The Digital Multimeter

Transcription

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 And further modified by Jim Hetrick and Annette Sieg. With funds from the Office of the Provost, UM-D, and NSF CCLI grant # DUE to RD and PZ.

2 Welcome to The Digital Multimeter Module! To complete the module you must work through the hands-on tutorial described in this document then pass a follow-up mastery test. The goal of the module is to leave you with the ability to confidently use a digital multimeter to measure, without any assistance, three quantities: 1) electric voltage, 2) electric current and 3) electric resistance. These important electrical instrumentation skills naturally appear in the context of the analysis, design, construction and use of electric circuits. That being so, electric circuit vocabulary and diagrams will appear throughout the module. Many of the terms and diagrams may appear unfamiliar if you haven t covered them in class yet. Keep in mind that you are not expected to obtain a full understanding of electric circuit theory from this module. The module has the specific and limited goal of teaching you the three electrical instrumentation skills described above. These skills will be valuable for your electric circuit labs when you take up the study of electric circuits in your course.

3 DIGITAL MULTIMETER (DMM) OVERVIEW

4 DIGITAL MULTIMETER (DMM) OVERVIEW Introduction The digital multimeter, sometimes referred to as simply the multimeter or DMM, is a versatile instrument. It contains three different meters in one case: 1. A voltmeter measures the electric potential difference between two points. (in volts). 2. An ammeter measures the electric current through a device (in amperes, or amps). 3. An ohmmeter measures the electric resistance of a device (in ohms).

5 DIGITAL MULTIMETER (DMM) OVERVIEW 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).

6 DIGITAL MULTIMETER (DMM) OVERVIEW Function Switch Overview The Function Switch has eight positions. The first is OFF. The meter should always be returned to this position when not in use. The next three marked with V measure voltage, the one marked Ω measures electrical resistance, the one marked checks for continuity, and the last two marked with A measure current.

7 DIGITAL MULTIMETER (DMM) OVERVIEW Function Switch Voltage V~ ( *) is for measuring alternating-current voltages, or simply AC voltage. V= ( *) is for measuring direct current voltage, or simply DC voltage. 300mV= ( *)is for measuring low voltages of direct current in the millivolt (mv) range. * * *

8 DIGITAL MULTIMETER (DMM) OVERVIEW Function Switch Ohms & Amps ( *) is used for measuring electrical resistance (in ohms). ( *) is for continuity testing. It will not be covered here. A~ ( *) is for measuring AC current (in amps). * A= ( *) is for measuring DC current (in amps). * * *

9 DIGITAL MULTIMETER (DMM) OVERVIEW Function Switch Summary 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)

10 DIGITAL MULTIMETER (DMM) OVERVIEW Starting Up When the digital multimeter is first turned on, it spends a few seconds conducting 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.

11 THE DESIGN BOARD AND POWER SUPPLY

12 THE DESIGN BOARD AND POWER SUPPLY Introduction Measurements with the digital multimeter will be made on an electric circuit you will build. The circuit will be assembled on the Design Board and powered by an adjustable DC Voltage Supply. (A battery could be used to power the circuit, but the DC Voltage Supply, which plays the same role as a battery, is being used instead.) DC Voltage Supply Design Board

13 THE DESIGN BOARD AND POWER SUPPLY Power Supply Connections Check that a red cable is connected from the red (+) terminal on the DC Voltage Supply to the red socket on the Design Board. Check that a black cable is connected from the black (-) terminal on the DC Voltage Supply to the black socket on the Design Board

14 THE DESIGN BOARD AND POWER SUPPLY Power Supply Settings The DC Voltage Supply has four knobs that can be turned. Check that the left-most knob is turned fully clockwise. Do not touch this knob for the remainder of the module. Check that the next knob to the right is turned fully counter-clockwise. Do not touch this knob for the remainder of the module.

15 THE DESIGN BOARD AND POWER SUPPLY Power Supply Settings The DC Voltage Supply has a display that can read voltage and current. Check that it is set to read voltage by ensuring the black slider-switch at the right of the display is in the up position.

