ECEN 1400
Lab 1
Resistors
ECEN 1400, Introduction to Analog and Digital Electronics Lab 1: Resistors 1 INTRODUCTION
The purpose of this lab is to learn the basics of resistive electrical circuits and how to
work with them in the lab. You will learn to create simple circuits and characterize them
with bench-top instruments. These laboratory experiments will parallel the simulations
that are part of the associated homework.
BEFORE LAB: Do the homework before lab on Friday and turn it in during
lab. This material is designed to prepare you for the lab, so it will help to get
the lab write up and read it before doing the homework to get an idea of what
is coming.
DURING LAB: As you proceed with this lab, you should make notes to be
included in your lab write up. All your notes, observations, circuit diagrams,
measurements and calculations should be made in this write up. Also, there
may be questions that are asked throughout a lab text. You should answer
these questions directly in your lab write up. Work neatly and complete as
much of the write up as you can during lab to minimize the amount of work
later.
For this lab, you may work in groups of two, or if there are enough lab
stations available, you may work alone. Throughout the semester, do not be
afraid to ask your TA a question or ask a group at another lab station for some
help. This does not mean you should copy lab results from another group. It
does mean that it is perfectly reasonable to ask someone else if your circuit
looks like it is properly wired. Or, to have someone check that you have
properly adjusted the settings on a piece of equipment.
AFTER LAB: Analyze, plot, synthesize or otherwise massage your data. Make
conclusions.
For your convenience, questions and lab procedures are introduced in a unique color.
2 COMPONENTS AND TOOLS REQUIRED
• From your kit:
o Breadboard
o Wires
o Wire-cutter and pliers
• On the lab bench:
o Variable DC power supply
o Bench-top multimeter
• From the TA:
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o Packet of three resistors
o Hand-held multimeter (checked out and returned to TA)
3 A GUIDED TOUR OF YOUR LAB STATION
You will be performing all of your work at a lab station, shown in Figure 1. Each station
includes several pieces of instrumentation, a signal generator and several sources of
power. At the bottom of the picture, sitting on the bench, are a variable AC power supply
and a variable DC power supply. Above these is a shelf that holds a signal generator, a
multimeter and an oscilloscope. For this lab we will concentrate on using the variable
DC power supply and the two multimeters. We will save the use of the oscilloscope, the
signal generator and variable AC power supply for future labs.
Figure 1: A lab station
3.1 DC POWER SUPPLY
Find your DC power supply, as shown in Figure 2. “DC” here stands for Direct Current
which means that the quantities provided by this instrument do not vary in time. The
opposite of this is “AC” which means alternating current. In the jargon of electrical
engineering, we will use these phrases even if we are talking about a voltage or a power
(that is, not just for current).
This power supply is capable of supplying a voltage in three ranges, 0 to +6V, 0 to +25V,
and -25 to 0V. The range of voltage you use depends on the circuit you are building. For
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this lab, we will be using the 0-6V range. As you might guess, for this range of voltages,
you would connect your circuit to the red terminal labeled “+6V” and the black terminal
labeled “COM”. Remember that voltages are potential energies and thus must be
relative to some other potential, just like altitude must be relative (e.g. to sea level). That
reference potential is, in this case, “COM”. So as you adjust the instrument, the red
terminal is between 0 and 6V higher in potential than the “COM” terminal. Note that
this “COM” potential is “floating” – it is not constrained. Sometimes we want to insure
that we know exactly what “COM” is and will thus short “COM” to “ground”, which is
the green terminal. This is connected to the case of the instrument and the third (offcenter) post of the wall plug which, in turn, is connected to the earth. In this lab, do not
connect anything to the green terminal.
Figure 2. Agilent variable DC power supply.
The voltage is then adjusted using the knob labeled with “+6V”. The display of the power
supply shows the output voltage and the amount of current being pulled from the power
supply. The pushbuttons just below the display allow you to adjust the range of voltages
shown on the display. Although the display shows the current being supplied in Amps, it
does not have the precision we will want in our measurements. So, we will often use a
multimeter (described below) to make more accurate current measurements.
