Electronic Systems Module 4 Digital
Acknowledgements General Motors, the IAGMASEP Association Board of Directors, and Raytheon Professional Services, GM's training partner for GM's Service Technical College wish to thank all of the people who contributed to the GM ASEP/BSEP curriculum development project 2002-3. This project would not have been possible without the tireless efforts of many people. We acknowledge: The IAGMASEP Association members for agreeing to tackle this large project to create the curriculum for the GM ASEP/BSEP schools. The IAGMASEP Curriculum team for leading the members to a single vision and implementation. Direct contributors within Raytheon Professional Services for their support of translating a good idea into reality. Specifically, we thank: Chris Mason and Vince Williams, for their leadership, guidance, and support. Media and Graphics department under Mary McClain and in particular, Cheryl Squicciarini, Diana Pajewski, Lesley McCowey, Jeremy Pawelek, & Nancy DeSantis. For his help on the Electrical curriculum volume, Subject Matter Expert, Ken Beish, Jr., for his wealth of knowledge. Finally, we wish to recognize the individual instructors and staffs of the GM ASEP/BSEP Colleges for their contribution for reformatting existing General Motors training material, adding critical technical content and the sharing of their expertise in the GM product. Separate committees worked on each of the eight curriculum areas. For the work on this volume, we thank the members of the Electrical committee: Jack Davis, Community College of Baltimore County - Catonsville Jim Halderman, Sinclair Community College Megan Kuehm, Community College of Allegheny County Frank Longbottom, Camden County College Jeff Rehkopf, Florida Community College at Jacksonville Randy Peters, Des Moines Area Community College David Rodriguez, College of Southern Idaho Ed Schauffler, Longview Community College Vince Williams, Raytheon
Contents Module 4 Digital Acknowledgements... 2 Objective... 4 Display... 6 Excercise... 10 Fluke 87 Components...11 Overload (OL)... 14 Measuring Voltage... 15 Measure Voltage Drop... 16 Measuring Resistance... 17 Measuring Amperage... 18 Exercise... 19
Objective At the end of this section, the student will be able to: Describe how the multimeter must be set-up and the leads connected to a circuit to measure voltage, voltage drop, current flow, and resistance Define voltage drop NATF Area VI A6: Demonstrate the proper use of a DMM during diagnosis of electrical problems. A9: Measure current flow in electrical/electronic circuits and components using an ammeter; determine necessary action. A10: Check continuity and measure resistance in electrical/electronic circuits and components using an ohmmeter; determine necessary action. 4-4
The Fluke 87 digital multi-meter (J 39200) combines the precision a digital meter with the speed and versatility of a high-resolution analog display. Powered by a 9-volt battery, the meter is sealed against dirt, dust, and moisture. Figure 4-1, Fluke 87 DMM - J 39200 4-5
Display In order to make it easier to work with large numbers, digital mult-meters use the metric system. The metric system is based on the number 10, making it possible to change the unit by moving the decimal point. Each metric system unit has a prefix that is used to describe the unit of measurement. The most common prefixes are the micro, milli, kilo, and mega. The meter will tell you the unit it is reading in the lower of the display. Figure 4-2, Measurement Prefixes Another way to remember this is that the decimal moves three places with each prefix. For example to convert from milliamps to amps, the decimal will move three places to the. Figure 4-3, Prefix Chart 4-6
Figure 4-4, Metric Prefixes 4-7
Mega Mega stands for one million and is abbreviated with a capital letter M. One mega ohm equals one million ohms. To convert any value from megohms to ohms, move the decimal point six places to the. For example, 3.5 megohms will convert to 3,500,000 ohms. Kilo Kilo means one thousand and is abbreviated with a lowercase letter k. A kilohm is equal to 1,000 ohms. To convert any value from kilohm to ohms, move the decimal point three places to the. For example,.657 kilohms will covert to 657 ohms. Base Units Base units are standard units without a prefix. Volts, ohms, and amperes are the primary base units used in electronics. Prefixes are added to base units to change the unit of measurement. Milli Milli stands for one thousandth and is abbreviated by the lowercase letter m. A millampere is one-thousandth of one ampere. To convert any value from milliamperes to amperes, move the decimal point three places to the. For example, 0.355 millimps will convert to.000355 amps. Micro Micro means on millionth and is abbreviated by the symbol u. A microampere is equal to one millionth of an amp. To convert any value from microamperes to amperes, move the decimal point six places to the. For example, 355 microamperes will covert to.000355 amps. 4-8
TO M (Mega) K (Kilo) BASE m (milli) U (micro) FROM M (Mega) 0 places 3 places to 6 places to 9 places to 12 places to K (Kilo) 3 places to 0 places 3 places to 6 places to 9 places to BASE 6 places to 3 places to 0 places 3 places to 6 places to m (milli) 9 places to 6 places to 3 places to 0 places 3 places to U (micro) 12 places to 9 places to 6 places to 3 places to 0 places Prefix chart: Start with the FROM column, and cross reference over to the proper TO column. 4-9
Excercise 4-10
Fluke 87 Components The meter has four main areas: the liquid crystal display, push buttons, rotary dial function switch, and inputs for the meter leads. Figure 4-5, Fluke 87 DMM Components Liquid Crystal Display 4-11
Liquid Crystal Display The meter s crystal display, or LCD, uses display segments and indicators. The display segments have a pointer that rolls across them indicating a measurement change. The display also uses indicators to abbreviate various display modes and meter functions. Figure 4-6, Fluke 87 DMM Liquid Crystal Display Push Buttons The buttons on the meter are used to perform additional functions. Future courses will cover additional button functions. For this course, the only button you need to understand is the range button. When the meter is first switched on and a measurement is made, it automatically selects a range and displays the word AUTO in the upper. Pressing the range button puts the meter in the manual range mode and displays the current scale in the lower. With each additional press of the range button, the next increment is displayed. Press and hold down the range button to return to the auto range mode. The yellow button can be used to back light the meter display. Figure 4-7, Fluke 87 DMM Push Buttons 4-12