16 THE DESIGN BOARD AND POWER SUPPLY Power Supply Voltage Turn on the DC Voltage Supply by pressing the green button at the lower left. For all of this module s activities the DC Voltage Supply should be set to supply 3.00 V. If its display already reads 3.00 V it is ready to go! If not, turn the two right voltageadjust knobs until the display reads 3.00 V. Do not touch these knobs for the remainder of the tutorial. Voltage Adjust Knobs

17 MEASURING DC VOLTAGE

18 MEASURING DC VOLTAGE Introduction In this portion of the module you will use the digital multimeter to measure an electric potential difference or voltage between two points. The basic unit of electric potential is the volt. The symbol for a volt is V. When the multimeter is used to measure electric potential difference (voltage) it is called a voltmeter.

19 MEASURING DC VOLTAGE Measuring the Supply Voltage Your first measurement will be using the digital multimeter to determine the potential difference maintained by the DC Voltage Supply. The DC Voltage Supply is connected across the terminals labeled A and B on the Design Board. Terminal A is at a higher potential with respect to terminal B, which is why terminal A is labeled with a + sign on the DC Voltage Supply. You will measure the potential difference between these terminals. It should be close to 3.00 V, the supply voltage. B A

20 MEASURING DC VOLTAGE Configure the Multimeter Step 1 To carry out this measurent you must configure the multimeter to measure a DC voltage. This involves three steps. First, turn the DMM s Function Switch to the V= position.

21 MEASURING DC VOLTAGE Configure the Multimeter Step 2 Second, connect voltage probes to the multimeter. At the bottom of its front cover the multimeter has four sockets for connecting probes. Only two are used at a time. To measure voltage, probes are connected to the two sockets on the right. Connect a red probe to the red socket on the meter (labeled V ) and a black probe to the socket labeled COM.

22 MEASURING DC VOLTAGE Configure the Multimeter Step 3 Third, the probes must be connected to the points across which the potential difference is to be measured. To measure the supply voltage, connect the red test lead from the DMM to point A, and the black test lead to point B. B A

23 MEASURING DC VOLTAGE Measure the Voltage Read the value on your display, which should be about V. The type of voltage is indicated by VDC to the right of the number displayed, which means a DC voltage as opposed to AC. B A

24 MEASURING DC VOLTAGE Review of Method First set the Function Switch to the desired position (V= in this case). Then connect the two probes to the proper terminals of the meter. Lastly, connect the probes to the points in the circuit across which the voltage change is to be measured, and read the displayed value.

25 MEASURING DC VOLTAGE Determining Polarity Now, reverse how the DMM s probes are connected to the Design Board. Connect the red probe to point B and the black probe to point A. Notice the display shows nearly the same numerical value, but now has a negative (-) sign in front of it. The DMM not only measures the magnitude of the voltage, but it also senses which terminal is at the higher potential. Positive readings indicate the red (or VΩ) terminal is at the higher potential, while negative readings indicate the COM terminal is at the higher potential. Now disconnect the probes from the Design Board. A B

26 MEASURING DC VOLTAGE Circuit Diagrams This figure is called a circuit diagram. It is a symbolic representation of a physical electric circuit. The symbol between points A and B represents the DC Voltage Supply (or a battery). The symbol between points C and D and E and F represents a resistance in the circuit. The solid black lines represent wires connecting the DC Voltage Supply and two resistances.

27 MEASURING DC VOLTAGE A Complete Circuit The connections form a closed loop called a complete circuit. Because a complete circuit is formed the DC Voltage Supply drives a steady flow of charge around the loop. The rate of charge flow is called electric current (labeled I). Charge flow through a resistance indicates a voltage change occurs across the resistance. You will build this circuit and use the DMM to measure the voltage change across each resistance.