An important note about all power supplies: What would happen if you connected a
short (or very low resistance) load to this supply? This is a voltage source, so the
instrument would try to supply as much current as needed to maintain the requested
voltage – this can be dangerous to you and your circuit. Thus, our DC power supplies
have what is called a current-limit. This is simply an upper limit on the maximum current
the meter will supply. When using the 0 to 6V range, this maximum is 5.0A. The current
limit for the 0-25V range is 1.0A.
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Question 1: Why do you suppose the current limit is lower in the second case?
3.2 THE MULTIMETERS
Multimeters are to electrical engineers as hammers are to carpenters – you will use this
instrument a lot. The first multimeter we will consider is one that you can check-out
from your TA for the lab period (see Figure 3). We will refer to this multimeter as the
handheld or portable multimeter since it is designed to be both handheld and portable.
This multimeter is not as sophisticated as the HP bench top multimeter (see Figure 4), but
is more convenient, particularly away from the bench.
Using a multimeter involves three simple steps. Do them in this order until you are
comfortable with the tools.
1. CHOOSE THE QUANTITY TO MEASURE: These multimeters have the
capability of measuring: 1) an AC or DC voltage, 2) an AC or DC current, 3)
a resistance and 4) the directionality of a diode. Before using either of the
meters you must decide what you want to measure and put the meter in a
mode that makes that measurement using the knobs or buttons on the front
panel.
2. CHOOSE THE RANGE TO MEASURE: The meter will amplify and display
your quantity with an appropriate number of digits. In order to do this, it
needs to know the range of the signal (10 mV or 2 kV?). You can damage the
meter if you try to measure a kilovolt while the meter is on the millivolt
setting. So, if you are not sure, chose a large range from the front panel, then
decrease the range once you are connected to get sufficient accuracy. If you
decrease the range so that it is smaller than the meter is set for, the meter will
give you an “out of range” error.
3. CONNECT THE TEST LEADS: First connect the leads to the appropriate
meter ports and then to your circuit. The meter ports are labeled both with
quantity type (V, A, or Ω) and range. If you are measuring 5 amps, do not
attach the probe to the port labeled mA.
Note. The equipment and meters in the lab, particularly the HP multimeters and
oscilloscopes, are expensive and delicate instruments. In some cases, they cannot be
replaced. Please be careful with them. One way to destroy a multimeter is to incorrectly
set the range. While the internal circuitry provides some protection from mistakes, it is
not foolproof. If a range is set to measure milli-amps, but you wire a circuit that uses 1
amp, it is possible to burn-out the meter. So, please check your range settings before
turning on the power. Most of this equipment is protected by fuses and we expect to blow
a few fuses during the semester. People who are blowing fuses on a weekly basis can
expect to start getting charged for their replacement. Later in this lab, we will warn you
about another common way to burn out a multimeter.
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3.2.1 THE HANDHELD MULTIMETER
This must be checked out from the TA and returned at the end of the lab period. You will
not need it every lab, but there are occasions where you need a second independent
measurement.
To use the meter, first turn it on. This is done simply by turning the dial to the type of
reading you wish to make. When the meter is turned on, the display should light up. With
this multimeter, and in general, do not power-up a circuit until you have wired it up and
put the multimeter in the proper mode to avoid damage to the meter.
Figure 3. Handheld or portable multimeter
At the very bottom of the meter are places to plug in banana plugs from your circuit. The
center jack, labeled “COM”, is used for all measurements. The jack on the right is used
for making all voltage measurements. For voltage measurements, the meter will
automatically adjust the displayed range to provide the most precise reading.
For current measurements, the two jacks on the left are used. If the measured current is
below 320mA, then use the upper jack; if it is above 320mA, use the lower jack. You
must also adjust the dial for the range of current you are measuring. If you are not sure
about how large a current you will measure, always start by assuming the largest value
and then work your way down.
