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MMT. Tutorial on Using a Multimeter (version 1)

Use this version of the tutorial if you have the meter shown below. Otherwise, click here.

Record your responses in the MMT WebAssign form. Follow the instructions below.

 It's important for you to go through this tutorial before doing any circuits labs. You need to know how to use your meter correctly in order to be able to get correct readings as well as to avoid burning out a fuse in the meter.

Prelab

Read textbook sections 21.1-3 and complete E.21.1 in advance of doing this tutorial.

Equipment

Multimeter
Set of resistors
4 fresh D cells
4 alligator clips or clip wires
Battery holder

A multimeter is a device for measuring currents and potential differences (sometimes termed voltages for short), both AC and DC, and for measuring resistance.  A photo of your digital multimeter is shown to the right  (Click on the photo if you want to see greater detail.) There are two probes. The red one is conventionally taken to be positive and the black one negative. The black probe always connects to the port labeled COM. The red probe can connect to 2 or 3 different ports depending on what you're measuring. We'll discuss those later.

Note the two white, oblong buttons. The one on the left should be out. This is the position for measuring direct currents and potential differences. That's the only kind you'll be measuring. The button to the right is the power button. Always turn off the meter when not in use. This will save the batteries.

The dial is divided into sections for measuring current (A), potential difference (V), and resistance (Ω). Each section has several ranges which we'll discuss. There's also a symbol, , near the top of the dial. This is a continuity checker. It's used to determine if there's a break in a wire. You'll try using that setting next.

 Care of your meter and of yourself The ends of the meter probes are sharp! Handle them carefully to avoid stabbing yourself. When the meter is not in use, turn it off, wrap the cables, and stow the probes neatly. When using the meter, make sure to connect the probes as per the instructions to follow in order to avoid blowing a fuse.  If at any time during your experiments, you feel the battery becoming warm, that means you have a mistake in your circuit connections. Disconnect the battery immediately and check your connections.

Part A. Using the continuity checker

Your meter should be turned off. If it's on, turn it off now. Always start with the meter off. Make sure your connections and settings are right before you turn it on.

Make sure the black probe is connected to COM. Check that the red probe is connected to the V, Ω port as in the photo above. Turn the dial to the continuity checker setting . Turn the meter on. Now touch the metal ends of the probes together. After a few seconds, the reading should drop to 0.00. If it doesn't, you may not have good contact between probes. Whenever you make measurements in the future, remember that you need to make establish good contact in order to get good readings.

Test one of your alligator clip wires in the same way. Clip one end of the wire to the positive probe and the other end to the negative probe. (In the future, we'll call them red and black, assuming that you're using the convention.) You should have continuity.

Part B. Measuring resistance

You'll be using resistors in the circuits labs. These are in your lab kit in the envelope. Take a close look at one of them. They have several colored bands. The colors and order of the bands can be read in order to determine the resistance in the ohms. Here's a link where you can find out what colors correspond to what numbers. The first two colored bands give the first two digits of the resistance. The 3rd band tells you the power of 10 to multiply by. For example, brown-orange-red translates to 1 for brown, 3 for orange, and 102 for red. So the resistance is 13x102 or 1300 ohm. Note that the value that you read from the color code has a percentage uncertainty (or tolerance) that is read from the 4th band.  Gold represents a tolerance of 5%, silver represents a tolerance of 10%. In your experiments, you won't pay attention to this tolerance, because you'll measure resistances accurately to the nearest ohm or better with the multimeter. The main reason to use the color code is so that you can quickly select a resistor close to the value that you want.

1. Select your 1-kΩ resistor. Give the color code of the resistor.

2. Now, you'll measure the resistance to a precision of 1 ohm with the meter. Prepare your meter as follows.

1. Check that the meter is turned off.
2. Check that the red and black probes are connected to the same places on the meter as when using the continuity checker (V, Ω for the red probe and COM for the black probe).
3. Turn the selector to the 2k setting.
4. Clip a red alligator clip to the metal end of the red probe and a black alligator clip to the metal end of the black probe.
5. Clip one alligator clip to each end of the resistor. It doesn't matter which alligator clip is clipped to which end of the resistor.
6. The connections are represented schematically by the diagram to the right. M represents the meter. You'll need to learn to read and draw these schematics.

If your meter is connected and prepared as per the above instructions, it should read the resistance to the nearest ohm. Record the value.

3. Leaving the resistor connected, turn the selector to the 200 Ω setting. What is the reading this time? The fact that the reading makes no sense indicates that this scale is too low for measuring the resistance.

4. Now turn the selector to the 20k setting. This reading should make sense but will probably be displayed with fewer significant figures than the reading on the 2k setting. Also, the units of the readings are kΩ rather than Ω. Record the reading.

For future measurements of resistance, remember the following:

 When measuring resistance, use the setting that provides the greatest measurement precision.
1. Remove the 1-kΩ resistor and clip the 10-kΩ resistor between the probes. What is the color code of this resistor?

2. Now select the appropriate setting for measuring the resistance of the 10-kΩ resistor. Give both the setting and the value of the resistance in units of kΩ .

Note that when measuring resistance, you didn't place a battery in the circuit. It's important to realize that if you place a battery in the circuit with the resistance, the meter will give the wrong reading. That is the most common mistake that people make in measuring resistance.

 Never have a battery connected to a resistor when you're measuring resistance.

