The Digital Multimeter

1
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.

2
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).

3
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).

4
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.

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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.

6
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).

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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.

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SummaryThe 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)

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Voltage Measurements

• This first series of measurements will be of DC
  voltages.
• Turn the function switch to the V position to
  read DC voltages.

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• 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.

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Circuit Box

• The battery is installed between the terminals
  labeled A and B at the left-hand side of the box.
• Terminal A is at a higher potential with respect
  to terminal B.

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• 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.

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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.

14
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.

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• 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.

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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.

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• 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.

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• 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.

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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.

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• 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.

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• 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.

22

• 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.

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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.

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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.

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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.

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• 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.

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• 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.

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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)

29
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.

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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.

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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.

32
Lets 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.

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• 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?

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• 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.

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• 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.

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• 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?

37

• 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.

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• 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).

39

• 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?

40

• 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.

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• 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 circuits configuration.

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• 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 I3

• 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.

43
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

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• 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.

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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.

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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

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Preparations forResistance 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.

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• 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.

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• 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.

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• 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.

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• 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 ?.

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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?

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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.

54

• 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.

55

• 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).

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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.

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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.