Note, again: When wiring a circuit, you should wire-up and adjust the mode of the
multimeter, before turning on the power to your circuit. Generally, think when you are
using equipment – get the instruments ready before applying power (pretty obvious, no?).
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A simple measurement
• Turn on the DC power supply. Make sure the output voltage is set to 0 volts.
Check and record the current limit. This limits damage to you or your
instruments in case of an accidental short circuit. Turn the supply off.
• Wire the power supply output to the multimeter to measure voltage. The “COM”
terminal on the power supply should be connected to the “COM” jack of the
handheld multimeter. The “6V” terminal should be connected to the voltage input
of the meter. COMMON MISTAKE: Use the correct terminals for a voltage
measurement on the multimeter. Ask if you are not sure!
• Turn on the multimeter and then the supply.
• Turn the “Voltage” knob on the power supply and check that the meter reading
corresponds to the gauge on the power supply. Play with the scales on the
multimeter to get more accurate readings for low voltages.
Question 2: What happens when you switch the connections, so that the “6V” terminal is
connected to “COM” on the multimeter and the “COM” of the power supply is connected
to the “V” of the multimeter? Think about this and predict the result, then do the
experiment. Explain.
3.2.2 THE BENCH TOP MULTIMETER
The HP multimeter is more sophisticated than the handheld one. Nonetheless, it still
requires that a user make several adjustments to make measurements without destroying
the meter.
The buttons on this meter are totally different from the handheld multimeter and there are
more places to plug things in. Fortunately, at this point we only need to use a few of the
buttons. Turn on the meter. You will notice that it takes a few seconds for the display to
clear. Press the “DC V” button. This sets the meter to measure DC voltage. To measure
DC current you need to press the “Shift” button, the blue one, followed by the “DC V”
button. You can see that just above the “DC V” button there is a “DC I” label in light
blue. Any other functions that are labeled in light blue can be invoked by pressing the
“Shift” button followed by the appropriate button below the label. Try putting the meter
in “DC I” mode and then switch back to “DC V” mode.
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Figure 4. Bench top multimeter
To adjust range, the “_” and “^” buttons are used. If you press these buttons you will see
the display change. Either the location of the decimal point will shift, or the “DCV” will
change to “mDCV”, that is, from DC volts to DC milli-volts.
As noted earlier, there are more places to plug into with this multimeter. First make sure
the button marked “Terminals” is sticking out. This indicates that connections will be
made from the front panel of the multimeter. For making voltage and current
measurements, we will be using the connections in the right-hand column. The hole in the
middle, labeled “LO”, is equivalent to the one labeled “Common” on the handheld
multimeter. You should connect the “-” terminal of the power supply to this connection.
For voltage measurements, the upper connection, marked “HI” is used. For current
measurements, the lower connection, marked “I” is used.
Another simple measurement
Perform the same measurements with the bench top multimeter as you did with the
handheld.
4 YOUR FIRST LITTLE CIRCUIT
Before starting this lab, you should have received a packet of three resistors from your
TA. Enter the number of your packet into your lab write up. (Each packet contains a
slightly different set of resistors.)
For the first several weeks of the semester, you will be building circuits on what is called
a breadboard. The breadboard in your kit is the white board with holes in it. Before you
can use the breadboard, you have to attach the three terminal posts to the board. These are
found taped to the underside of the breadboard. Orient the breadboard so the holes for the
terminal posts are at the top. In this position, it is most convenient to put the green
terminal to right of the black and red ones.
If you look at the breadboard with the terminal posts at the top, notice the pattern of holes
in the board. There are two rows of holes at the top and two rows of holes at the bottom.
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In each row the holes are in groups of 5. Despite this grouping, all the holes in a row are
connected inside the board. However, in the middle of the board there is a break, which
indicates that the left half of a row is not connected to the right half of a row. The way
these connections work is shown in Figure 5. In the center of the board, each column of
5 holes is connected. The horizontal space between the columns is for placement of chips
and other components. The holes within are column are used to connect to other columns
to wire together various components.
