The Digital Multimeter

Document Sample
The Digital Multimeter Powered By Docstoc
					     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
• 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             *
• V= ( ) is for direct current           *
  voltage, or DC
• 300mV= ( )is used to
  measure low voltages of
  direct current in the
  millivolt (mV) range.
    Function Switch - Ohms/Amps
• The W position ( ) is
  normally used to measure
  electrical resistance (in ohms).
• The ®|-))) position ( ) is for
                        * will
  certain applications that
  not be covered here.
• A~ ( ) is used to measure AC
       * (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.
        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
•   W 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
• The connections to devices such
  as batteries or resistors are made
  via the two terminals on the
  lower right of the base of the
•    Connect a long red test lead to
    the red input terminal on the
    meter (labeled VW) 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 left-
  hand 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
           Review of Method
• We first set the function switch to the desired
  (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 VW terminal.
   • Connect long black lead to COM
   • Connect the leads across the device.
   • Read the meter and record result in
      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

• 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

• 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

• 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%)
• 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
 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 = I 2 + 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 = I 1
               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.

              • W for ohms
            • kW for kilohms
           • MW 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 W 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 VW
  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

• These measurements are
  made by placing the leads
  across the resistor to be
• 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 kW,
  or 3,280 W.
     The Digital Multimeter
     Measuring Resistance

• Set FUNCTION switch to W
• Connect long red lead to V W terminal
• Connect long black lead to COM
• Connect the leads ACROSS the device
• Read meter and record W, kW, or MW
 Resistances In the Circuit Box
You should get the following to within a 10%
• R1 - 1000 W
• R2 - 2200 W
• R3 - 3300 W

• 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
• Remove all power sources

• Turn the Function switch to the W position.
• Connect the long leads to the VW 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.
• 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.

Shared By: