Transcript The Digital Multimeter

The Digital Multimeter
Science Learning Center
University of Michigan – Dearborn
Modified from a presentation written by Dr. John Devlin by: Donald
Wisniewski, Dawn Wisniewski, Huzefa Mamoola, Shohab Virk,
Saadia Yunus
Under the direction of: Dr. Ruth Dusenbery, Dr. Paul Zitzewitz and
Mr. Henry Povolny
With funds from the Office of the Provost, UM-D, and NSF CCLI
grant # DUE 9952827 to RD and PZ.
Quick Overview
• The digital multimeter is one of the most
versatile instruments, containing three
different meters in one.
• 1. A voltmeter measures the electrical potential
across a device (in volts).
• 2. An ammeter measures the amount of
electrical current through a device (in amperes,
or amps).
• 3. An ohmmeter measures the electrical
resistance of a device (in ohms).
Digital Multimeter Layout
• The top portion of the meter contains
the digital readout area, which
resembles the digital display of many
pocket calculators.
• Below the digital readout is a large gray
knob, called the FUNCTION switch.
This switch determines which function
the multimeter will perform (voltmeter,
ammeter, or ohmmeter).
Function Switch
• There are eight positions to
choose from on the function
switch.
• The first is OFF. The meter
should always be returned to this
position when not in use.
• In general, the three V markings
measure voltage, the next
measures electrical resistance, the
one marked |-))) checks for
continuity, and the last two read
AC and DC currents.
Function Switch - Voltage
• The V~ (*) is set to measure
alternating-current voltages,
or simply AC voltage.
• V= (*) is for direct current
voltage, or DC
measurements.
• 300mV= (*)is used to
measure low voltages of
direct current in the
millivolt (mV) range.
*
*
*
Function Switch - Ohms/Amps
• The  position (*) is
normally used to measure
electrical resistance (in ohms).
• The |-))) position (*) is for
certain applications that will
not be covered here.
• A~ (*) is used to measure AC
current (in amps).
• A= (*) is used to measure DC
current (in amps).
*
*
*
*
Starting Up
• When the digital multimeter is first turned on, it will go
through a self-analysis of its battery and its internal circuits.
• While this is proceeding, the meter will light up almost all of
the digital segments including a tiny battery symbol in the
upper left hand portion of the display.
• If you turn it on and it does not look like the image below,
notify the SLC personnel.
Summary:
The Digital Multimeter
Function Switch
•
•
•
•
•
•
•
V~ for AC voltage
V= for DC voltage
300 mV for low DC voltages (millivolts)
A~ for AC current
A = for DC current
 for resistance
|-))) for continuity (not used in this module)
Voltage Measurements
• This first series of
measurements will be of
DC voltages.
• Turn the function switch
to the V= position to
read DC voltages.
• The connections to devices such
as batteries or resistors are made
via the two terminals on the
lower right of the base of the
meter.
•
Connect a long red test lead to
the red input terminal on the
meter (labeled V) and a long
black lead to the black input
terminal (labeled COM for
common terminal).
• You will now be ready to begin
making measurements. Start by
measuring the electrical potential
difference of the battery in your
circuit box.
Circuit Box
• The battery is
installed between the
terminals labeled A
and B at the lefthand side of the box.
• Terminal A is at a
higher potential with
respect to terminal B.
• To measure the potential across the battery, connect the red test
lead from the meter to point A on the circuit box, and the black
test lead to B.
• Read the value on your display. You should obtain a value of
about 9 volts, since that is the potential of the battery that
powers the circuit boxes.
• The type of voltage is indicated by ‘VDC’ to the right of the
number displayed, which means ‘volts across a direct current
circuit’.
Review of Method
• We first set the function switch to the desired
position
(V= in this case).
• Then we connect the long leads to the proper
terminals of the meter.
• Lastly, we connect the meter across the device
in the circuit and read the display.
