Guide to Using the Multimeter

Transcription

1 Guide to Using the Multimeter 33-01

2 This guide was produced by the Société de formation à distance des commissions scolaires du Québec. Production Coordinator : Development : Translation : Scientific Proofreading : Proofreading : Layout : Alain Pednault Dorcotek inc. Direction de la production en langue anglaise Services à la communauté anglophone Ministère de l'éducation Interscience Francine Cardinal Michèle Gagné Société de formation à distance des commissions scolaires du Québec, 13 All rights for translation and adaptation, in whole or in part, reserved for all countries. Any reproduction by mechanical or electronic means, including microreproduction, is forbidden without the written permission of a duly authorized representative of the Société de formation à distance des commissions scolaires du Québec.

3 This guide accompanies the multimeter, which is included in the experiment kit. The description and instructions may differ slightly for instruments made by another manufacturer. The safety rules, however, apply to all multimeters. 1. DESCRIPTION OF THE MULTIMETER Your multimeter allows you to measure the voltage (voltmeter function), current intensity (ammeter function) and resistance (ohmmeter function) of certain components of an electric circuit. Its specialized functions also allow you to perform other types of tests, such as circuit continuity and diode polarity tests. This guide is limited to the study of the measurement of voltage, current and resistance, since these three types of measurements are the most useful in analyzing circuits. Also, they are the only measurements you will need to make in the experiments in this course. Your multimeter is powered by a 9-V cell, without which it cannot operate. The cell is inside the instrument; to reach it, you must remove the back of the multimeter by unscrewing the two screws. Near the cell is a 0.2-A, 250-V fuse, which protects the multimeter against surges. The multimeter has three main parts: the digital display, the function and range selector, and the jacks. Figure 1 Display U 0m 0 U m 0k m m Selector k k 0 Jacks On the front of the multimeter, the digital display is on top, the selector is in the middle, and the three jacks are in the bottom right-hand corner. guide to using the multimeter 3

4 1.1 The Digital Display The display represents the measurements taken by the multimeter. hen the power is on, it indicates a number that corresponds to the measurement; the unit will depend on the function selected: volts or millivolts for voltage, amperes or milliamperes for current, and ohms or kilo-ohms for resistance. These units are accepted worldwide and are part of the international system of measurement (SI) used in all countries. Your multimeter's display can display numbers from 0 to 1999, as well as any combination of these numbers and a decimal point. It can also display the negative sign and the number "1" may appear on the left when the measurement exceeds the maximum allowable in any given range. 1.2 The Selector The selector is used to choose the function (voltmeter, ammeter or ohmmeter) and the most appropriate range for the measurement. The choice of function determines the section in which the selector must be positioned to take the measurement. The choice of range determines the exact position of the selector when the reading is taken. There are seven distinct sections around the dial: Ω, DCV, ACV, (0 - ma), (10 A) and two specialized functions identified by symbols. The functions corresponding to the first five sections are indicated below. hile reading, have your multimeter handy to establish the correspondence between the sections and their function. To measure a resistance, select a position in the "Ω" section. To measure a DC voltage, select a position in the "DCV" section. To measure an AC voltage, select a position in the "ACV" section. To measure a direct current whose intensity is less than ma (0. A), select a position in the section identified in black as "." To measure a direct current whose value is between ma and 10 A, select a position in the section identified in red as " - 10 A." Note: In the experiments, you will use only the sections Ω, DCV, (0 - ma) and (10 A). You will not be called upon to take measurements in AC voltage. Once the section has been determined, you need to select the range, that is, the exact position at which the selector must be placed to take the measurement. The choice of range determines the accuracy of the mea- surement taken. The number indicated at a given posi- 4 guide to using the multimeter

5 tion corresponds to the maximum value that can be measured in that range: for example, in the "DCV" section, "" is for measuring DC voltages under V; however, " m" is the best choice for voltages under mv. Note: In the experiments, the procedure always indicates the range to use for a given measurement. If you want to take measurements in another context, however, follow the instructions in Section 3 of this guide to determine the most appropriate range. 1.3 The Jacks Your multimeter has three jacks, labelled, from top to bottom, "10 A," "V-Ω-mA" and "." The jacks are used to connect the multimeter to the circuit or component to be measured. To connect the multimeter to the circuit, you will find two probes, that is, two special wires with a banana plug on one end to be connected directly to the multimeter and a thin metal rod at the other, to be applied to a point in the circuit. Like any other electrical component, the multimeter must be connected at two points in the circuit. That is why there are two wires. For all measurements, one wire, usually the black one, is connected to the "" jack of the multimeter. The second jack depends on the section you selected: if you are working in the " - 10 A" section, to measure a current between ma and 10 A, you must connect the second wire (red) to the jack labelled "10 A;" for all other measurements, connect the second wire to the middle jack, labelled "V-Ω- ma." Thus, the circuit is connected either between the "V-Ω-mA" and "" jacks or between the "10 A" and "" jacks. The combination "10 A"/"V-Ω-mA" should be avoided, since it might damage the multimeter. 2. Safety instructions Measurements of voltage, current and resistance are by nature different. Even if your multimeter is a quality instrument, protected against surges, you must pay attention to a few important rules in order to avoid damaging it. Measurements of voltage can be taken safely. An error in the connections will give the wrong measurement, but will not affect the multimeter. Measurements of resistance are easy to take, but you must always make sure that the component to be measured is not already live. To take the measurement, the multimeter provides a certain voltage at its terminals in order to inject a current into the component to be measured. It is therefore the ideal instrument for measuring the resistance of a resistor or a wire. If the resistor is already live, however, the multimeter cannot correctly measure the resistance. Measurements of current are more delicate than measurements of voltage or resistance. guide to using the multimeter 5