Figure 5. Breadboard connections. Lines indicate holes that are connected. COMMON MISTAKE:
placing components vertically in one of the rows above such that the component is actually shorted.
Figure 6. Correct way to place a simple component such as a resistor.
The rows of holes at the top are typically used to provide power and ground to your
circuit components. So for example, you may decide to use the second row across the top
to provide the positive voltage to your circuit. You would screw a wire to the red terminal
post and then plug the wire into a hole in the row. Since all the holes are connected, you
can tap into this power with other wires. You would then use a cable with banana plugs to
connect from your breadboard to the power supply. To wire your circuit to ground, or the
lower voltage, the “-” on the power supply, you would use a similar approach, but use a
different horizontal row. Remember, you need to use a jumper at the center to connect the
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two halves a row if you expect to use power from the second half of a row. Forgetting to
include this jumper is a common mistake.
Create the circuit
For this part of the lab, select one of the resistors from your packet and, using your
breadboard, wire the following circuit:
Figure 7. First simple circuit.
•
•
•
Before you begin wiring, turn on the power supply and make sure that the voltage
is turned down to 0 volts. Then turn the power supply off until you have finished
your wiring.
Wire the circuit on your breadboard and connect the two multimeters using one
for voltage and one for current.
Before you make any connections to the power supply, make sure the power
supply is turned off. Double-check that your wiring is correct before turning on
the power. An incorrectly wired circuit usually destroys a part or component and
can cause damage to a power supply or measurement instrument. Even though
there are fuses in this equipment, sometimes by the time the fuse has blown, the
damage is already done.
Warning. One of the easiest ways to destroy a multimeter is to set it to measure current
and then connect it in parallel with a power supply. This can easily happen if you wire a
multimeter to a circuit to read a voltage, but accidently wire it to the input for current. Or
you might have just mis-wired the meter into your circuit. Please double check that you
have modes and wiring set properly!
•
•
Once you have the circuit wired, and have double checked that it is properly
wired, turn on the power supply. Now make measurement of the current flowing
through the resistor at 1, 2, 3, 4 and 5 volts. Make a table in your notebook that
records the voltage and current for your circuit. (If your current values are not
changing, double check that you have turned-up the current on the DC power
supply.)
Plot a curve of your data with voltage on the horizontal axis and current on the
vertical axis. Fit a line to this data (excel does this).
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Question 3: Based on your data, what is the value of this resistor?
Check your results
Your measurements of the I/V curve of Lab 1, part A are a rather tedious way to establish
the resistance of an unknown resistor. You may have noticed, the resistors that you were
given last week have a series of colored bands on them. In fact, most small resistors have
such markings. These markings are used to specify the value of a resistor. The key for
interpreting how these bands are read is shown in Figure 8.
Figure 8. Resistor code meanings.
The first three bands specify the resistor value. The fourth band is used to specify the
tolerance, or precision with which the resistor was made. The first two bands are labeled
d1 and d2. They represent the first two digits of the resistor value and there is an assumed
decimal point after the second digit. The third digit is called the multiplier and depending
how you choose to remember its encoding, it can be thought of in two ways. Whichever
way you think of it, the value for the resistance can be written as:
R = d1d 2 .0 × m = d1d 2 .0 × 10 d3
The different colors used on the bands represent different decimal digits. The mapping of
colors to digits is shown in Table 1. Notice that the multiplier column simply corresponds
to the value of m=10d3 for the given color-digit encoding. So, if you memorize the color
and corresponding digit, you can determine the value of most resistors. The only colors
for which things don’t work in this way are gold and silver. For these two colors, you just
have to know that the multipliers are 0.1 and 0.01.
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Table 1. Resistor color codes and standard values.
The fourth band, if it appears on a resistor, corresponds to a tolerance value. As can be
seen in the table, there are three possible values for tolerance. If there is no band, the
tolerance is ±20%. A silver or gold band specifies the tolerance as given in the table. A
complete example is shown in Figure 9.