Determining Polarity
• Leave the Function switch in the position just
used, but disconnect the test leads from the
circuit box. You will now reverse the
connections of the long leads to the circuit box.
• Connect the red test lead to terminal B on the board, and the
black test lead to terminal A.
• Notice the display shows nearly the same numerical value,
but now has a negative (-) sign in front of it. The multimeter not
only measures the magnitude of the voltage, but it also senses
which terminal is at the higher potential.
• Positive readings
indicate that the red
terminal is at the
higher potential,while
negative readings
indicate that the black
(or COM) terminal is
at higher potential.
Schematic Circuit Diagram
• This is a schematic (or abstract) circuit diagram. Do
not worry if you have not seen this before. It is really
quite common and will be explained in detail in your
physics course this term. We will just give you a brief
introduction to such diagrams.
• The device between points C and D is a resistor. A resistor
reduces electric potential when there is a current through it.
• Your circuit box contains a battery and 3 resistors (R1,R2,R3) that
are all soldered in place and connected to terminals. Since the box
contains no internal wiring, you will have to connect these devices
in a closed circuit.
• Connect a short wire from
point A to point C on the
circuit board.
• Then connect another short
wire from point D to point E.
• Finally, connect a third short
wire from point F to point B.
• You have just set up a simple
series circuit which includes
a battery and two resistors
connected in series.
Measuring Voltage
• Check to see if your meter is still set to the V= position,
and the leads are disconnected from the box.
• Now, connect the red test lead to point A, and the black
to point B. Record your results as VAB, the voltage
between points A and B, that is the battery voltage.
• Next, disconnect the two meter leads from the circuit box.
• Now place the free end of the red test lead to point C and
the free end of the black test lead to point D.
• Because the meter is now connected across resistor R1, we
will be measuring the potential difference across it. Record
this value as VCD.
• Now disconnect the two leads from the box, and
reconnect the red lead to point E and the black lead to
point F.
• This configuration measures the value of potential
across resistor R2. Record your result as VEF.
• Add the voltage results for VCD and VEF.
• The loop law states that the sum of potential
changes around a circuit is zero. In this circuit the
loop law gives the following equation.
VCD + VEF = VAB
• If this rule does not hold within 10% of your
measurements, you have probably measured
something wrong. If so, redo the measurements.
• When you are finished, disconnect all your wires
and turn the meter off.
Summary of Voltage Measurements
•
•
•
•
Measuring DC Voltage:
Set Function switch to V=.
Connect long red lead to V terminal.
Connect long black lead to COM
terminal.
• Connect the leads across the device.
• Read the meter and record result in
volts.
Current Measurements
• When measuring electrical currents through
devices, it is important to remember that the
ammeter must be connected in an entirely
different fashion from that used for voltage
measurements.
• It MUST be connected in series with the circuit.
Diagram of a Simple Circuit
• The device between points A and B is a battery.
• The device between C and D is a resistor.
• In this circuit, the battery will cause a current, or flow of
electric charge, to pass out one end of the battery, through the
resistor and into the other end of the battery. The current
direction is represented by the arrows around the circuit. We
will use the letter ‘I’ to designate the current.
• Assemble this circuit with the circuit box. Connect a
short wire from point A to point C and then another
short wire from D to B. This completes the circuit with
the battery and resistor R1.
• Set the FUNCTION switch to the A= position.
• Connect a black lead to the COM terminal at the lower
right.
• Connect a red lead to the 300mA at the lower left corner of
the meter. We use this terminal for low current (milliamp
range) measurements only. This will be used for all
measurements using this circuit box. If we needed to
measure larger currents, we would use the 10A terminal
instead.
• In order for the ammeter to be able to measure ‘I’, we must
have this current pass through the ammeter. We want the
current to go, from the battery, into the multimeter through
the red lead and exit through the black lead.
