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Lab Date:__/__/__ Lab Time:____ Partner:_________________ Name:___________________ Attendance verification:____ Laboratory 1 & 2 Combined: An Introduction to the Digital Multimeter EE-1301: Modern Electronic Technology Read the discussion about digital multimeters on the following pages, then answer the following questions? 1. Record the name and model number of your multimeter? 2. List seven types of measurements that can be taken using your multimeter? (see p. 7) 3. Briefly explain how measuring probes or test leads are used? Your answer should include how to connect the leads to the multimeter and how to connect them to the object being measured! (see p. 8) 4. In your own words, describe an “overrange” condition? Your answer should include how you recognize “overrange” on the display? (see p. 8) 5. Briefly explain how one chooses the proper range for an electrical measurement? (see page 89) Hand in only the first six page! Keep the rest of the pages in your course pack for future reference. 1 Laboratory 1 & 2 Combined: An introduction to the digital multimeter. Measuring battery voltages: 6. Set your multimeter to the 20 volt DC voltage range. With the V lead on the “+” terminal and the COM lead on the “-” terminal, measure the voltage of two AA-size zinc-carbon dry batteries, and record the values (including units) below? _____________ 1st battery _____________ 2nd battery 7. Arrange your batteries into a series connection as shown at the right, and measure the series voltage? _________ The series voltage should be the sum of the individual voltages. Is your measured value roughly the sum of the individual battery voltages? 8. Arrange your batteries into a parallel connection as shown at the right, and measure the parallel voltage? __________ (You may have to use short pieces of wire, coins, or keys to form the parallel connection.) The parallel voltage should be the same as the individual voltages. Is your measured value roughly the same as the individual battery voltage? 2 Laboratory 1 & 2 Combined: An introduction to the digital multimeter. 9. Measuring known resistance values: Use a multimeter to measure the resistances of the following known resistor elements. Always set the range to the lowest value that does not give an OVERRANGE in the display! This gives the greatest accuracy in the reading. Terminals Value (listed) Value (in ohms) Measured Value (include units) Multimeter Range (units) #71 and #72 100 100 ohms _______________ _____ #73 and #74 470 470 ohms _______________ _____ #75 and #76 1K 1,000 ohms _______________ _____ #77 and #78 2.2K 2,200 ohms _______________ _____ #79 and #80 4.7K 4,700 ohms _______________ _____ #81 and #82 10K 10,000 ohms _______________ _____ #83 and #84 10K 10,000 ohms _______________ _____ #85 and #86 22K 22,000 ohms _______________ _____ #87 and #88 47K 47,000 ohms _______________ _____ #89 and #90 100K 100,000 ohms _______________ _____ #91 and #92 220K 220,000 ohms _______________ _____ #93 and #94 470K 470,000 ohms _______________ _____ 10. Body resistance: Set your multimeter to the 2M or 20M range. Tightly grasp the red probe tip in one hand and the black probe tip in the other. (This is safe.) With dry hands, what is your resistance from hand to hand? _______________________ What is your resistance with wet hands? __________________ Do you feel anything?____________ Note: You should not feel any electrical effect in this measurement. The current in the ohmmeter is a few millionths of an ampere, whereas the “threshold for sensing”--the smallest current you can feel--is about one thousandth of an ampere. That is, the ohmmeter current is about one thousandth of what you can feel, so you do not feel it. 3 Laboratory 1 & 2 Combined: An introduction to the digital multimeter. 11. Revisiting the selection of range: The Science-Fair 130-in-one Electronic Project Labs have two sets of batteries. Each set consists of three 1.5 volt AA-size carbon-zinc dry batteries connected in series to produce a nominal voltage of 4.5 volts DC. With the red lead on the terminal for the first number and the black lead on the terminal for the second number, measure the following terminal voltage on different ranges and record the values (including units) below? V119,121 = __________ 1K Range V119,121 = __________ 200 V Range V119,121 = __________ 20 V Range V119,121 = __________ 2 V Range V119,121 = __________ 200mV (equal to 0.2 V) Range Circle the readings that are overrange, and mark the lowest range that does not go overrange? As previously stated in the discussion about selecting range, the most accurate reading--that is, the reading with the most significant figures--occurs on the lowest range that does not go overrange. The use of higher ranges to read a selected quantity is not incorrect; but the answer is not as accurate as an answer from a lower scale that does not go overrange. 