The Digital Multimeter (DMM)
Name
Section
Since Physics II covers electricity and magnetism, the analysis of both DC and AC circuits is required. In
the lab, you will need to measure voltages and currents (and sometimes resistances) in the circuits that are
used. The DMM will handle all of these tasks, but you must first learn how to use one correctly.
Theory
Figure 1 shows a typical DMM. The display is for measured values. The dial is used to select a function
(what you want to measure) as well as the scale (over what range of values you wish to measure). This
DMM has an on/off switch; some come on when you turn the dial to a function/range. Note the connection
terminals along the bottom. Which of these you use will vary depending on what function you wish the
DMM to have - the term multimeter means that it can be used for a variety of measurements. These vary
depending on the particular meter, but all DMMs should measure potential (AC and DC), current (AC and
DC), and DC resistance.
A DMM is a pretty rugged device, but it is possible to damage it; therefore, exercise caution when using
one. Remember that each function requires not only different connections to the DMM, but also a different
placement in or out of a circuit.
You also need to have some idea of the relative magnitude of the value(s) you wish to measure so that you
can scale the DMM properly. Choose the scale most appropriate for your measurement. You want to use
the lowest scale you can (more accuracy) - but you do want to exceed the limits of that particular scale and
possibly damage the DMM. Changing scales changes internal resistances in the DMM to achieve the desired
full scale capability; this can be a problem when you need to take a series of readings over a range of values
(the readings may be somewhat discontinuous at the point you changed scales). If measuring a variable
voltage, for example, choose a scale that will incorporate all values from lowest to highest.
If by chance you ever see a 1 (only) at the far left of the display, or an OL - you have exceeded the maximum
for the scale you are on (this should not happen if you are careful). Switch immediately to a higher scale
before you damage the DMM.
Apparatus
DMM(s), Batteries, Resistors, Bulbs, Wires, Switch.
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Figure 1: DMM
Procedure
Using the DMM to Measure Resistance (Ohmmeter)
This is the easiest quantity to measure. All you need to do is connect the DMM directly to the circuit
element whose resistance you want to know. The circuit element must not be part of a live circuit. The
DMM uses its own internal source of potential to generate a current through the element, which it then uses
to determine the resistance it offers (it then displays this value for you).
Look for the section around the dial labeled Ω (or ohm) - this is the function area for the ohmmeter. The
numerical values listed at each setting represent the maximums for the different scales. For example, at 200
you could measure any resistance between 0 − 200Ω; 20k means you could measure any resistance between
0 − 20kΩ(20, 000Ω).
There are three resistors on the circuit board, and have their resistances printed on them. Record these
resistances in Table 1.
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Resistor
Given
Resistance
(Ω)
Measured
Resistance
(Ω)
1
2
3
Table 1: Resistance
If you want to use the DMM to measure their resistance, which scale do you think would be appropriate?
Why?
Make sure that the AC/DC switch (if there is one) is set to DC - we will look at AC circuits later in the
semester. Connect one lead to the COM terminal at the bottom of the DMM, and another lead to VΩ.
Connect the other ends of the leads to your Resistor 1, and record the displayed value as its Measured
Resistance in Table 1. Before testing the other two resistors, switch the leads at the resistor. Does the
measured resistance change?
Now measure the resistance of each of the other two resistors, recording these values in Table 1. Do not
be concerned if your measured resistances differ from the given resistances. Resistors (along with all other
circuit elements) are constructed only within certain tolerances. For example, a resistor may be rated as 5Ω
- but what this really means is 5Ω ± 5% (or 10% or 20%).
Before going on, measure a resistance as above, then switch to progressively larger resistance scales on the
DMM. What happens?
This is why it is important to properly scale the DMM for your measurements!
Using the DMM to Measure Potential (Voltmeter)
Leads at the DMM are connected as for the ohmmeter (COM and VΩ). Look for the section around the dial
labeled V (or DCV) - this is the function area for the voltmeter. The 20V scale means you could measure
voltages from 0 - 20V; the 200m scale means you could measure voltages only from 0 - 200mV (200 × 10−3 V).
There are three D cells (batteries) on the circuit board. What is the potential offered by a single D cell? If
you are unsure, it should be listed on the battery.
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Which scale do you think would be appropriate to measure the potential of a single battery?
Connect the other ends of the leads to one of the batteries. What is the magnitude and sign of the measured
potential?