Measuring the Current I
• Disconnect the end of the short lead from point C and
join the free end of the short lead to the long red lead
from the multimeter. These connected leads remain
hanging free, unattached to any of the terminals on the
circuit box.
• Connect the long
black lead of the
multimeter point C
to complete the
electrical circuit.
• The meter should
read between 8.0 mA
and 10.0 mA (that is,
within 10%)
Note
• We have temporarily interrupted the current through
the resistor and forced that current through the meter
before going through the resistor. The current through
the meter is the same as that through the resistor. The
ammeter is connected in series with the resistor.
• Would you have obtained the same result if you had
measured the current out of the resistor? Try it.
• All current measurements are to be performed in this
manner.
Schematic diagram showing a
current measurement
• Open up the circuit at the point of interest and connect
the meter between the open points. The ammeter is
indicated by a circle with the letter A inside of it.
Summary of Current Measurements
• Set the function switch to A=.
• Connect the long leads to the 300 mA and the
COM terminals.
• Connect the meter in series with the device
being measured by opening up the circuit and
inserting the meter between the open points.
• Read the display and record the result. When
the 300 mA terminal is used, the units of your
results are milliamps.
Let’s try a more complicated circuit.
• Before you begin, disconnect all of your
previous wiring.
• Place a short lead between points A and C on
your board.
• Place another short lead between D and E, and
then another one between E and G.
• Finally connect a wire from point H to point F
and then another wire from F to B.
• Your wiring should look like this.
• Note that there are double plug connections at points E and F.
• Ask yourself: How would the meter have to be connected to
the circuit board in order to properly measure all of the
current that passes through resistor R2 only?
• The correct answer is that the circuit would have to be
opened up at point E and the meter connected between
the open points.
• The current into point E goes to R2. If we insert the
meter at this point all of the current through R2 will first
go through the ammeter.
• Now, connect the meter in this fashion, by removing both of
the plugs that go into point E and connecting both of them
to the long, red meter lead. This combination of 3 plugs
will not be attached to anything else.
• Finally, connect the
long, black meter lead
to point E to complete
the circuit.
• Your reading should
be between 2.1 and 2.7
mA for the current
through resistor R2.
• Before measuring the current through the 3rd resistor,
disconnect the 2 meter leads and return the 2-plug pair
to point E as before. This restores the circuit to its
original configuration.
• How would you connect the meter to the circuit to
measure the current through circuit R3?
• The correct answer is shown diagrammatically.
• Open up the circuit at point G.
• Connect the long red meter lead to the end of the
single, short wire from E.
• Connect the long black meter lead to point G. Note
that the free end is a double plug.
• In this configuration, all of the current through the meter
will also have to go through R3.
• Read the display to find the value of the current. Record
this result as I3.
• Your answer should be between 1.4 and 1.8 milliamps (mA).
• After recording your value, disconnect both meter leads
from the circuit box and return the end of the short
lead to point G as before.
• Now we shall measure the current through resistor R1.
• How would you would do this?
• The answer is: Open the circuit at resistor R1.
• Open the circuit at point C by disconnecting the short lead at
point C. Connect the long red meter lead to the end of the short
lead and connect the long black meter lead to point C.
• Note that the current through R1 will now be the same as through
the meter. Double check your wiring, and record the value
obtained for the current as I1.
• Your meter should read between 3.6 and 4.4 mA.
• Disconnect both meter leads from the circuit and
return the end of the short wire to point C to restore
the original circuit’s configuration.
• A second important circuit law says that the current
through resistor R1 is equal to the sum of the current
through resistor R2 and R3, or
I1 = I2 + I 3
• Check your numbers to see if this holds for your case. The
agreement should be within about 10% uncertainty.
• If you do not obtain this result, measure I1, I2, I3 again,
being very careful with your connections.
Current Through the Battery
• To measure the current through the battery, we
perform the same procedure as for the resistors.
• We open the circuit at the battery terminal and insert
the meter between the open points. One possible
connection is as follows:
• Disconnect the wire at point B, and connect that wire to
the long red meter lead. Connect the long black meter
lead to point B.
