Electrical Fundamentals-I. Module 2: Electrical Measurement

Transcription

1 Electrical Fundamentals-I PREPARED BY Academic Services Unit August 2012 Institute of Applied Technology, 2012

2 Module Objectives Upon successful completion of this module, students should be able to: 1. Define voltage and give its unit of measurement. 2. Describe the function of two types of voltmeters and give their schematic symbol 3. Describe how to use a voltmeter to measure voltage 4. Describe the function of two multimeters: analog and digital. 5. Define series and parallel circuit 6. Describe the voltage characteristics in series and parallel circuits 7. Use a DMM to measure the voltage drop and current in series and parallel circuit. 8. Define current and give its unit of measurement 9. Describe the function of two types of ammeters and give their schematic symbol 10. Define resistance and give its unit of measurement 11. Describe the function of two types of ohmmeters and give their schematic symbol 12. Measure resistance in series and parallel circuits 13. Describe two methods of measuring continuity Module Contents: Topic Page No. 2.1 Voltage Measurement Introduction to Series and Parallel Circuit Current Measurement Resistance Measurement Two Methods of Measuring Continuity Lab Activity Lab Activity Lab Activity Review Exercise 35 2

3 2.1 Voltage Measurement In order for anything to move, a force must be applied to it that is greater in one direction than another. For example, if forces A and B (figure 2.1) are equal the object will not move. If force A is greater than force B the object will move to the right. If force A is less than force B the object will move to the left. In other words, what causes the object to move is not the value of the forces on it, but the difference between them. Figure 2.1: Opposing Forces Voltage is the force that causes electrons to move (!ow) in an electrical circuit. Another name for voltage is electromotive force. The voltage value is measured as a difference in potential (force) between two points (figure 2.2). The basic unit used to measure the voltage (potential difference) in a circuit is the volt. The abbreviation for a volt is V. 3

4 Figure 2.2: Potential Difference Two Types of Voltmeters A voltmeter is a device that measures the voltage between two points in an electrical circuit. The value of this voltage is displayed on an analog display or on a digital display, as shown in figure 2.3 and the schematic symbol shown in figure 2.4. Figure 2.3: Analog and Digital Voltmeters Figure 2.4: Schematic Symbol of a Voltmeter 4

5 How to Use a Voltmeter Two points are needed to measure voltage because voltage is the difference between the potential of two points in an electrical circuit. The value measured depends on what two points you use. In the circuit shown in "gure 2.5, a power supply provides a source of both high and low potential. The difference in potential (voltage) from one side of the power supply to the other is the force that causes current to!ow. In this circuit: Point A is the point of highest potential because it is closest to the high potential terminal of the power supply. Point C is the point of lowest potential because it is closest to the low potential side of the power supply. Point B is somewhere between A and C in potential because it is separated by a resistor on each side. Figure 2.5: Different Points Have Different Potentials The actual voltage measured at any point depends on what other point you use as a reference. If you measure from point A to point C, you will get a higher value than measuring from point A to point B because point A is farther from C than B in potential. (figure 2.6) 5

6 Figure 2.6: Measuring Potential of Point A with Reference to Points B and C Most voltage comparisons will use one of these two methods. 1. Compare the point to a common reference of known value (e.g. A to C). 2. Measure the voltage across the component(s). (e.g. A to B) In application, a known reference point is called a ground. Grounds are usually a point connected to the power supply (such as point C above). To measure the voltage between two points you must place the voltmeter test leads as shown in "gure 2.7. The lead that is attached to the positive terminal of the voltmeter should be attached to the point with the higher (+) potential. The lead that is attached to the negative terminal of the voltmeter should be attached to the point with the lower (-) potential. In the example shown in "gure 2.7, the voltmeter is measuring the voltage at the point between resistors R1 and R2 referenced to the ground of the power supply, which is the negative terminal. 6

7 Figure 2.7: Proper Placement of the Test Leads to Measure Voltage Multimeters: Analog and Digital A multimeter is a multi-purpose device that can measure voltage, current and resistance. Multimeters are available with either an analog display or a digital display. Figure 2.8 shows analog and digital multimeters. Analog multimeters, are used when there are very fast changes in a reading because the needle responds faster than a digital readout. A multimeter with a digital display is called a digital multimeter (DMM). The measurement readings of a DMM are much easier to read because they are displayed in digital form, much like the readout on your calculator. Analog VOM Digital Multimeter Figure 2.8: Digital and Analog Multimeters 7

