ECE263L / Linear Circuit Theory I Laboratory
Laboratory #1 / Measuring Voltage and Current with a Multimeter
Objectives:
The objectives for this laboratory assignment are shown below. If you do not think you can
accomplish all three objectives by the end of the laboratory session, please see your lab
instructor.
1. Explain how to use a multimeter to measure DC voltage and current in a circuit.
2. Demonstrate how a multimeter can be used to measure DC voltages and currents in a circuit.
3. Use Cadence’s OrCAD software to simulate DC circuits and measure voltages and currents.
Background:
Multimeters (or digital multimeters often called DMMs) are some of the most common tools
used by electrical engineers and computer engineers to analyze and debug circuits. They are
particularly useful for measuring resistances, voltages, and currents.
Most ECE263 students have some experience using
multimeters to measure voltages and resistances.
However, most of our students have had limited
opportunities to measure currents with a multimeter.
In addition, it has been a couple months since
ECE100 and GE100 wrapped up, so we’ll make sure
everyone has all the info they need.
We recognized that multimeters can look
intimidating. They typically have a large screen, a
good sized dial, a couple buttons, and several holes on
their front. Plus, they are often covered with words,
numbers, lots of abbreviations, and maybe even a
couple icons. To make matter worse, there is no
standard as to how multimeters look, so we cannot
just show you one universal example. The multimeter
in Figure 1 probably looks like yours, but it is just an
example….
However, there is a lot of logic about how
multimeters are designed, and once you figure how to
use one multimeter, it is usually not hard to move to
another device with a different look-and-feel.
Figure 1. Example Multimeter
Measuring Resistance. Let’s begin by how you use a multimeter to measure resistance. To do
this, make sure your multimeter is turned on. Next, we need to tell the multimeter we want to
measure resistance (instead of voltage, current, or some other type of value). To do this, you
typically have to adjust the dial so that it is in the Ohmmeter range. On most (not all)
multimeters, look for the  symbol and a range of values that typically ranges from 20 to
20,000,000 (20M) or more.
The number shown is the largest resistance value the multimeter can measure at a particular
setting. For example, in Figure 2(a), the multimeter can measure values up to 200,000,000 (or
200M on the dial). As the multimeter dial is rotated to lower values of resistance (for example to
200k or even 2k), the measurement becomes more accurate. Therefore, the multimeter can
measure the smallest resistances with the most accuracy.
Next, make sure your cables are plugged in correctly. The black cable is always plugged into the
COMMON socket. For resistances, make sure your red cable is plugged into the socket labeled
with the  symbol (see Figure 2(b)).
Ω symbol
Figure 2(a). Ohmmeter Range on a Multimeter. Figure 2(b). Cable Locations to Measure
Resistance.
Finally, before you start measuring resistances, you need to remember to always measure
resistances when they are in isolation and not part of a larger circuit. As soon as you put the
resistor into a circuit, the multimeter may not be able to accurately measure its resistance. We
will see an example of this later on in the lab.
Measuring Voltage. Before you can measure a voltage, you need to first tell the multimeter
what type of voltage you want to measure: DC (constant voltages) or AC (alternating voltages).
Each type of voltage has its own range on the multimeter. Typically, they are indicated as V~
for AC voltages and V─ for DC voltages (see Figure 3).
In the example multimeter in Figure 3, we see that the DC voltmeter range is 200mVdc to
1000Vdc and the AC voltmeter range is 2Vac to 750Vac. Most of ECE263 will be spent looking
at non-AC circuits, so you will primarily be using the DC voltage section.
Figure 3. Voltmeter Range on a Multimeter.
Just as with measuring resistances, the multimeter will be able to measure lower voltages more
accurately than larger values. Therefore, if you have DC voltage potential of about 100mV, you
will get the most accurate measurement if you turn the dial to the 200mV setting.
To measure voltages, you need to remember you are essentially measuring an electrostatic
potential difference between two points in a circuit. For example, take a look at the circuit in
Figure 4.
In ECE100 and GE100, you would typically
have reported the voltages at nodes A, B,
and C as 3.5V, 7V, and approximately 6.4V,
respectively. However, it is important to
note that all three of these values are
measured with respect to the 0V potential at
node D (our ground).
In ECE263, we will often be looking at
voltages not in reference to ground. It may
take a couple weeks to get used to doing it
this way, but voltage measurements will
become a lot more useful when we can
reference them to anything (and not just
ground).
B
C
A
D
Figure 4. Example Circuit to Measure Voltages
To measure voltages, you also need to make sure your cables are plugged into the correct
sockets. In most multimeters, the cables will plug into the same sockets as when you are
measuring resistances. The black cable is plugged into the COMMON socket, and the red cable
is plugged into a socket denoted by “V” (usually labeled right next to the  symbol for
resistances, see Figure 5).
V symbol
Figure 5. Cable Locations to Measure Voltage.
With a multimeter, it is important to remember that when you are measuring voltages, you are
measuring one potential relative to another potential. You can think of it as measuring your
height with a tape measure. I can put the tape measure beside me, and measure my height as the
distance from the top of my head to the floor. Note, it is important to put the tape measure next
to me to do this, NOT INSIDE OF ME. (That may seem like a strange thing to say now, but
trust me, it will make sense soon.)
So, let’s look at how we can measure a bunch of different voltages from the Figure 4 circuit
using our multimeter.
