second that flows through a wire the current. Current is measured in
Amperes (or amps for short), defined as a Coulomb per second. Note that the
water current in a stream or the electrical current in a wire is the same
everywhere, as long as the water or charge isn't pooling up somewhere. (The
electrical equivalent of a pool is a capacitor, which we will be studying later in
the term.)
How could you measure how much water was flowing in the stream?
You could imagine that some kind of paddle wheel device could be made to
measure the flow in terms of how fast the water made the paddle wheel turn.
Commercial devices that do the job are called flowmeters. To measure the total
flow rate, you'd have to make sure all of the water in the stream went through
the flowmeter. An ammeter is built differently, but it works like a flow meter for
electrical current.
Resistance
Now imagine letting water flow from the top of a hill to the bottom of a
hill through a hose. How many gallons per second would flow through the
hose? Well, it depends on (1) the height of the hill and (2) the length and
diameter of the hose. An electrical object that works like this hose is called a
resistor. The current that flows through the resistor is proportional to the
voltage difference across the ends of the resistor. Double the voltage difference
and the current doubles; halve the voltage difference and the current halves. The
proportionality constant is called the resistance. This relationship is called
Ohm's law:
V2-V1 = I R
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where V2-V1 is the voltage difference across the resistor. Resistance is measured
in Ohms (symbol ). An ohm is defined as a volt per ampere.
Exercises
A digital multimeter (DMM) is a modern device that can measure voltage
differences, current, and resistance. Check yours out. It has an LCD display that
dims after awhile if you forget to turn it off, to save on batteries (which is nice,
believe me!). You can pick different modes of operation, for DC and AC volts,
DC and AC amps, and for Ohms. DC stands for direct current, AC stands for
alternating current. But they are used to describe voltage, too. They really
describe whether the quantity you are measuring (voltage or current) is constant
in time (DC) or varying in time (AC).
Measurements of volts, amps, or ohms all involve using two wires
coming out of the DMM. But you have to be careful to attach them to your
circuit correctly, or what you measure won't be what you think you are
measuring. In your first exercise you will put the DMM through its paces and
check out Ohm's law.
Resistance measurements
Check out the decade resistance box. Inside there are banks of resistors
that you can connect together using the switches. The switches are arranged by
powers of ten. Figure out how you think it works and set up a resistance of
11,205 ohms. To check it, measure the resistance using your DMM. Put your
DMM in Ohms mode. Plug in the black and red probe wires in the V- and
common plug holes (not the amps plug hole). At the other end of the wires are
the probes. Connect the red and the black probe leads together. What resistance
do you measure? Where do you think this resistance comes from? This is the
smallest resistance you can measure with this device. Now make your resistance
measurement. Does it agree with your setting on the decade switches, within
reason? Dial in a small resistance. How does the fractional error compare for
small and large resistances?
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DC Ohm's law measurement
Check out your power supply (it's the black box). It has both DC and AC
outputs. With the power supply off, connect the DC output of the power supply
to the decade resistance box as shown below:
Power
Supply
Decade
Resistance
Box
Don't turn on the power yet. What value of resistance is dialed in on your
decade box? What would happen if you put a large voltage across a small
resistor? If at any point you smell this happening, kill the power on your supply
as quickly as you can. But with your help we can avoid frying the smaller
resistors on the decade boxes. Start out by setting the decade box at 100 ohms.
OK, now you are set to drive some current through the resistor, but you
aren't set up to measure anything. Think about how to go about measuring
current and voltage. For current, we want all the current that goes through the
resistor to also go through a DMM. The DMM gets inserted into the circuit just
like the resistor. We say the resistor and the DMM are in series. Serial processing
is like ants marching in single file - the electrons have to first go through the
DMM and then though the resistor (or the other way around, the order doesn't
matter). To measure voltage, you want to put one probe on one side of the
resistor and the other probe on the other side of the resistor (like using a ruler).