8 2.2 Introduction to Series and Parallel Circuit Series Circuit A series circuit is one in which there is only one path for current to travel, as shown in "gure 2.9. The total current in a series circuit!ows through each component. It starts at the positive terminal of the power supply and!ows through one component after the other and then back to the negative terminal of the power supply. Figure 2.9: A Series Circuit Parallel Circuit A parallel circuit is a type of circuit that has more than one path for current to!ow, as shown in "gure The current!ows from the positive terminal through the circuit until it reaches node 1. It then splits with part of the current!owing through branch A and part of the current!owing through branch B. The two branch currents then rejoin each other at node 2 and!ow back to the power supply s negative terminal. Figure 2.10: A simple parallel circuit 8

9 Voltage characteristics in Series and Parallel Circuits In a series circuit, the total voltage supplied by the power supply is divided among the loads, as shown in "gure Each load in the circuit uses a certain amount of the voltage as the current travels through it. This leaves less voltage available for the next load to use. The voltage drop across any component is the difference in voltage referenced to ground between the two terminals of the component. For example, the voltage drop across load 1 is 4 volts because the voltages at its two terminals referenced to ground are 10 V (V A ) and 6 V (V B ). Figure 2.11: Loads in Series When a parallel circuit is connected to a constant voltage power supply, as shown in "gure 2.12, the same amount of voltage is available to each of the branches of the circuit. In fact, the voltage available to each branch is equal to the source voltage. 9

10 Figure 2.12: Voltages in Parallel Circuit As you can see in "gure 2.12, the voltage at points 1,2, and 3 referenced to ground are equal since they are all actually the same point. The same is true for points 4, 5, and 6. This means that the voltage drop across each branch in a parallel circuit is always the same. In this case, the voltage drop is 10 volts. Conduct Lab Activity-1 on page

11 2.3 Current Measurement Electrical Current is the!ow of electrons through a circuit. It carries the electrical energy to the point of use. Current is measured in units of amperes. The abbreviation symbol (amps) is an A following a numeric value. For example, a current of 5 amperes is written as 5A. Sometimes current values can be very small, such as 1/1000th of an amp (0.001A). In this case, the unit used is milli-amp or ma (the pre" x milli means 1/1000th). For example, 0.001A can be written as 1mA. Current can either!ow in one direction (DC) or alternately in two directions (AC). Two Types of Ammeter An ammeter is a device that measures electrical current. It can have an analog display, like the one in "gure 2.13, or a digital display. Figure 2.13: Analog Ammeter Figure 2.14 shows the schematic symbol for an ammeter. 11

12 Figure 2.14: The Schematic Symbol for an Ammeter How to Use an Ammeter To measure current through a component, the ammeter is placed in series with the component, as shown in "gure This is different than measuring voltage where a meter s leads are connected across the component (in parallel). Caution: Incorrect connection of an ammeter can result in damage to the ammeter and/or the power supply. Always make sure the ammeter is connected in series. Figure 2.15: Measuring Current vs. Measuring Voltage Activity 1. Current Measurement Simulation Perform Activity 3 by referring to page-45 of Amatrol-Basic Electrical Circuits, Learning Activity Pack 2. 12

13 Current Characteristics in Series and Parallel Circuits The current in a series circuit!ows from the positive terminal of the power supply to the negative terminal through each component in the circuit. Unlike voltage, the current does not change as it!ows through a series circuit. The same amount of current!owing from the power supply s positive terminal!ows through each component, as shown in "gure Figure 2.16: Loads in Series While it is common to connect input devices in series, it is not common to connect loads in series. One reason is that if one of the loads fails while the circuit is in operation, all of the loads will lose power. Figure 2.17: Removing a Lamp from a Series Circuit Many older Christmas tree light strings were connected in series. When one light burned out, they were all out. You had to "nd out which light was burned out and replace it. 13