28 MEASURING DC VOLTAGE Build the Circuit To build this circuit the two resistances must be incorporated. The resistances are supplied by objects called resistors. Find the resistors labeled R1 and R2 and place them as shown. A C D R1 Using three of the short black connector wires establish connections between the points A-C, D-E and F-B. B E F R2 This completes what is called a series circuit. The battery and two resistors are in series with one another.

29 MEASURING DC VOLTAGE Measuring V AB Check that the DMM s Function Switch is still set to the V= position. Now, connect the red probe to point A, and the black probe to point B. Record your results as V AB, the voltage between points A and B. This is the supply voltage. Is it the same as when the resistors were not connected? B A R1 R2

30 MEASURING DC VOLTAGE Measuring V CD Next you will measure the voltage across R1. To do so connect the red probe to point C and the black probe to point D. The meter is now connected across resistor R1, so you are measuring the potential change across it. Record this value as V CD. C D R1 R2

31 MEASURING DC VOLTAGE Measuring V EF Next you will measure the voltage across R2. To do so connect the red probe to point E and the black probe to point F. The meter is now connected across resistor R2, so you are measuring the potential change across it. Record this value as V EF. E F R1 R2

32 MEASURING DC VOLTAGE A Consistency Check Add the voltage results for V CD and V EF. Kirchoff s Voltage Law, called 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. V CD + V EF = V AB 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 the two DMM probes and the three small black connector wires and turn the meter off.

33 MEASURING DC VOLTAGE Summary of Voltage Measurement Set Function Switch to V=. Connect red probe to V terminal. Connect black probe to COM terminal. Connect the probes to the two points across which the voltage change is to be measured. Read the meter and record result in volts.

34 MEASURING DC CURRENT

35 MEASURING DC CURRENT Introduction In this portion of the module you will use the digital multimeter to measure electric current, a quantity that gives the rate of charge flow. The basic unit of electric current is the ampere, or amp. The symbol for an amp is an A. A current of 1 A at some point in a circuit indicates charge is flowing past that point at a rate of 1 Coulomb every second. When the multimeter is used to measure electric current it is called an ammeter. It is critical to learn that to measure current the multimeter must be connected in an entirely different fashion than for measuring voltage! It is not enough to simply change the Function Switch! Read carefully and note the difference as you proceed.

36 MEASURING DC CURRENT Build the Circuit Build the circuit shown below using resistor R1 and two short black connecting wires. The single resistor is in series with the DC Voltage Supply. A This is a complete circuit and there will be a steady flow of charge. The rate of charge flow is electric current, labeled with an I in the diagram. Soon you will measure I. B C D R1

37 Configure the Multimeter Step 1 To measure DC current the multimeter must be reconfigured. First, the Function Switch must be set to measure DC current. Do so by turning the Function Switch to the A= position. MEASURING DC CURRENT

38 MEASURING DC CURRENT Configure the Multimeter Step 2 Second, the red probe must be relocated, because a different pair of sockets on the multimeter is used to measure current. Insert the red probe in the socket on the lower left labeled 300 ma. This socket is used for measuring currents up to a maximum of 300 ma. (Higher currents, up to 10 A, can be measured with this meter by using the socket labeled 10 A.) The black probe should remain inserted in the COM socket.

39 MEASURING DC CURRENT Configure the Multimeter Step 3 Third, the probes must be connected to the circuit, but in a fashion that is different from a voltage measurement. The circuit must be broken to connect the ammeter. Do so by first removing the short black wire that connects point A to point C. Then A Wire has been removed. C R1

40 MEASURING DC CURRENT Configure the Multimeter Step 3 replace the short black wire with the ammeter by connecting the multimeter in its place. That is, connect the red probe of the multimeter to point A and the black probe of the multimeter to point C. A C R1

41 MEASURING DC CURRENT Measure the Current Once again a complete circuit has been established, but now the ammeter is part of the circuit, connected in series with R1 and the DC Voltage Supply. By placing the ammeter in series, charge flowing through the circuit also flows through the meter where its rate of flow is measured. Record what the display reads. This is the current in units of ma. It should read between 2.73 and 3.33 ma. R1

