The Digital Multimeter (DMM)

Transcription

1 The Digital Multimeter (DMM) Since Physics 152 covers electricity and magnetism, the analysis of both DC and AC circuits is required. In the lab, you will need to measure resistance, potential (voltage), and current. The DMM will handle all of these tasks, but you must first learn how to use one correctly. Theory Figure 1 shows a typical DMM. The dial is used to select the function (which of the quantities listed above you wish to measure), as well as the scale (range of values). The display is for the measured values. The leads of the DMM are connected to the terminals along the bottom, and vary depending on the function. Figure 1: A typical DMM. Discussion of each function is given in the Procedure section below. 1

2 Apparatus Power supply, Pasco AC/DC Electronics Laboratory (100Ω < R S < 200Ω, R M 10KΩ, R L > 20KΩ), Patch cords, DMM. Procedure Using the DMM to Measure Resistance (Ohmmeter) 1. Measuring resistance is the easiest function, and as such provides an opportunity to discuss some basic aspects of the DMM. 2. If not already, plug the black lead of the DMM into the COM terminal. This is the common or reference lead. Plug the red lead into the Ω terminal (most likely it will be labeled /Ω). This is the test lead. You are now ready to measure resistance. 3. Look at the resistance (Ω) function area of the dial. You will notice several options for the scale. The figure below shows a typical setup. The number indicates the range of values that will be displayed. The letters k and M are the SI multipliers kilo (10 3 ) and mega (10 6 ). Table 1 gives details. Table 1: Explanation of Resistance Scales Scale Display Multiplier Range Example alues Ω = 134.3Ω 2k , 000Ω = Ω = 1456Ω 20k , 000Ω = Ω = 19, 870Ω 200k , 000Ω 95.6 = Ω = 95, 600Ω 2M , 000, 000Ω = Ω = 1, 002, 000Ω 20M , 000, 000Ω = Ω = 12, 240, 000Ω 4. Select the 200 scale. Touch the leads of the DMM to the small resistor (one lead to one side, the other to the other side). This is how you measure resistance - by connecting the leads directly to the resistance. Note that the resistance is not part of a live circuit. This is because the DMM has its own internal potential. When you connect a resistance, it completes a circuit with this internal potential in order to determine the value of the resistance. Record the value displayed by the DMM, and the resistance this represents (do not forget units). 2

3 5. Reverse the leads at the resistor and record the resistance displayed by the DMM. As you can see, when measuring resistance, it does not matter how you connect the leads. 6. Measure the medium resistor. Record what is displayed by the DMM and then switch immediately to the 20k scale. The value you recorded above is what is displayed by the DMM when you have gone off-scale; i.e., attempted to measure a quantity that exceeds the range you have selected. This happened here since you attempted to measure a resistance greater than 200Ω while on the 200Ω scale. You are now on an appropriate scale. Record the value displayed by the DMM and the resistance this represents. 7. Ask your lab instructor for the value of the large resistor. Which scale do think is appropriate for measuring this resistance? Rescale the DMM to the scale you chose and measure the large resistor. 8. Finally, return to the 200 scale and small resistor. Re-record the resistance displayed by the DMM. Switch to 2k and record the value and resistance. Repeat for the 20k and 200k scales. As you can see, the accuracy of the measurement decreases as the scale becomes larger. You should always select the smallest scale into which your resistance fits in order to obtain the most accuracy. If you do not know what the resistance is, then start on the largest scale and switch to progressively smaller scales until you have the most accurate value (stopping before you go off-scale, of course). 9. Turn off the DMM. 3

4 Using the DMM to Measure Potential (oltmeter) 1. The leads remain the same as with the ohmmeter - COM and /Ω. Look for the function area of the dial (DC, not AC; if it does not say DC, look for the symbol ). List the scales available on your DMM [m is milli (10 3 ) and µ is micro (10 6 )]. 2. Locate the binding posts on your power supply that will provide a constant potential (voltage) of about 5. Ask your lab instructor if you need help with this. 3. Turn on the DMM and select the 20 scale. Why is 20 the best choice here? 4. Turn on the power supply; touch the black lead of the DMM to the black (negative) binding post and the red lead to the red (positive) binding post. Record the measured potential. 5. Reverse the leads at the power supply and record this measured potential. You should have gotten the same magnitude of potential, but a negative sign when you switched the leads. The reason for this is that the DMM measures the difference in potential between its two leads - the red relative to black. The positive side of a supply is at a higher potential than the negative side, but you had the black lead of the DMM connected to the positive side. Therefore, the red lead was at a lower potential and the DMM displayed a negative value. The first potential was positive, as the red lead was connected to the positive side of the power supply. Most of the time you are only interested in the magnitude of the potential (i.e., without regard to sign). Unlike an analog voltmeter, you will not damage the DMM if you connect it such that you get a negative value - just record it as positive. Turn off the DMM and power supply. 6. The diagram below is known as a schematic diagram. It shows an electrical circuit consisting of the power supply, small resistor R S, and medium resistor R M. These circuit elements are in series, as there is only one path for the current around the circuit. The longer of the two lines of is the positive side of the power supply. R M R S 4