Part C. Measuring potential differences

When you measure potential difference, the reading is the difference between the potential at the red probe and the potential at the black probe.  Any measurement of voltage is actually a difference of potential between two points of a circuit. When you measure potential difference, you simply touch the probes (or connect alligator clips) to the two points of the circuit in question. More about that in a moment.

A diagram of a simple circuit with a battery and a resistor is shown to the right. Four points are marked for reference. The long side of the battery symbol is the positive terminal of the battery. Conventional (positive) current goes from positive to negative in the circuit. That is, current goes from A to B to C to D to A. In the future, when we talk about current, we mean positive current (even though we know that the actual charge carriers are electrons.)

One type of battery holder--we'll call this the Type I holder--that you may be using for the circuits labs is shown below. You can connect any number of batteries up to four. Note that metal tabs separate the batteries. Make sure all batteries face the same way in the holder; that is, the positive end of one battery connects to the negative terminal of the next one. The photo shows that only one battery is connected to the circuit with the clip leads. If you moved the black clip to the middle tab, then you would have 2 batteries in the circuit. For this tutorial, you'll just use one battery as shown in the photo. (Click on the photo for a larger version.)

The Type II battery holder consists of 4 blue or yellow plastic boxes. You can interlock boxes in order to have 1, 2, 3, or 4 batteries in series. They interlock either by sliding or snapping. Snapping is easier, if your boxes have snaps on the ends. Otherwise, see the figures below for the interlocking method. Figure 1 shows two boxes aligned with the ends that will slide together. Figure 2 shows the boxes being connected. When you slide the battery holders together, be careful of the sharp edges of the plastic! It takes quite a bit of force to slide them together. Figure 3 shows the boxes connected. Note the metal terminals in the gap between the boxes that touch each other. (Click on any photo for a larger version.)

 Figure 1 Figure 2 Figure 3 Type 2 holders, separated Type 2 holders, being connected Type 2 holders, connected

Connect two boxes together for the next set of measurements. If you want to use just one battery, you don't have to disconnect the boxes. You just clip wires to the metal tabs on one of the boxes. If you want to use two batteries in series, you connect wires to the two outermost tabs.

1. In order to prepare to measure potential difference, do the following.

1. Check that the probes of the meter are connected to the meter the same as in Parts A and B. That is, red to V,Ω and black to COM. The meter should be off.

2. Using a single 1.5-volt battery, the same 1-kΩ resistor as in the previous part, and four alligator clips, construct the circuit shown below. Use two of the alligator clips to connect the resistor to the terminals of the battery. Use two more clips to connect the probes of the meter to the terminals of the battery. Use the red clip for the red probe and the black clip for the black probe. Connect the red clip to the positive side of the battery and the black clip to the negative side.

1. Turn the multimeter selector to the 2 V setting. Then turn the meter on.

2. You'll measure a series of potential differences that we'll represent as V12 = V1 - V2.  The first potential in the difference is that of the red probe, and the second potential is that of the black probe. If, for example, you were measuring VAD, you'd expect a measured potential difference of about +1.5 volts, but that will depend on how fresh your battery is.
 In order to measure any potential difference, V1 - V2, connect the red probe to point 1 and the black probe to point 2. Think red minus black.

Now measure and record the potential differences indicated in the table below.

 Point 1 Point 2 V12 (V) A D <_> D A <_> B A <_> C B <_>
1. For the next measurement, use two batteries in series with the same resistor. Measure only VAD this time. You'll need to turn the selector to the 20 V setting. Record the value of VAD that you measured with 2 batteries in the circuit.

Turn your meter off and go on to the next part.

Part D. Measuring current

The multimeter is used in a very different way to measure current. For measuring potential differences, the multimeter is placed in the circuit in parallel.  When measuring current, the multimeter must be placed in the circuit in series. Suppose that we want to measure the current at point P shown in Figure 1. To do so, the circuit must be opened at P as shown in Figure 2. Then the meter is inserted in the circuit at the point at which the circuit was opened.

 Figure 1 Figure 2 Figure 3

 When measuring current, don't connect the meter directly to the ends of the battery. Always have a resistor in the circuit as described below.

Now do the following to prepare the meter and circuit.

• Your meter should be off. There are two ports on the meter for measuring current. The port you'll use is labeled A in white. (The higher current port, which you will not use, is labeled 10A in oranged.) Move the red probe to the lower current port.
• Use the same 1-kΩ resistor and the same battery that you used first in Part C. Now open your battery-resistor circuit at point P as shown in Figure 2. Just unclip the alligator clip attached to the resistor.
• Connect the multimeter in series as shown in Figure 3. Specifically, connect the red probe to the open circuit connection closest to the battery, and connect the black probe to the open circuit connection closest to the resistor. When you're finished, you'll have a chain that includes the battery, the meter, and the resistor.
• Turn the selector to the 20m setting and turn on the meter. You should get a reading.
1. Give the value of the current. Note that the units used by the meter are milliamperes (ma). However, write the current in units of amperes (A).

2. Remove the meter from the circuit and reconnect the battery directly to the resistor.  Now repeat your measurement technique for point Q shown in the figure below.  That is, open the circuit at Q and insert the meter.  Record the current reading in units of amperes.

 Whenever measuring current, open the circuit at the place where you want to measure current and connect the meter in series. Be sure the red probe is connected to the positive side of the circuit.

People make more mistakes in measuring current than any other circuit measurement. That's because they try to use the meter in the same way as for measuring potential difference. Unfortunately, using the meter incorrectly can burn out a fuse. You would then have to replace the fuse before you could continue experimenting.

Turn the meter off and store your materials. Then continue below.

Part E. Reflection