6 For measurements of current, the multimeter must be inserted in series with the component in which the measurement is to be taken, so that the same current flows through the multimeter and the component. Ideally, the ammeter function of the multimeter has 0 resistance, so as to avoid increasing the total resistance of the circuit. If you make the mistake of connecting the multimeter in parallel with a component of the circuit, the instrument will short-circuit the component; the total resistance of the circuit will decrease and, at the same time, the current in the multimeter may increase sufficiently to damage it. Most often, however, the fuse will burn and the instrument remains intact. Note: The fuse is not connected to every function of your multimeter. If it is burned, the display will still work; you can take measurements of voltage and resistance. Measurements of current, however, will be affected. 3. How to take measurements 3.1 Resistance Your multimeter measures resistances between 0.1 and Ω. As mentioned in the preceding section, it is essential that the component to be measured not be already live. To measure the resistance of a component: 1) connect the red wire provided with the multimeter to the "V-Ω-mA" jack and the black wire to the "" jack; 2) apply the two probes to the terminals of the component whose resistance you are measuring; Figure 2 U 0m 0 U m m 0k m k k 0 U 0m 0 U m m 0k m k k 0 6 V a) Measurement of resistance b) hen you measure resistance, the component to be measure should not be live 6 guide to using the multimeter

7 3) if you already know the most appropriate range for the measurement to be taken, turn the selector counterclock- wise to the corresponding position in the "Ω" section; if you do not know the most appropriate range, turn the selector counterclockwise to "0 K" in the "Ω" section. If the value of the resistance is small, the multimeter will display a very small number or zero. This is normal, since you chose a very large range. Turn the selector counterclockwise successively through " K", " K", "0" and "". hen you have passed the range allowing for maximum accuracy, the multimeter will display the number "1" on the left. Turn the selector clockwise to the previous position and you will obtain the most accurate reading. 3.2 DC Voltage Cells and batteries generate DC voltage. Multimeters can measure DC voltages between 0 and V. Measurements of voltage must always be taken in parallel with the component to be measured. A faulty connection, however, would not cause any ill effects. To measure DC voltage: 1) connect the red wire provided with the multimeter to the "V-Ω-mA" jack and the black wire to the "" jack; 2) apply the two probes to the ends of the component whose voltage you wish to measure; Figure 3 U 0m 0 U m m 0k m V k k 0 Measurement of DC voltage guide to using the multimeter 7

8 3) if you already know the most appropriate range for the measurement to be taken, turn the selector counterclock-wise to the corresponding position in the "DCV" section; if you do not know the most appropriate range, turn the selector counterclockwise to "" in the "DCV" section. If the value of the voltage is small, the multimeter will display a very small number or zero. This is normal, since you chose a very large range. Turn the selector counterclockwise successively through "," "," "0 mω" and " mω." hen you have passed the range allowing for maximum accuracy, the multimeter will display the number "1" on the left. Turn the selector clockwise to the previous position and you will obtain the most accurate reading. 3.3 Direct Current Your multimeter allows you to measure direct current between 0 and 10 A. Note that the procedure differs depending on whether you are measuring a current between ma and 10 A or a current under ma. If you are uncertain of the intensity of the current you are measuring, begin by using the 10-A range. If you believe that the current may exceed 10 A, do not use the multimeter, since you may damage it. To measure a direct current between ma and 10 A: 1) connect the red wire provided with the multimeter to the "10 A" jack and the black wire to the "" jack; 2) disconnect the circuit at the point at which you wish to take the measurement; 3) connect the multimeter in series by applying a probe to each of the disconnected extremities of the circuit; Figure 4 U 0m 0 U m m 0k m V k k 0 a) Measurement of a current between ma and 10 A. 8 guide to using the multimeter

9 U 0m 0 U m m 0k m 6 V k k 0 b) hen you measure current, you risk damaging the multimeter if you connect it in parallel. 4) turn the selector clockwise to "10 A" in the "" section. If the value of the current is under 0.2 on the 10-A range, you may follow the procedure for measuring a direct current between 0 and ma described below. If the multimeter displays a number greater than 0.2, it is displaying the most accurate reading possible. If the multimeter is displaying the number "1" on the left, you have passed the appropriate range. You cannot take the measurement, since this multimeter, as mentioned previously, does not allow you to take measurements over 10 A. To measure a direct current under ma: 1) bconnect the red wire provided with the multimeter to the "V-Ω-mA" jack and the black wire to the "" jack; 2) disconnect the circuit at the point at which you wish to take the measurement; 3) connect the multimeter in series by applying a probe to each of the disconnected ends of the circuit; Figure 5 U 0m 0 U m m 0k m 6 V k k 0 Measurement of a current under ma guide to using the multimeter 9

10 4) if you already know the most appropriate range for the measurement to be taken, turn the selector clockwise to the corresponding position in the "" section; if you do not know the most appropriate range, turn the selector clockwise to " m" in the "" section. If the value of the current is small, the multimeter will display a very small number or zero; the range chosen may be too large. Turn the selector counterclockwise successively through " m," "0 μ" and " μ," until the multimeter displays the num- ber "1" on the left. This indicates that you have passed the range. Turn the selector to the previous position and you will obtain the most accurate reading. Note: If the multimeter displays "0" in every range, check the connections. If everything seems normal, check the fuse (see Section 1 of this guide). If it is burned, it must be replaced 10 guide to using the multimeter