14 In contrast, the current in a parallel circuit is divided among the branches of the circuit, as shown in "gure The amount of current that!ows in each branch depends on the amount of resistance in that branch. Figure 2.18: Loads in Parallel If loads are connected in parallel and one fails, the others will continue to work, as shown by the lamps in "gure This is because the same current does not!ow through each branch. For this reason, newer strings of Christmas lights are connected in parallel. Figure 2.19: A Typical Parallel Light Circuit Most circuits in commercial and residential buildings are wired in parallel. Therefore, all of the outlets and sockets do not require something to be plugged into them to complete the circuit. If series wiring is used, each outlet and socket would need to have something plugged into it and be turned on to complete the circuit. Conduct lab activity 2 and 3 on pages 25 and 30 14

15 2.4 Resistance Measurement Resistance is the measure of a component s ability to resist the!ow of current in a circuit. When current passes through a component that has resistance, the current!ow is slowed. The units of measure for resistance are ohms. Ohm is abbreviated by the uppercase Greek letter omega (#). For example, a 30 ohm resistor is written as R = 30#. Many resistors have a very high resistance. These higher values are usually measured in kilo-ohms (K#) which means 1000 times the value, or megaohms (M#) which is 1,000,000 times the value. For example, a 15K# resistor is 15,000 ohms. An ohmmeter is a device that measures the resistance between two points in an electrical current. An ohmmeter can also be either analog or digital. Figure 2.20 shows the schematic symbol for an ohmmeter. Figure 2.20: The Schematic Symbol for an Ohmmeter How to Use an Ohmmeter To measure resistance, the test leads of the ohmmeter are placed in parallel with the component to be measured. The test lead connections for measuring resistance are the same as for measuring voltage. You measure across the component as shown in "gure It does not matter which lead is connected to each side of the component since resistance has no polarity. 15

16 Figure 2.21: Measuring Component Resistance To measure a component when it is connected in a circuit you should always turn off the power and disconnect one terminal of the component from the circuit, as shown in "gure Otherwise, you will get an erroneous resistance measurement. Figure 2.22: Measuring Resistance in a Circuit Resistance Characteristics for Series and Parallel Circuit When loads are connected in series, their individual resistances add together to increase the total resistance in the circuit. As shown in "gure 2.23, the resistance of the circuit is larger with three resistors in series than with just one resistor. 16

17 Figure 2.23: Series Resistances increase Total Resistance However, when loads are connected in parallel, the total amount of resistance actually decreases when more resistors are added. In "gure 2.24, the total resistance of three 10# resistors in parallel is only 3.33# while the total resistance of one resistor is 10#. Figure 2.24: Parallel Resistance Decreases Total Resistance Conduct lab activity 4 on page 36 17

18 2.5 Two Methods of Measuring Continuity Continuity describes a situation in which there is a continuous or complete path for current to!ow in an electrical circuit. Since all electrical components in a circuit are connected with a conductor of some kind (wires or conductive traces on a printed circuit board), wires must have continuity or the circuit will not operate. Figure 2.25: A Circuit Connected with Wires Two methods to test for continuity using DMM: Measuring Resistance Using a Continuity Tester! Measuring Resistance If a conductor has continuity, the display shows a very small resistance value such as 0.01 ohms. If the display shows an out-of-range indication, there is an open in that conductor and there is no continuity. An open has infinite resistance. Using a Continuity Tester A continuity tester, a function in DMM s, indicates continuity with a buzzing sound. When you place the leads across a conductor or component that has continuity, a beeping or buzzing sound is made by the continuity tester. 18

19 2.6 Lab Activity 1 Objective: To measure the voltage drop across each component in series and parallel circuits. Procedure: 1. Connect the series circuit shown in figure Figure 2.27: Series Test Circuit 2. Perform the following substeps to turn on the power supply. a. Place the AC/DC selector switch in the DC position. b. Turn on the power supply. 3. Set the DMM to the highest DC voltage setting. 4. Place the test leads across resistor R1, as shown in figure Make sure the red test lead from the DMM is connected to the side of the resistor connected to the positive (+) terminal of the power supply. Measuring the voltage drop between two points is the same as 19