42 MEASURING DC CURRENT Determining Direction of Flow Reverse how the DMM s probes are connected to the Design Board. Connect the red probe to point C and the black probe to point A. Notice the display shows nearly the same numerical value, but now has a negative (-) sign in front of it. The DMM not only measures the magnitude of the current, but it also senses the direction of flow. A positive reading indicates conventional charge flow into the 300 ma socket and out of the COM socket. A negative reading indicates conventional charge flow into the COM socket and out of the 300 ma socket. A C R1

43 MEASURING DC CURRENT What Does it Mean? In this circuit the DC Voltage Supply, the ammeter, R1, and the wires form a single complete loop. We say all of these elements are in series. A SERIES RULE TO REMEMBER: The current through all elements in series is the same! Therefore, the current you just measured is not just the current through the ammeter, but is also the current through R1, all the wires, and the DC Voltage Supply. Reset the circuit by disconnecting the ammeter from the circuit and reconnecting the short black wire between points A and C.

44 MEASURING DC CURRENT A Consistency Check Let s propose a test to check this rule. If you measure the current on the other side of R1, the rule says you should get the same result. But don t start the test right away. You will conduct this test using a different technique for connecting the ammeter to the circuit. Read carefully, learn the new technique, and note the difference. R1 What is the current here?

45 MEASURING DC CURRENT The Two-Point-Break A key step in creating a test to measure current in a circuit is breaking the circuit to insert the ammeter in series. In the measurement you just completed this was accomplished by removing the wire connecting points A and C and replacing it with the ammeter. This remove-and-replace method should be recognized as a twopoint-break. The circuit was broken at two points: at point A one end of the black wire was disconnected, and at point C the other end of the wire was disconnected. A C R1

46 MEASURING DC CURRENT The One-Point-Break An ammeter can also be connected to a circuit with a one-point-break. Let s try it. To measure the current on the other side of R1, first disconnect the other short black wire at point D. This is the one-pointbreak. Do not disconnect the other end from point B; this wire will remain part of the circuit. Then B D R1

47 MEASURING DC CURRENT The One-Point-Break connect the ammeter s red probe to point D and its black probe to the free end of the short black wire. (This requires two hands so both hooks can be made to protrude simultaneously.) D R1

48 MEASURING DC CURRENT Measure the Current Once again, but this time using a one-point-break, a complete circuit has been established with the ammeter inserted in series. Record what the display reads. The current on this side of R1 should be the same as what you measured on the other side. Is it? R1

49 MEASURING DC CURRENT Two Ways to Break a Circuit To recap, the diagrams below contrast the two-point-break and the one-point-break methods for connecting an ammeter. Both accomplish the same goal in slightly different ways. Be aware of the difference. symbol for ammeter two-point-break one-point-break

50 MEASURING DC CURRENT Summary To measure DC current with the multimeter: Set the function switch to A=. Connect the red probe to the socket labeled 300 ma (or the socket labeled 10 A if the current is greater than 300 ma.) Connect the black probe to the socket labeled COM. Connect the meter in series with the elements through which the current is to be measured. Use either the onepoint-break or two-point-break method. Read the display and record the result. When the 300 ma socket is used, the units of your results are milliamps. (When the 10 A socket is used the units are amps.)

51 MEASURING DC CURRENT Voltage vs. Current The circuit diagrams below contrast the connections needed to measure the voltage across R1 (left) and the current through R1 (right). The voltmeter is connected in parallel with (across) R1. No break in the circuit needed. (Easy!) symbol for The ammeter is connected in series (in line) with R1. voltmeter The circuit must be broken. (More difficult.) Measuring Voltage Measuring Current R1 R1

52 MEASURING RESISTANCE

53 MEASURING RESISTANCE Introduction In this final portion of the module you will use the digital multimeter to measure electrical resistance. Electrical resistance is a property of objects that depends on many things including an object s size and shape, its structure and composition, and the temperature. The basic unit of resistance is the ohm. The symbol for an ohm is a capital omega, Ω. When the multimeter is used to measure electrical resistance it is called an ohmmeter.

