NAME DATE CLASS LAB 1 Introduction to Meters and Voltage Measurement In trodu cti on to Meters an d VoltageMeasu rement Objectives After completing this lab, you will be able to • measure voltage using digital and analog multimeters, • determine accuracy of a measured value for both analog and digital instruments, • explain polarity as it pertains to voltage measurement, • set the zero adjust of an analog multimeter, • read analog meters with minimal parallax error. Equipment Required Digital multimeter (DMM) Volt-ohm-milliammeter (VOM) Variable dc power supply 9-V battery PRELIMINARY Before you start, review the section on meters and dc voltage measurement in A Guide to Lab Equipment and Laboratory Measurements at the front of this manual. 1 2 Introduction to Meters and Voltage Measurement EQUIPMENT USED Instrument Manufacturer/Model No. Serial No. DMM VOM Power supply Table 1-1 TEXT REFERENCE Section 2.6 MEASURING VOLTAGE AND CURRENT DISCUSSION In principle, measuring voltage is easy—you simply connect a voltmeter across the circuit element whose voltage you wish to determine, then read the value from the meter as indicated in Figure 1-1. However, there are important practical considerations. In this lab, you will look at a number of these; you will for example, learn what meter connections to use when making a voltage measurement, how to select appropriate ranges, how to interpret polarity, how to read complex VOM scales, and so on. Learning how to use meters and make measurements correctly is an important skill. We begin with a look at digital multimeters and voltage measurements. MEASUREMENTS PART A: Measuring dc Voltage with a DMM DC voltage measurements are made as illustrated in Figure 1-2. (Although a hand-held DMM is illustrated, the connection is the same for a bench DMM.) Note that color coded leads are used, red for the Figure 1-1 Measuring voltage in a dc circuit. Lab 1 3 Safety Checklist • Always turn power off before making changes to your circuit. • Select the function (and range if necessary) before connecting the meter. If you do not have autoranging and you don’t know the approximate voltage, begin with the highest range and work your way down. • Keep your fingers behind the finger guards on the probes when making measurements. (Although this is not essential here because of the low voltages used, it is a good idea to develop safe work habits from the outset.) Figure 1-2 Making a dc voltage measurement with a DMM. Steps 1 and 2 may be reversed. (Adapted from The ABCs of DMMs. Courtesy of John Fluke Mfg. Co., Inc. Used with permission.) VΩ or (+) lead and black for the COM lead. (Check your meter—as noted earlier, if it uses a combined voltage/resistance/current input, the jack will be designated VΩA instead of VΩ.) 1. Connect the probes to the DMM as in Figure 1-2, i.e., the red probe to the V2 (or V2A) jack and the black probe to the COM jack. (If your meter has alternate designations, check with your instructor.) Select Volts DC and set to the appropriate range if your meter is not autoranging. (20 V is adequate for this test.) Connect the DMM to the variable power supply. Adjust to 10 volts. Note the polarity of the meter reading. Meter reading _____________ 2. Reverse the test probes and note the new reading, including its sign. Meter reading _____________ 4 Introduction to Meters and Voltage Measurement 3. Repeat tests 1 and 2 at E = 0.5 volts. Indicate meter readings and your observations concerning range changing, i.e., did your meter automatically change range or did you have to do it manually? ___________________________________________________________ ___________________________________________________________ 4. Add the 9-V battery to the circuit as in Figure 1-3. Take voltage readings with E set to 0 V, 5 V, 10 V, and 15 V and record data in Table 1-2. Based on these results, write an equation for V in terms of E and the 9-V battery. V= E V 0V 5V 10 V 15 V Figure 1-3 Circuit for Step 4, a series aiding circuit. Table 1-2 5. Reverse the 9-V battery as in Figure 1-4 and repeat Step 4. Record data in Table 1-3. V= E V 0V 5V 10 V 15 V Figure 1-4 Circuit for Step 5, a series opposing circuit. Table 1-3 PART B: Measuring dc Voltage with a VOM DC voltage measurements using a VOM are made as illustrated in Figure 1-5. As with the DMM, the meter will indicate a positive (or Lab 1 5 Figure 1-5 Making a dc voltage measurement with a VOM. Steps 1 and 2 may be reversed. upscale) value when the VΩ lead is connected to the positive side of the circuit and the COM lead is connected to the negative side. Unlike the DMM however, you cannot interchange leads; if the leads are reversed, the meter will attempt to read down scale, and you risk damaging the instrument. Zeroing a VOM With no voltage applied, a VOM should indicate zero. However, analog meters sometimes go out of adjustment. (You should always check the zero position of your meter before you make any measurements.) To set the pointer back to zero, disconnect the meter from the circuit, place the meter in its operating position, then turn its zero adjust screw until the pointer rests exactly over the zero mark on the scale. Now carefully reverse the screw a slight amount to introduce some play into the system. However, be careful not to disturb the pointer as you do this—it should remain at zero throughout the process. 