We say this DMM and the resistor are in parallel. Parallel processing allows the
electrons to take two different routes, either through the resistor or the DMM.
(To avoid disturbing the flow through the resistor, the DMM has a very high
internal resistance so not much current flows through it.)
Drawing detailed sketches of actual circuits is tedious. It is convenient to
draw circuit diagrams (or schematics) to represent the circuits that you build and
test. There are symbols for every electrical component you can imagine (many of
which you will never see or use). Wires are shown as lines. Below are the
symbols for a battery, a power supply, and a resistor. To represent a device like
a DMM, you can simply draw a box and put a letter in it (like A for amps or V
for volts). In the following box, draw a schematic for your simple circuit with the
ammeter DMM and the voltmeter DMM in place.
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+
Battery
+
Power
Supply
Resistor
Do you and your lab mates agree you've got the circuit diagram set up
right? If you are not quite sure, get your instructor to check it out. Then wire up
your circuit. Remember that the probes for measuring volts and ohms get
plugged into different holes than for measuring current. When you are ready,
turn down the "volume" knob on the power supply and turn the unit on. Check
to make sure there is no voltage across or current through your resistor, and that
no smoke is coming out of the decade box. Then slowly turn up the power
supply until you see some current flow. Make 10 measurements of the current
and voltage covering the range from 0 to 0.1 amps. Repeat the measurement for
a 10 ohm resistor.
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trial 1: resistance= _______________
current
(amps)
trial 2: resistance = _______________
voltage
(volts)
current
(amps)
voltage
(volts)
Plot your results on graph paper or on the computer (and insert into your
notebook). Quote a mean resistance and determine and quote an experimental
uncertainty for each trial (see the appendix on quick and dirty statistics again, if
necessary). Ohm's law says that the resistance should be constant as the current
and voltage vary. What feature of your graph shows this? How does your
measurement compare to the resistance measurement on the DMM?
AC Ohm's law measurement
What would happen if you sloshed the current back and forth through the
resistor instead of setting up a steady current? It's not obvious, but resistors
don't care whether the flow is steady or changing. We can check this by using
the same kind of test circuit you have already built, except use a power supply
that puts out a time-varying voltage. This exercise will introduce you to the
function generator and the AC modes on the DMM.
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Look over your function generator. It is a device that makes a timevarying voltage, which we sometimes call a "waveform." Identify the knobs and
switches on your function generator that allow you to choose the shape of the
waveform, its frequency, and its amplitude.
To make the AC Ohm's law measurement, construct the same circuit as
before with two differences: (1) use the function generator in place of the power
supply, and (2) change your DMM's from DC mode to AC mode, for both
voltage and current. Using one of the same resistance values as before, repeat
the Ohm's law measurement using sine waves at about 100 Hz and at 10 kHz.
trial 1: frequency = _______________
current
(amps)
trial 2: frequency = _______________
voltage
(volts)
current
(amps)
voltage
(volts)
Graph both data sets and determine a value for the resistance. Calculate
the uncertainty in the measured values of the resistance. Do you get the same
value as before? Does the data look as good as the DC data set? Is Ohm's law
followed?
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Before you leave:
Show your instructor your data sets. Explain how you determined the
uncertainty in the measured values of the resistance.
Explain to your instructor how to use the DMM to measure voltage, and how to
use it to measure current. Hazard a guess on how the DMM measures resistance.
Before the next lab:
Type a report on the work that you did in this lab. Your report should be about
one page long and double spaced with two attached plots. The two plots to
include are the DC and AC current vs. voltage plots that you already made.
These can be drawn by hand or completed on the computer. Begin your report
with an introductory paragraph summarizing the conclusions in the body of the
report. The body of the report should describe the data that is used in the plots,
discuss what the plots show, and compare the results of the two plots including
uncertainties. End your report with a concluding paragraph. This report is due at
the start of next week’s lab. Be sure to proofread your writing for proper
grammar, punctuation, sentence structure, and general clarity.
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