Rotary Switch Various meter functions are selected by turning the meter s rotary switch. Each time the rotary switch is moved from the OFF position to a function setting, all display segments and indicators tun on as pat of a self-test routine. Meter Lead Inputs Depending on the measurement you wish to make, the meter leads will have to be placed in the correct terminal. Notice the insides of the input terminals are color-coded red or black. The positive lead can go in any of the red inputs. The COM, or common terminal, is used for most measurements. The black lead will always remain in the COM terminal. From to the first input terminal is for measuring amps. This input is fused at 10 amps. Figure 4-8, Fluke 87 DMM Meter Lead Inputs 4-13
The next position is for measuring milli or micro amps. No more than 400 milliamps can be measured when the rotary switch is in this position. If you are unsure of a circuit s amperage, you may want to start out with the red meter lead in the 10-amp input. The final input terminal is for measuring voltage, resistance, and checking diodes. Overload (OL) Figure 4-9, Fluke 87 DMM Overload While making some measurements you may see OL, or overload. The following conditions can lead to an overload display: In auto range, a resistance reading showing an open circuit In manual range, a resistance showing an open circuit or incorrect scale selected In manual range, a voltage reading that exceeds the range selected When doing a diode check 4-14
Measuring Voltage Figure 4-10, Measuring Available Voltage To check for available voltage: Apply power to the circuit. Place the meter leads with the black lead in the COM input and the red lead in the Volt/Ohm input. Place the rotary switch in the DC voltage position to measure DC volts, or AC voltage position to measure AC volts. Place the black meter lead on ground-side of the component and the red meter lead on the feed side of the component. Checking for available voltage indicates the voltage that is available to components; it indicates continuity or the voltage drop before the component. Measure voltage can also be used as a quick way to locate resistance in a circuit. The measurement shown is voltage drop. Anything that has resistance will use up voltage and, therefore, have a voltage drop. Wires and switches do drop a very small amount of voltage. So, anything other than an extremely small voltage drop across a wire or switch indicates a problem. Lights, motors, and other load devices drop a lot of voltage since they are performing work. 4-15
Measure Voltage Drop To check voltage drop: Apply power to the circuit Place the meter leads with the black lead in the COM input and the red lead in the Volt/Ohm input. Place the rotary switch in the DC voltage position. Place the leads across a component or part of the circuit. This measurement indicates the voltage that the component uses up. The higher the voltage reading, the higher the resistance compared to other parts of the circuit. The Fluke meter is a high impedance meter. This means the meter will not significantly increase the current flow in the circuit being measured. Voltage measurements should be made with the circuit working. This makes the meter ideal for use in circuits controlled by solid state devices like computers or microprocessors. Figure 4-11, Measuring Voltage Drop 4-16
Measuring Resistance Figure 4-12, Measuring Resistance To measure resistance: Make sure power in the circuit is OFF. This is necessary since the meter measures resistance by applying a small voltage to the circuit. Place the meter leads in the COM and Volt/Ohm inputs. Place the rotary switch in the Ohm position. Place the meter leads across the component you want to check. It is important that your fingers are not touching the tips of the probes since body resistance can affect the test. Keep in mind that the meter provides a small amount of voltage to a circuit when checking resistance, so be careful when checking the resistance of certain electrical components, such as solid slate or electronic circuits. This applied voltage may damage these circuits. Important: Remember that resistance readings can be a good indication of how components are functioning when the circuit is turned off. The component may act differently with current passing through it. For example, the resistance of a bulb is much lower when no power is supplied to the bulb. Using a meter to read the resistance will only test the continuity of the filament and not the working resistance of the bulb. 4-17
Measuring Amperage Figure 4-13, Measuring Amperage To measure amperage: Place the meters leads with the red lead in the AMP input and the black lead in the COM input. Place the rotary switch in the milliamp/amperage position. Then create an open in the circuit, preferably at the fuse or connector. Place the meters leads to complete the circuit, so current will flow through the meter. Caution: If the current flow exceeds the rating of the meter fuse, the fuse will blow since current flows through the meter (the meter is like a jumper wire). Apply power to the circuit to measure amperage. Caution: Never place the meter leads across a component when measuring amperage. When measuring current flow, the Fluke meter is designed with low resistance to not affect the current flow in the circuit. When measuring current in a circuit, always start with the red lead of the DVOM in the Amp input (10A fused) of the meter. Only move the red lead into the ma/ua input after you have determined the current is below the ma/ua input maximum current rating (400mA). The meter has a "buffer" which allows it to momentarily measure current flows higher than 10A. This buffer is designed to handle the "surge" current when a circuit is first turned on. 4-18
Exercise Read each question carefully and answer by filling in the blanks. 1. The meters leads must be placed across a component with the power to measure resistance. 2. The meters leads must be placed across a component with the power on to measure on to measure 3. When the multimeter is set up so that it is part of the circuit, you are measuring 4-19