20 measuring with reference to ground. Measured voltage = (VDC) It should be approximately be 2 VDC. Figure 2.28: Measuring Voltage Dropped Across a Resistor 5. Adjust the voltage range on the DMM until you reach the lowest voltage range that is still above the reading on the meter. 6. Repeat step 4 for resistors R2 and R3. If the voltage is displayed with a negative sign, reverse the test leads. R2 Measured voltage = (VDC) R3 Measured voltage = (VDC) The voltage drops across R2 and R3 should be the same, approximately 5 VDC. This is because their resistance is the same. Also, because their resistance value is higher than R1, their voltage drop is higher. You can also determine voltage drops across components by measuring a point before and after each component with reference to ground. The difference between the two voltage readings will be the voltage drop for that component. Proceed to the next step to do this. 7. Perform the following substeps to determine the DC voltage drops in the circuit in figure 2.29 by measuring voltages with reference to 20

21 ground. The DMM connections to measure the voltage at point A are shown in figure 2.29 as a guide. Figure 2.29: Measurement of Point Voltage Referenced to Ground A. the voltage referenced to ground at the two points: before and after resistor R1. These points are labeled A and B in figure Point A Voltage (before R1) = (VDC) Point B Voltage (after R1) = (VDC) B. Calculate the voltage drop across R1. This is the difference in voltage between points A and B. Voltage Drop R1 = (VDC) 21

22 The voltage should be 2 VDC. This should be the same as measured in step 4. C. Repeat substeps A and B for R2. Point B Voltage (Before R2) = (VDC) Point C Voltage (After R2 = (VDC) Voltage Drop R2 = (VDC) D. Repeat substeps A and B for R3. Point C Voltage (Before R3) = (VDC) Point D Voltage (After R3) = (VDC) Voltage Drop R3 = (VDC) Note: If there is only one load in a circuit, the voltage dropped across it should always equal the source voltage. 8. Turn off the power supply. 9. Prepare the DMM to measure AC volts. 10. Place the AC/DC selector switch in the AC position. 11. Turn on the power supply. 12. Repeat step 4 to measure the AC voltage drop across resistor R1. R1 Measured Voltage Drop = (VAC) 13. Repeat step 7A to measure the AC voltage at points A and B with respect to ground. Then calculate the voltage drop as you did in step 7B. Point A Voltage = (VAC) Point B Voltage = (VAC) Voltage Drop R1 (VAC) 22

23 The values may be slightly higher than those obtained in steps 4 and 6. The reason for this is discussed in the AC lessons. However, you can see that AC and DC voltages react the same in a series circuit with resistors. 14. Repeat steps 7C and 7D for R2 and R3. Point B Voltage (VAC) Point C Voltage (VAC) Voltage Drop R2 (VAC) Point C Voltage (VAC) Point D Voltage (VAC) Voltage Drop R3 (VAC) 15. Turn off the power supply. 16. Disconnect the circuit. 17. Connect the parallel circuit shown in figure In the next few steps, you will test the voltage drop characteristics of parallel circuits. Notice that the schematic in figure 2.30 shows the nodes a little differently than how they are actually connected in the pictorial. pictorial schematic 23

24 Figure 2.30: Parallel Circuit 18. Place the AC/DC selector switch in the DC position and turn on the power supply. 19. Prepare the DMM to measure DC volts. 20. Measure the voltage drop across each branch of the circuit. You can do this by measuring the voltage across the terminals of each component. For example, the voltage drop across branch 1 is the voltage drop across resistor R1, as shown in figure Branch 1 Voltage Drop (R1) = (VDC) Branch 2 Voltage Drop (R2) = (VDC) Branch 3 Voltage Drop (R3) = (VDC) Notice that the voltage drop across each branch is the same. This is true of parallel circuits. In this case, the voltage drop is the same as the source voltage, which is approximately 12 VDC. 21. Turn off the power supply. 22. Now switch the power supply to AC. 23. Turn on the power supply. 24. Set the DMM to measure AC volts and repeat step 20. Branch 1 Voltage Drop (R1) = (VAC) Branch 2 Voltage Drop (R2) = (VAC) Branch 3 Voltage Drop (R3) = (VAC) The voltage should be approximately 12 VAC. Again, AC and DC voltages act the same in a parallel resistance circuit. 25. Turn off the power supply, disconnect all wires and store all components. 26. Turn off the DMM. 24