6 Introduction to Meters and Voltage Measurement Parallax Errors Parallax results when you view a meter at other than at right angles to its face. To illustrate, consider Figure 1-6. Assume the pointer is exactly over the 150 V mark. When viewed straight on as in (a), you read 150 V; when viewed slightly to one side as in (b) or (c), you get a small error. To avoid parallax, some meters use a mirror-backed scale. When you align the pointer with its reflection, you reduce the possibility of parallax error. Figure 1-6 In all cases, the pointer is exactly over the 150 V mark. MEASUREMENTS If you have trouble interpreting VOM readings, review the section on analog scales in A Guide to Lab Equipment and Laboratory Measurements at the front of this manual. 6. a. Zero your VOM as described above, then connect both it and a DMM to the power supply as in Figure 1-7. b. Using the VOM to measure the voltage, carefully set the power supply to 2 V. (Don’t look at the DMM, as the objective here is to see how close you can come using the VOM.) When you have set the voltage to your satisfaction using the VOM, read and record both voltmeter readings in Table 1-4. c. Repeat test b) for E = 10 V, 15 V, and 20 V plus some off division values such as 5.7 V, 13.1 V, and 17.6 V. d. With a calibrated and properly zeroed VOM, you should have been able to set the supply accurately to the voltages that correspond to primary and secondary scale markings. However, if you are like most people, you probably didn’t do so well for the off division values. Now repeat, using the DMM to set the volt- Lab 1 E VOM 7 DMM 2V 5.7 V 10 V 13.1 V 15 V 17.6 V 20 V Figure 1-7 Use both meters. Table 1-4 ages. Note how easy it is. This is of course one of the reasons why the DMM has largely superseded the VOM in practice. PART C: Accuracy of Measured Values As noted earlier in A Guide to Lab Equipment and Laboratory Measurements, no meter can be guaranteed 100% accurate. However, DMMs are considerably better than VOMs. Recall: • • VOMs are rated in terms of full scale. A typical specification is ± 2% of full scale, which means that at half scale, the error could be ±4%, while at less than half scale, it is even worse. (This is why you should make measurements as close to full scale as possible.) The accuracy specification for a DMM is given in terms of its actual reading and, even for low-cost instruments, is typically ±(0.5% plus 1 digit) or better. Meter Ranges The VOM referenced in this lab has 2-V and 10-V ranges, etc. If your meter does not have exactly the same ranges, use the closest ones available. For example, if your meter has a 2.5-V scale instead of a 2-V scale, use it. MEASUREMENTS 7. a. Using the circuit of Figure 1-7, set the supply to E = 2.00 V using the DMM. b. Read the voltage on the 2 V scale of the VOM. V = _________ c. Read the voltage on the 10 V scale of the VOM. V = _________ 8 Introduction to Meters and Voltage Measurement d. Assuming 0.5% possible error for the DMM and 2% full scale for the VOM, compute between what guaranteed limits the actual voltages lie for the readings of Steps 7(a), (b) and (c). PROBLEMS 8. What color probe should you connect to the VΩ terminal? To the COM terminal? 9. Using a DMM, what happens when you connect the VΩ probe to the negative side of a circuit and the COM probe to the plus side? 10. Using a VOM, what happens when you connect the VΩ probe to the negative side of a circuit and the COM probe to the plus side? 11. The DMMs of Figure 1-8 have autopolarity. What does each indicate? (Show your answer in the DMM display block.) Lab 1 Figure 1-9 What is the meter reading? 9 Table 1-5 12. For the VOM of Figure 1-9, determine the voltage indicated for each setting of the range selector shown in Table 1-5. Range (V) 250 100 20 10 2 Figure 1-8. Indicate the reading of each meter. Value 10 Introduction to Meters and Voltage Measurement 13. For the meter of Figure 1-10, draw in the pointer position and the range selector setting for the most accurate reading if the voltage being measured is 18 volts dc. 14. A DMM indicating 15.00 volts has an accuracy specification of (0.2%+1 digit). Between what two values does the true value of the measured voltage lie? ___________________________________________________________ Figure 1-10 Add the pointer and range setting.
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