3.5V
7V
7V
7V
6.4V
0V
6.4V
3.5V
0V
0V
(a)
6.4V
3.5V
(b)
(c)
Figure 6. Measuring Voltages with the Multimeter. The Multimeter Shows How Much Higher
the First Voltage (Connected to the Red Cable) Is Compared to the Second Voltage (Black
Cable).
In Figure 6.a, we are measuring the voltage VAD = VA - VD = 3.5V - 0V ~ 3.51V. The voltage at
node A (VA) is approximately 3.5V above the voltage at node D (VD).
In Figure 6.b, we are measuring the voltage VBD = VB - VD = 7.0V - 0V ~ 6.98V. The voltage at
node B (VB) is approximately 7V above the voltage at node D (VD).
In Figure 6.c, we are measuring the voltage VCD = VC - VD = 6.4V - 0V ~ 6.36V. The voltage at
node C (VC) is approximately 6.4V above the voltage at node D (VD).
Note, since the voltages VAD, VBD, and VCD are all measured relative to ground, sometimes, you
might seem them written as VA, VB, and VC, respectively. This is typically what you would have
seen in GE100 or ECE100.
Now, let’s take a look at what happens when the voltage is not measured relative to ground.
7V
7V
7V
3.5V
6.4V
6.4V
3.5V
6.4V
3.5V
0V
0V
0V
(a)
(b)
(c)
Figure 7. Measuring Voltages with the Multimeter. Again, the Multimeter Shows How Much
Higher the First Voltage (Connected to the Red Cable) Is Compared to the Second Voltage
(Black Cable).
In Figure 7.a, we are measuring the voltage VBA = VB - VA = 7.0V - 3.5V ~ 3.51V. The voltage
at node B (VB) is approximately 3.5V above the voltage at node A (VA).
In Figure 7.b, we are measuring the voltage VDA = VD - VA = 0V - 3.5V ~ -3.5V. The voltage at
node D (VD) is approximately -3.5V above the voltage at node A (VA). (That is, VD is +3.5V
below VA.) This is going to seem confusing for some people at first, but remember, once we get
it figured out, it will make things easier. I promise. : ) The good news is that the way we
measure and calculate the voltages is always the same. We always use the same procedure.
In Figure 7.c, we are measuring the voltage VAC = VA - VC = 3.5V - 6.4V ~ -2.86V. The voltage
at node A (VA) is approximately -2.9V above the voltage at node C (VC). (That is, VA is +2.86V
below VC.)
Note, in all these examples, we put the multimeter NEXT TO the circuit in Figure 4 to measure
the voltages. We did not have to move any circuit parts around to measure the voltages, nor did
we “insert” the multimeter into part of the circuit. This is in contrast to how we measure current.
Measuring Current. Before you can measure a voltage, you need to first tell the multimeter
what type of current you want to measure: DC (constant currents) or AC (alternating currents).
Each type of current has its own range on the multimeter. Typically, they are indicated as A~ for
AC currents and A─ for DC currents (see Figure 8).
In the example multimeter in Figure 8, we see that the AC ammeter range (on the left) is 2mAac
to 10Aac and the DC ammeter range is 20mAac to 10Aac. Most of ECE263 will be spent
looking at non-AC circuits, so you will again primarily be using the DC voltage section.
Just as with measuring resistances and voltages, the multimeter will be able to measure lower
currents more accurately than larger values. Therefore, if you have a DC current of about 10mA,
you will get the most accurate measurement if you turn the dial to the 20mA setting.
DC Settings
AC Settings
Figure 8. Ammeter Range on a Multimeter.
To measure currents, you also need to make sure your cables are plugged into the correct
sockets. The black cable is still plugged into the COMMON socket. However, the red cable
might be plugged into a number of different sockets - depending on what type of multimeter you
have. For our example multimeter, look for the socket labeled “mA” as shown in Figure 9. (The
“10A” socket would be used for higher currents, up to 10A.)
Figure 9. Cable Locations to Measure Small Currents.
Remember, current is a measurement of the quantity and direction of electricity flowing in a
circuit. To measure a current, we need to physically insert the multimeter into the circuit to
measure the flow THROUGH the multimeter. For example, in Figure 10, we show how one of
the wires in the Figure 4 circuit has been removed so the current can flow THROUGH the
multimeter to measure it. In all cases, the multimeter will report the current flowing INTO the
red cable and OUT OF the black cable.
Measures current
flowing into
red cable
Figure 10. Multimeter Inserted INTO the Circuit to Measure the Current Flowing Down
Through the Resistors.
With a quick check of Ohm’s Law, we can check to see if the multimeter got it right:
I=
V
7V
=
= +6.36mA
R 1.1k 
It looks like there is a small amount of error here, probably due to the resistor values not being
exactly 100 and 1k and the voltage decaying on the batteries.
However, if we reverse the multimeter connections, we instead measure the current flowing up
THROUGH the resistors.
Measures current flowing
into red cable
Figure 11. Multimeter Inserted INTO the Circuit to Measure the Current Flowing UP Through
the Resistors.
With a quick check of Ohm’s Law, we can again check if the multimeter got it right:
I=
V
−7V
=
= −6.36mA
R 1.1k 
Again, it looks like there is a small amount of error here, probably due to the resistor values not
being exactly 100 and 1k and the voltage decaying on the batteries. However, the multimeter
definitely shows us that there is a NEGATIVE current flowing up THROUGH the resistors.