25 2.7 Lab Activity 2 Objective: To measure current in both a series and a parallel circuit. Procedure: 1. Perform the following substeps to measure the current of a series circuit with a DMM. A. Turn the Function/Range switch of the DMM to the OFF position. B. Plug the red test lead into the 10A (right) terminal. C. Plug the black test lead into the COM terminal. D. Turn the Function/Range switch to the 10A position in the DC Current section. 2. Connect the series circuit with the DMM connected as shown in figure This allows you to measure the current between the positive terminal of the power supply and R1. Figure 2.31: Series Circuit 3. Turn on the power supply and record the current reading. Current Reading = (Amps) It should be 0.38 Amps. 4. Turn off the power supply. 25

26 5. Now, move the meter between R1 and the lamp, as shown in figure Figure 2.32: Series Circuit 6. Turn on the power supply and record the current reading. Current Reading = (Amps) The current at this point should be the same as in step 3 because the same amount of current flows through each component in a series circuit. 7. Turn off the power supply. 8. Now, place the meter between the lamp and the ground terminal of the power supply, as shown in figure Figure 2.33: Series Circuit 9. Turn on the power supply and record the current reading. 26

27 Current Reading = (Amps) The current at this point should be the same as in steps 3 and 6. This demonstrates that the current in a series circuit is the same at any point in the circuit. 10. Turn off the power supply and disconnect the circuit. In the following steps, you will measure the current at different locations in a parallel circuit. 11. Perform the following substeps to measure the current between the positive terminal of the power supply and the parallel loads. A. Connect the parallel circuit, as shown in figure Figure 2.34: Parallel Circuit B. Turn on the power supply and record the current reading. Current Reading = (Amps) The current should be approximately 1.58 amps. The current measurement at this point is the total current for the circuit. C. Turn off the power supply. 12. Perform the following substeps to measure the current in branch 1 of 27

28 the parallel circuit. A. Place the meter in branch 1, as shown in figure Figure 2.35: Parallel Circuit B. Turn on the power supply and record the reading. Branch 1 Current Reading = (Amps) The current in branch 1 should be less than the current you measured in step 11. Since current is split between the branches of a parallel circuit, this is only part of the total current in this circuit. The remainder of the current should be flowing in the other branch. C. Turn off the power supply. 13. Perform the following substeps to measure the current in branch 2 of the circuit. A. Place the meter between the lamp and the ground terminal of the power supply, as shown in figure To do this, you will need to disconnect the wire between the lamp and the ground terminal. Placing the meter here allows you to measure the current that is running through the lamp in branch 2. 28

29 Figure 2.36: Parallel Circuit B. Turn on the power supply and record the reading. Branch 2 Current Reading = (Amps) The reading should again be less than the reading from step 11. The current here is the remaining portion of the total current that is not flowing through branch 1. The amount of the total current that flows through each branch of a parallel circuit depends on the resistance that is present in each branch, as you will learn later. C. Turn off the power supply. D. Turn off the DMM and remove the test leads. 15. Disconnect the circuit and store all components. 29

30 2.8 Lab Activity 3 Objective: To measure the resistance of each component in series and parallel circuits. Procedure: 1. Connect the series circuit shown in figure Figure 2.40: Series Circuit 2. Perform the following substeps to measure the resistance of each load. A. Disconnect the circuit from the positive terminal of the power supply. CAUTION: DO NOT make resistance measurements in a circuit if the power is on. The meter can be damaged. B. Set the DMM to measure resistance. C. Disconnect the terminals of load 1 from the circuit. D. Measure the resistance across load 1 (R1). Resistance Load 1 = (Ohms) Load 1 should be approximately 10 Ohms (if the 10-Ohm resistor) or 25 Ohms (if the 25-Ohm resistor). E. Reconnect the load s terminal to the circuit. F. Repeat substeps B-E to measure the resistance across load 2 (R2). Resistance Load 2 = (Ohms) Load 2 should be approximately 10 Ohms (if the 10-Ohm resistor) 30