54 MEASURING RESISTANCE Measuring Resistance Step 1 You will use the digital multimeter to measure the resistance of resistors labeled R1, R2 and R3. This requires four steps. First, set the DMM s Function Switch to measure resistance. Do so by turning it to the Ω position. 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.

55 MEASURING RESISTANCE Measuring Resistance Step 2 Second, connect the probes to the DMM for a resistance measurement. The same sockets as for a voltage measurement are used for a resistance measurement. Insert the red probe in the VΩ socket. Insert the black probe in the COM socket.

56 MEASURING RESISTANCE Measuring Resistance Step 3 Third, electrically isolate the object whose resistance is to be measured. For example, let s measure the resistance of the resistor labeled R1 first. R1 must be completely isolated before its resistance can be measured. This means if it is still connected in a circuit on the Design Board you must disconnect all wires from it. Resistance of an object cannot be measured when the object is still in a circuit, even if the circuit s power source is off. It must be removed from the circuit. R1

57 MEASURING RESISTANCE Measuring Resistance Step 4 Fourth, connect the DMM s probes across the object whose resistance is being measured. Here connect the red probe to one side of R1 and the black probe to the other side of R1. Note that, just like a voltage measurement, this step connects the multimeter in parallel with the object. Read the display showing R1 s resistance. It should be within 10% of 1000 Ω. R1

58 Repeat this procedure to measure the resistance of the resistor labeled R2. Its resistance should be within 10% of 2200 Ω. MEASURING RESISTANCE Measuring R2 R2

59 MEASURING RESISTANCE Measuring R3 Repeat this procedure to measure the resistance of the resistor labeled R3. Its resistance should be within 10% of 3300 Ω. (Note: the multimeter automatically switches to the kω scale when the resistance exceeds about 3000 Ω.) R3

60 MEASURING RESISTANCE Summary Turn the DMM s Function Switch to. Insert the red probe in the V socket. Insert the black probe in the COM socket. Electrically isolate the object whose resistance is to be measured. Connect the probes across (in parallel with) the object. Read the display and record the resistance in, k, or M.

61 MEASURING RESISTANCE Equivalent Resistance Next you will use your resistancemeasurement-skills in a more complex example. C Sometimes it is necessary to know the combined resistance of a group of resistors, called the equivalent resistance of the group. The ohmmeter is capable of measuring this resistance as well. D E R1 G The circuit diagram shows one way of connecting resistors R1, R2 and R3 together. F R2 R3 H What is the equivalent resistance of this combination?

62 MEASURING RESISTANCE Connect the Resistors Lay out the three resistors on the design board. Using three short black connecting wires establish links between the pairs of points: D- E, E-G and F-H. Do not connect the resistors to the DC Voltage Supply. You will not be powering this circuit. Remember, you will be measuring resistance with the multimeter which means the object, in this case a combination of three resistors, must be electrically isolated. F C D E R1 G R2 H R3

63 MEASURING RESISTANCE Measure the Equivalent Resistance Check that the DMM is still configured to measure resistance. Then connect the red probe to point C and the black probe to point F. Read the DMM s display; it shows the resistance between points C and F. This is the equivalent resistance of the three resistors in this configuration. C D E F R1 G R2 H R3 Your result should be between 2088 Ω and 2552 Ω.

64 CONCLUSION

65 CONCLUSION Mastery Test You have completed the tutorial portion of the Digital Multimeter Module. To complete the module you must pass the Digital Multimeter Mastery Test. Disconnect all of your wiring. Turn the Function Switch on the multimeter to the OFF position. Turn off the DC Voltage Supply. Return resistors R1, R2 and R3 to the SLC personnel. The Design Board, DC Voltage Supply and short black connecting wires remain at the station. When you are ready to take the Mastery Test obtain a new set of testing resistors, two 3-way splitters, the test instructions, and answer sheet from the SLC personnel.