5 7. Wire the circuit above - referencing the schematic as you go. Pick a place to start (the positive side of is a good choice). As you can see, it is connected to R S, so run a patch cord from the positive side of to one side of R S. R S is connected to R M, so run a wire from the free side of R S to one side of R M. Finally, R M is connected to the negative side of, so run a patch cord from the free side of R M to the negative side of. The circuit is now complete. A more realistic representation is shown in Figure 2. Figure 2: A simple series circuit. 8. Turn on the DMM and use the same scale as above. Touch the leads across R M ; i.e., one on each side. The DMM is now in parallel with R M, as you have provided the current with another path. This shown schematically below. DMM R M R S Turn on the power supply and record the potential displayed. 9. Measure the potential across R S. 10. Based on your value for the potential across R S, do you think that there is a more appropriate scale with which to measure it? Do so, and below, state the scale you chose and the potential displayed. 11. Turn off the DMM and power supply. 5

6 Using the DMM to Measure Current (Ammeter) 1. Current is the most difficult of the three parameters to measure. This is because most DMM s have two different terminals with which to connect the red lead. Which one you use depends on the magnitude of the current you wish to measure. Generally, one is for large currents (the maximum the DMM can handle) and the other is for all other currents. 2. Which terminal(s) of the DMM do you think you will be using to measure current? Which will you use to measure smaller currents? 3. This is the one you will use most often - as the currents you will be working with are 2A or less (likely in the ma range). List the scales available on your DMM. 4. Use the circuit from the previous procedure. As you will see (or have seen), the total resistance of a number of individual resistances connected in series is the their sum. What is the total resistance offered by your circuit? 5. You can easily blow a fuse in the DMM if you use it incorrectly to measure a current. You should always have a rough idea of the magnitude of the current you are about to measure. As mentioned previously, your only other option would be to start on the largest scale and progressively switch to lower scales until you obtain the most accurate value. Use Ohm s law = IR to calculate the current in your circuit, since you know both and R. Record this value below. 6. Which scale do you think would be appropriate for this measurement? 7. Unlike the voltmeter where you connect the leads in parallel across a conductor, with the ammeter you must break the circuit at the point at which you wish to measure the current and place the DMM in the circuit; i.e., in series. For example, suppose you want to measure the current in your circuit between R M and R S. This is shown schematically below. 6

7 R M DMM A R S 8. Proceed as follows. Turn on the power supply and DMM, selecting the current scale you specified above. Remove the wire between R M and R S. Touch one lead of the DMM to the (now) free end of R S and the other to the (now) free end of R M. What is the measured current? [As before, if it is negative, record as positive.] Note that the original circuit connection was from R M to R S and it still is - the current just flows through the DMM along the way. Note also that the value above is the current at this particular point in the circuit only. If you want to know the current at another point in the circuit, you would need to break the circuit at that point and insert the DMM. Replace the wire that you removed between R S and R M, restoring the original circuit. 9. Keep the same scale and measure the current at the positive side of the power supply; i.e., DMM between and R S. R M R S DMM A What value did you obtain? It should be the same current. If not, make sure the DMM is in series and not parallel (remember that you need to break the circuit between and R S and insert the DMM). 10. Turn off the DMM and power supply. 7

8 Pre-Lab: The Digital Multimeter (DMM) Name Section Answer the questions at the bottom of this sheet, below the line - continue on the back if you need more room. Any calculations should be shown in full. 1. When you want to measure a DC resistance, which terminals do you connect to on the DMM? Where and how in a circuit is the DMM placed when you want to measure resistance? 2. When you want to measure a potential (voltage), which terminals do you connect to on the DMM? How do you place the DMM in a circuit when you want to measure potential? 3. When you want to measure a current, which terminals do you connect to on the DMM? How do you place the DMM in a circuit when you want to measure current? 8