Switch the leads at the battery and record the magnitude and sign of this measured potential. Is this value
different than before? How so?
You will not damage the DMM as you would an analog meter if you connect the leads backwards (you just
get a minus sign) - technically, the COM lead should be connected to the negative side of the potential.
(a)
(b)
(c)
Figure 2: Batteries in Series
Figure 2(a) shows the schematic representation of a single battery. The longer of the two lines is the positive
side of the battery and the shorter one is the negative. 2(b) shows 2 batteries connected in series. Think of
a flashlight that takes multiple D cells. All batteries are inserted into the flashlight in the same direction. It
will use the positive of the very first battery inserted and the negative of the very last battery - in between,
the batteries are connected positive to negative. Connect 2 batteries in series; the positive of one battery
connected (use a wire) to the negative of a second battery. The batteries are now connected in series. Switch
to the 20V scale and measure the potential of this series combination by connecting the DMM to the free
positive and negative terminals of the batteries. What is the measured potential, and why did you switch
to the 20V scale on the DMM?
Add a third battery so that you have 3 in series and measure the potential offered. What do you notice
about adding batteries in series?
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(a)
(b)
Figure 3: Batteries in Parallel
Figures 3(a) and 3(b) show 2 and 3 cells connected in parallel, respectively. In this arrangement, all positives
are connected together, and all negatives are connected together. Measure the potential that each arrangement offers (connect one of the DMM leads to the positive side and the other to the negative). Record your
values below. What do you notice about batteries connected in parallel? What do you think might be the
advantage of connecting batteries in parallel?
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Figure 4: Simple Bulb Circuit
You can also measure potentials in a circuit. Connect the circuit shown in Figure 4, which consists of a
battery, a switch, and a bulb connected in series. Use the DMM to measure the potential of the battery
when the switch is open, then again when the switch is closed. How do these values compare? Why do you
think this is so?
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Measure the potential across the bulb with the switch open and closed. How do these values compare?
Measure the potential across the open and closed switch. Whats going on here?
Using the DMM to Measure Current (Ammeter)
Measuring current is the most difficult function of the DMM. Before, you were able to connect the ohmmeter
or voltmeter directly to the circuit element being tested (in parallel). However, you need to measure the
current at a particular point in the circuit. Therefore, you must break the circuit and insert the ammeter
where you want to measure the current (in series).
Additionally, there are two ways of connecting leads to the DMM when you want an ammeter. As before,
one of the leads will be in the COM terminal; however, you have a choice for the other lead. One of the
terminals will be for large currents, the other for smaller currents. Look at the connection terminals at the
bottom of your DMM. If there are four of them, there will be the two you have been using already, and two
more for measuring current. If there are only three terminals, then smaller currents are measured via the
VΩmA terminal.
How many connection terminals are along the bottom of your DMM? Which one is for smaller currents? For
larger currents?
Look for the section around the dial labeled A (or DCA) - this is the function area for the ammeter. There will
most likely be more scale settings here than for the voltmeter or ohmmeter - µ is micro (10−6 A). Maximum
currents are typically 10A or 20A. If there are two numbers at a particular scale setting, then it can be used
either way (first value if connected for smaller currents, second value if connected for larger currents).
If this sounds confusing, it really is not. If you want to measure from 0 to the maximum, connect the leads
for larger currents; otherwise, connect for smaller currents (always). This is where you can blow a fuse in
the DMM or damage it. Be careful not to measure large currents when you are connected for measuring
smaller currents!
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AAA Cr
10Ω
Ar
r r
Br
Figure 5: Resistor Circuit
Reconnect the circuit used in the previous procedure, with the 10Ω resistor in place of the bulb. Set the
DMM for larger currents, and measure the current in the circuit at each point (A, B, and C) shown in Figure
5. Remember, you must break the circuit at each point and put the DMM there. Record your results in
Table 2.
Use Ohms Law
V
R
to calculate the current I in this series circuit. If the potential V is in volts (V), the resistance R in ohms
(Ω), then the current will be in amperes (A).
I=
What do you think would be a more appropriate scale for measuring the current in this circuit? Reset the
ammeter and measure the current again on this scale. Add the data to Table 2 (specify your units for this
measurement).
Point
Large Scale
Current
(A)
Small Scale
Current
(
)
A
B
C
Table 2: Current
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