• Record this value as IB. For this particular circuit:
IB = I1
current through battery
is the same as
current through resistor R1.
• If this is not the case for you, go back and measure I1
and IB again.
General Procedure for
Measuring Electrical Currents.
• First, set the function switch to A= in order to measure
DC currents.
• Second, connect the long leads to the 300mA and the
COM terminals on the multimeter if you are measuring
milliamp currents.
• Third, connect the meter in series with the device by
opening up the circuit at the device and inserting the
meter between the two points so that all of the current
going through the meter also goes through the device.
• Fourth, read the value and record the results.
Resistance Measurements
• The final portion of this study unit will be concerned
with resistance measurements. Electrical resistance is
an intrinsic property of almost every electrical device
and is measurable by the multimeter.
• The basic unit resistance is the ohm. When the
multimeter is used to measure electrical resistance, it is
called an ohmmeter.
• SYMBOLS FOR RESISTANCE UNITS
•  for ohms
• k for kilohms
• M for megohms
Preparations for
Resistance Measurements
• Disconnect all wiring from the meter and
circuit box.
• Individual resistors must be measured
separately from any other device in the circuit.
• All power sources must be disconnected when
taking resistance measurements.
• Turn the function switch to the  position. You will use this
position for all of your resistance measurements.
• In this position, the display will show an “O.L.” reading
when first turned on. This indicates that there is an “over
load” or off scale resistance. This occurs when the
resistance is higher than the meter is capable of reading,
such as when no device is connected.
• The long leads must also be connected properly to measure
resistance. The long red lead must be connected to the V
terminal, while the long black lead must be plugged into the
COM terminal.
• Please note that these are
the same connections that
were used when recording
voltage readings.
• Once the “O.L.” reading
has been obtained and the
long leads are attached
properly, you are ready to
begin making resistance
measurements.
• These measurements are
made by placing the leads
across the resistor to be
measured.
• Note that while measuring either voltage or resistance, the meter
is connected across or in parallel with the device.
• For example, connect the red test lead to point C on your circuit
board, and the black test lead to point D to measure the
resistance of R1.
• Within a 10% uncertainty range, R1 measures 1000 ohms.
• Disconnect your meter leads, and reconnect them across R2.
• The value shown here is 2,184 ohms.
• Repeat this procedure to determine R3. It will show 3.28 k,
or 3,280 .
The Digital Multimeter
Measuring Resistance
• Set FUNCTION switch to 
• Connect long red lead to V  terminal
• Connect long black lead to COM
terminal
• Connect the leads ACROSS the device
• Read meter and record , k, or M
Resistances In the Circuit Box
You should get the following to within a 10%
range:
• R1 - 1000 
• R2 - 2200 
• R3 - 3300 
• If you did not obtain these values, repeat your
measurements carefully.
• Sometimes it is necessary to know the combined
resistance of a group of resistors. The ohmmeter is
capable of measuring this resistance as well.
• Displayed here is a special combination of resistors R1,
R2, and R3. This is the circuit you will assemble.
• Connect a short lead from D to E, another from E to G, and a
third from F to H.
• Now find the resistance between C and F (RCF).
• RCF should read around 2,320 ohms (2.32 kilo ohms), or be
within a 10% difference (between 2088 and 2552 ohms).
Summary of Resistance
Measurements
• Remove all power sources
• Turn the Function switch to the  position.
• Connect the long leads to the V and COM
terminals of the multimeter.
• Connect the meter across the device.
• Read the scale and record the results, noting
the units in the readout.
Conclusion
• You should now be ready to take the mastery
test for this study unit on the multimeter.
• Disconnect all of your wiring and turn the
function switch to the OFF position to prevent
depletion of the battery inside the multimeter.
• Return only the circuit box to the SLC
personnel to obtain the post-test and test-box.