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Electricity and Magnetism
Lesson 12: Multimeters
Lesson 12: Multimeters
Multimeters are used to measure various components of electricity: voltage, resistance, and
current. Multimeters can be used in many different applications; for instance, in industry,
homes, laboratories, and for hobbies. The three main electrical components multimeters
measure are voltage (volts), current (Amps), and resistance (Ohms). But before we get into
measuring those components, let’s look at how to set up the multimeters.
Note: This lesson is based off of the Knight Electronics Multimeters. If our class is using a
different brand of multimeters, the basic instructions are the same, but with slight
variations in numbers and specific placements.
The multimeter should come with a set of two test leads: a red lead and a black lead. The
black lead is generally associated with ground or as labeled on the multimeter COM
(common). You should connect the black lead to the COM jack on the multimeter. In the
electrical world, ground is coined common because it is a “common return path” for
electrical current. Ground allows for a potential that causes electrons in a circuit to begin
moving. The lead plugged into the common jack is also called the negative lead, but more
on that later. The other test lead packaged with the multimeter is a red test lead. The red
test lead can be plugged into either of the three remaining jacks: V, A, or 20ADC. For most
projects in the class, you will plug the red test lead into the V or A jacks. The V jack is
used for measuring all voltages and resistances, hence the V for Volts and  for Ohms. The
A and 20A jacks are used when measuring current. The lead coming from any of these three
jacks is considered the positive lead.
Once you have the test leads connected to the multimeter, you need to indicate to the
multimeter what quantity you are trying to measure. The multimeters used in this class
have capabilities to measure voltage, current, and resistance. A dial is located at the center
of the multimeter. There are six portioned off sections on the multimeter around the dial.
Starting in the top left moving clockwise: hFE, ,
(voltage from a DC source), V~, A~,
and
(current from a DC source). In this class, you will not be concerned with using
hFE.
The first section on the multimeter is . This section is used when measuring resistance.
The numbers aligning the perimeter of the dial in the section are 200, 2k, 20k, 200k, 2M,
20M. The units of the resistance are Ohms (), where the k behind a number indicates that
the measurement is a kiliOhm and M indicates a MegaOhm. The dial should be placed on
the number closest to the value being measured, but not less than the value being
measured.
Note about measuring resistance: Resistance is a measure of difference. Placing the test
leads across the component on the positive and negative sides will yield the desired output.
Let’s look at the
section. You should turn the dial to this section when measuring
voltage coming from a direct current source. (The Boe-Bot outputs voltage from a direct
current source.) Notice the numbers along the perimeter of this section: 200m, 2, 20, 200,
1000. These numbers all have units of Volts. If the number is followed by an m, then the
NASA-Threads
Electricity and Magnetism
Lesson 12: Multimeters
units are milliVolts. The same applies for the V~ section, however, the voltage source
originates from an alternating current source.
Note about measuring voltage: Much like resistance, voltage is measured as a difference, so
placing the test leads over the component being measured on the negative and positive side
will yield the desired value.
Next, we will discuss the
section on the multimeter. The section is used when
measuring direct current values. Similarly for the
section, it is lined with the
numbers: 20, 2, 200m, 20m, 2m, 200. The base units for current are Amperes or Amps.
The  indicates that the measurement is microAmps and the m indicates the measurements
are in milliAmps. The same applies for the A~ section, however, the current originates from
an alternating current source. When measuring values close to 20A, but not exceeding 20A,
the red test lead should be plugged into the 20A jack. Turn the dial to the appropriate
, A~ and use accordingly.
Note about measuring current: Unlike voltage, current is not a measure of difference. In
order to measure current, the test leads must be positions such that the multimeter is part
of the circuit. Tell the students to picture current passing through the circuit. The test leads
must be connected such that the current passes through the red lead into the multimeter
and out of the black lead back to the circuit.
Dial placement: Regardless of the component being measured, the numbers aligning the
perimeter of the dial all indicate the upper bound that can be measured. For instance, if you
are measuring a resistor that should be 110 , you should place the dial at 200 in the 
section. This is because 200  is greater than 110 , but is also the closes to the 110  out
of the available values greater than 110 . Another example is if yuou were measuring a
circuit that should have a voltage source of 220 VDC. One may be inclined to place the dial
on the 200 V location in the
section. However, this would be incorrect because 220 V
is greater than 200 V and thus, the reading will be erroneous. The dial would need to be
placed at the 1000 V location in order to obtain a proper reading.
Troubleshooting
If you have properly set up your multimeter and no
readings are being shown in the display, be sure to check if
the multimeter has been turned on. There is an on/off
switch that must be flipped in order for the multimeter to
take measurements. This is a simple solution that is often
over looked.
If the multimeter is turned on and the readings are not
displaying properly, a fuze may be blown. Blown fuzes are a
common issue with multimeters. Many times this happens
when measuring current and the leads are not placed
properly into the circuit. To change a fuze, you must
unscrew the back of the multimeter. (If the screw holding
the back is not visible, it can be found behind the “Q.C. PASSED”
NASA-Threads
Electricity and Magnetism
Lesson 12: Multimeters
sticker.) The fuse, shown in picture to the right, should be able to be pried out easily. Once
removed, place a new fuze in the open spot and then replace the backing of the multimeter.
Units
1 Mega
(M)
106
1 kilo
(k)
103
1 hecto
(h)
102
1 deka
(da)
101
(base
unit)
1
1 deci
(d)
10-1
1 centi
(c)
10-2
1 milli
(m)
10-3
1 micro
()
10-6
Activity
Have the students create the following simple circuit on their Boe-Bot:
+
Vin
I
220Ω
-
Measure the resistance across the resistor, the current passing through the circuit, and the
voltage drop across the resistor. (When measuring the resistor, do not turn the power on
the Boe-Bot, but do turn the power on to measure the current and voltage.)
Activity 2: Change the 220  resistor to a 1 k resistor and repeat the measurements.
Change Vin to Vdd and repeat. Add a second resistor into the circuit and measure the
components using the multimeter.