Multimeter Instructions Voltmeter A voltmeter is used to measure the voltage drop across a circuit element or set of circuit elements. For our purposes, we will just consider measuring the voltage across a single light bulb. The most important thing to remember is that Figure 1: Measuring the voltage the voltmeter should be connected in across a resistor. parallel with the light bulb. That means you can always connect the voltmeter last. You do not have to disconnect any circuit elements in order to properly add a voltmeter to a circuit. A voltmeter is designed to have a very high resistance so that it does not affect the rest of the circuit. In a circuit diagram, a voltmeter is represented by a circle with a “V” inside Figure 2: A multimeter being used to measure as shown in Figure 1. a DC voltage. To use the multimeter as a voltmeter, put leads into the “COM” jack and the “V” jack. (“COM” is short for “common”. Any time that you use a multimeter, you should have a lead in the “COM” input jack.) Then turn the knob until it points to “VDC” or “VAC” (Figure 2). You will usually be measuring a DC voltage. Ammeter An ammeter is used to measure the current through a circuit element. (Once again, we will consider a light bulb.) The most important thing to remember is that an ammeter must be connected in series with the light bulb. This means that you must break the circuit in order to add an ammeter. That is, you must disconnect a lead or two in order to use an Figure 3: Measuring the current through a resistor. ammeter in a circuit. If you add an ammeter without disconnecting anything, you have added it incorrectly. Ammeters are designed to have very low resistance. That means that connecting an ammeter in parallel with a circuit element will cause a short circuit, possibly blowing a fuse in the ammeter. You must be very careful with Figure 4: A multimeter ammeters. Always feel free to ask your TA for help if you are unsure about the being used to measure a current. Always use use of an ammeter. the 10A setting in this course. In a circuit diagram, an ammeter is represented by a circle with an “A” in it as shown in Figure 3. To use the multimeter as an ammeter, put leads into the “COM” and the “10A” jack. Then turn the knob until it points to the “10A” setting (Figure 4). This setting can measure currents up to 10 A, which is much larger than any current you will encounter in this class. In theory, these multimeters can also be used to measure smaller currents more accurately using the μA or mA setting. However, the fuse for these settings has almost certainly been blown. As mentioned previously, it is easy to blow a fuse if you are not an ammeter expert. Rather than continually replacing the fuse for these setting, we ask that all students use the 10A setting to measure all currents. Never use the low-­‐current setting in this course. Ohmmeter An ohmmeter is used to measure the resistance between two points. Often an ohmmeter is used to find whether or not a good electrical connection exists between two points. To use the multimeter as an ohmmeter, put leads into the “COM” jack and the “Ω” jack. Then turn the dial to the “Ω” setting. If the multimeter reads “OL” then there is no electrical connection between the two input jacks. This might indicate that a resistor has Figure 5: No electrical connection between the input jacks (left) and a very good electrical connection between the input burned up or that a fuse has blown among jacks (right). other things. If there is a good electrical connection between the input jacks, the multimeter will quantify the resistance for you (Figure 5). Notice that two test leads create a path with a very low resistance, much less than 1 ohm. The ohmmeter is extremely useful as a troubleshooting device. If you think a test lead may be bad, plug one end into each input jack and check the resistance. If you think a light bulb is burnt out, check its resistance. AC vs. DC Many multimeters can measure both AC and DC quantities. The DC voltmeter/ammeter is often a separate setting from the AC voltmeter/ammeter. For the Extech multimeters that we use in lab, the “Mode” button switches between AC and DC current measurements. In the Circuits lab, we will only measure DC quantities. In the Speakers lab, we will measure AC quantities. In the other labs, it’s up to you to decide!