31 or 25 Ohms (if the 25-Ohm resistor). G. Repeat substeps B-E to measure the resistance across the load 3 (R3). Resistance Load 3 = (Ohms) Load 3 should be approximately 10 Ohms (if the 10-Ohm resistor) or 25 Ohms (if the 25-Ohm resistor). 3. Now measure the total resistance of the circuit, as shown in figure Measured total resistance = (Ohms) Figure 2.41: Measuring Total Resistance The measured total resistance should be higher than each of the three individual resistances. Resistances in series circuits add together to increase the total resistance of the circuit. 4. Disconnect the circuit. 5. Connect the parallel circuit shown in figure

32 Figure 2.42: Parallel Circuit 6. Perform the following substeps to measure the resistance of each branch of the parallel circuit in figure A. Disconnect the wire leading from the negative terminal of the power supply to R1, as figure 2.42 shows. When you measure the resistance of a branch in a parallel circuit you must disconnect one side of the branch from the circuit. B. Measure the resistance across R1, as shown in figure R1 = (Ohms) The resistance of R1, which is the resistance of branch 1, is approximately 10 ohms. Figure 2.43: Measurement of Resistance in a Parallel Circuit 32

33 C. Reconnect the wire from R1 to the ground terminal. D. Repeat substeps A-C to measure the resistance in branch 2 (resistor R2). R2 = (Ohms) The resistance of R2, which is the resistance of branch 2, is approximately 25 ohms. E. Repeat substeps A-C to measure the resistance in branch 3 (resistor 3). R3 = (Ohms) The resistance of R3, which is the resistance of branch 3, is approximately 25 ohms. 7. Measure the total resistance across the entire circuit, as shown in figure NOTE: Make sure that the power supply IS NOT ON. Figure 2.44: Total Resistance Measurement in a Parallel Circuit 8. Record the total resistance reading. RT = (Ohms) The total resistance of the three loads in parallel should be approximately 5.6 ohms. This value is lower than the lowest resistance branch (branch 1) of the circuit. In fact, the total resistance of any parallel circuit is always lower than 33

34 the resistance of the branch with the least resistance. As you continue to add branches to a parallel circuit, the overall resistance will continue to drop. If you remove a branch from a parallel circuit, the overall resistance will increase. 9. Disconnect R3 from the circuit (remove a branch). 10. Measure the total resistance of the circuit across R1 and R2, as shown in figure Figure 2.45: Total Resistance Measured with R3 Removed 11. Record the total resistance reading. RT = (Ohms) The total resistance should now be 7.14 ohms. This is higher than with the three resistors in parallel. However, the total resistance is still lower than the lowest individual resistance (10 ohms). 12. Disconnect the circuit and store all components. 13. Turn off the DMM and remove the test leads. 34

35 2.9 Review Exercise Section A: 1. is the existence of a potential difference between two points in a circuit. 2. Voltage is measured in units called. 3. is the abbreviation symbol used for voltage in equations. 4. The measurement readings of a(n) are much easier to read than an analog multimeter. 5. The black lead from the negative (common) terminal of the DMM should be connected to the point with the potential in the circuit. 6. The three basic units that can be measured using a multimeter are, current, and resistance. 7. A(n) is a common connection point, or a known reference point, in an electrical circuit. 8. Current is measured in units called. 9. is the abbreviation symbol for current. 10. When measuring current, the ammeter must be connected in with the components of the circuit. 11. The current in a(n) circuit fl ows from the positive terminal of the supply to the negative terminal through each component in a circuit. 12. The current in a(n) circuit is divided among the branches of the circuit. 13. Resistance is measured in units called. 14. The symbol for ohms is. 15. Since all electrical components in a circuit are connected with a conductor of some kind, the conductor must have or the circuit will not operate. 35

36 16. A continuity tester is composed of a(n) and a sounding device. 17. When measuring resistance, the user should measure the component. 18. When measuring the resistance of a component in a circuit, the user should one side of the component from the circuit. Section B: 1. Redraw the circuit below and add one more lamp connected in series. a) Will the lamp become brighter or dimmer when more lamps are connected in series? b) What happen to the ammeter reading now? c) If one of the lamps in the circuit fails what happen to the other lamp? d) Repeat the questions with a lamp connected in parallel

37 a) What is the voltmeter reading across R1 = R2 = R3 = Will there be any effect in the voltmeter reading across R1 and R2 if the resistor R3 is now removed from the circuit? Why? b) Ammeter is now connected in Branch 1, 2 and 3. What can you say about the ammeter reading in each branch? 37