Figure 9. Example resistor with colors, from left to right, of d1 = Brown = 1, d2 = Black = 0, d3 =
Yellow = 4 and tolerance = Gold = 5%. The resistor is thus a 10.×10,000 = 100KΩ ± 5%. Image
source: Wikipedia.
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The reason for these tolerance values is that not all resistors are produced with the same
care, which translates to expense. The actual resistance value can vary from the one
specified by the first three bands. If the fourth band is silver, for example, the
manufacturer is telling the user that the resistor may vary from its specified value by
±10%. Depending on your application you may want to use a more or less precise value
for a resistor. As you go from no color to silver to gold, resistors get more expensive
because their value is more carefully controlled.
If you get confused about which end of the resistor has the first band, there are a couple
observations that can help you decide. First, if there are four bands, then the right-most
one will be silver or gold. Although a silver or gold band can be a multiplier, silver or
gold is never used as the first digit of a resistor value. Second, if there are only three
bands, the bands will clearly be closer to one end or the other of the resistor. In this case,
the band closest to the wire coming out of the resistor will be the first digit.
Read your resistor color codes (10 points). Given this background on bands, check the
bands on the three resistors you were given last week. Write down the color of the bands,
and then using the tables above, give the resistor value. Is there a difference between the
specified values and the ones you computed from last week’s lab? Are the values you
computed within the tolerances specified by the manufacturer?
What if you’re colorblind? Measure it!
Sometimes the colors of the bands are difficult to read. In addition, the shades of the
colors will vary from one manufacturer to another. There is another relatively simple way
to determine the value of a resistor. You may have noticed that all of the multimeters that
we used last week have an Ω symbol at the outlets where the banana plugs are inserted
and on the buttons. This is because the multimeters can be used to determine a resistance
value. To make a resistance measurement, you should wire the resistor to a multimeter as
though you were making a voltage measurement. You do not use the power supply to
make this measurement. Notice that at the jacks where you were plugging in cables to
make voltage measurements, an Ω symbol is also shown.
Once you have connected the resistor to the multimeter, put the multimeter into a mode
that displays resistance. For the HP multimeters, find and press the buttons that are
marked with an Ω. You should also set an appropriate range for the resistance value. For
the hand-held multimeter, move the dial to the position marked with an Ω. Again, note
that you do not need a power supply for this measurement – the meter provides the
required power.
Measure your actual resistances (5 points). Using one of the multimeters, measure the
resistance value of each of the three resistors from last week’s lab. How well do the
measured values agree with the ones you computed last week? How different are the
measured values from the specified values given by the color codes? Are they within
tolerance?
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5 EXTRA CREDIT: CREATE A VOLTAGE DIVIDER
At this point you should have determined the resistance values of the three resistors. They
should be three different values. Let’s call these resistors Ra, Rb and Rc from smallest to
largest resistance.
• Using these resistors, build the circuit shown in Figure 10.
• As before, you should vary the voltage and make measurements of the current at
1V increments from 1V-5V.
• Confirm the measured voltage “V” by computing the equivalent resistance of
resistors Rb and Rc.
• For a voltage value of 5 volts at the DC supply, compute the current that you
expect to see flowing through Rb. Note: the current through Rb is not the same as
the current flowing through Ra. Show all your work.
Question 4: Do your measurements confirm this current flow?
Figure 10. Second circuit.
6 EXTRA CREDIT: RESISTANCE OF AMMETERS AND VOLTMETERS
Voltmeters are placed across the component whose voltage you want to measure (that is,
in parallel with the component) to measure the potential difference of the two sides of the
component. Ammeters are placed before or after the component you want to measure
(that is, in series with the component) to measure the current going through the
component. It turns out, each of the meter types has an associated resistance. Given that
the use of either meter should not change the voltages or currents in the circuit it is
measuring, what is the ideal resistance (really low, doesn’t matter, really high) of an
ammeter and a voltmeter and why? Carefully use the handheld meter to measure the
bench-top meter and confirm.
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