12. Continuity: The continuity test is a simple test to check for the presence of an electrical connection between two points. Set your multimeter range switch on the continuity test, place the test probes in the COM and V connections, and touch the test leads together. You should hear an audible buzzing sound which indicates a continuous connection between the test leads. Ask the TA for help if you are unable to hear the buzzer. Attach the alligator clips on the test leads to terminals #137 and #138 (the key switch). What happens when the key switch is depressed? Attach the alligator clips to terminals #134 and #135 on the slide switch. What can you say about continuity when the switch is in position A? What about position B? Is there continuity between terminals #134 and #136 in either position? 4 Laboratory 1 & 2 Combined: An introduction to the digital multimeter. Slide switch discussion: Terminal #135 is considered to be a common terminal relative to terminals #134 and #136. Continuity between #135 and #134 or between #135 and #133 depends on the switch position. The same can be said for terminal #132 relative to terminals #131 and #133. Because the switch has two common terminals and two switch positions, it is called a “double-pole, double-throw” (abbreviated DPDT) switch. 13. Diode test: Diodes are semiconductor devices that only allow current to pass in one direction. In previous tests, the resistance measurements did not depend on how the test leads are connected to the device under test; but when you test a diode, there is a polarity difference. If the diode is good, you should obtain a valid resistance reading in one direction (the forward conducting direction); and in the other direction (the reverse non-conduction direction), you should show an over-range indication. If the diode is bad, both readings are the same--either a valid reading or an over-range indication. Set your multimeter range switch on the diode test range and perform the following diode tests? In the diode test, do not change ranges when the display shows the OVERRANGE. Be sure to indicate if each device is good or bad! Germanium diodes: Diode test125, 126 = _______ Diode test126, 125 = ________ Good/Bad Silicon diode: Diode test129, 130 = _______ Diode test130, 129 = ________ Good/Bad LEDs: Diode test31, 32 = _______ Diode test32, 31 = ________ Good/Bad Diode discussion: The silicon diodes have less reverse-bias leakage than germanium diodes, but germanium diodes have a lower forward conduction voltage than silicon diodes, as shown by the lower value of the forward-bias diode test value. Both silicon and germanium devices should show over-range in the reverse direction. The LEDs (light-emitting diodes) also have a forward/reverse conduction characteristic. In reverse bias, the LEDs should exhibit an over-range indication; but in forward bias, there should be a proper reading. In an LED in forward bias, the recombination of current carriers in the GaAs (gallium arsenide) substrate produces visible red light. Performing the diode test should result in visible light when you test the LED in the forward direction. The recombination of carriers in forward bias in silicon and germanium diodes also produces light, but the emitted light is in the invisible infra-red part of the electromagnetic spectrum. 5 Laboratory 1 & 2 Combined: An introduction to the digital multimeter. 14. In your own words, briefly state the purpose of this laboratory exercise? 6 Laboratory 1 & 2 Combined: An introduction to the digital multimeter. Laboratory 1 & 2 Combined Discussion: Digital Multimeters There are many manufacturers of digital multimeters, and there are at least three brands available for use in the EE-1301 laboratory. All three of them provide the user with generally the same capability. This “introduction” is an outline of how to use these multimeters to make electrical measurements. As the name “multimeter” implies, the instrument provides the user with the capability to measure multiple electrical properties. Properties such as electrical pressure or voltage (measured in volts), the flow of electric current (measured in amperes), and the resistance to flow (measured in ohms) can all be measured with the same instrument. The adjective “digital” refers to the number format of the measured electrical information as it is presented to the user. An “analog” multimeter uses the deflection of a needle to indicate the magnitude of the quantity being measured. Digital multimeters have the following common features: An “on/off” switch. An internal battery or other power source. A “common” input terminal. A “volt & ohm” input terminal, usually written as V . An “ampere” input terminal. A “milliampere” input terminal. A “selector” button mechanism or rotary switch (or both) to choose which property to measure and the proper range. A “digital display” panel which includes a “polarity” indicator and an “overload” indicator. A set of leads to connect the multimeter to the circuit. Seven types of measurements: AC and DC volts from 200 millivolts full scale to 1000 volts full scale. AC and DC current from 200 microamperes to 10 amperes full scale. DC resistance from 200 ohms to 20 megohms full scale. The continuity buzzer is useful for testing for the presence of an electrical connection (continuity) between the test leads. The diode test is useful for checking the forward and reverse characteristics of PN junction diodes, LEDs, and transistors. 7 Laboratory 1 & 2 Combined: An introduction to the digital multimeter. Connecting the test leads: Voltage: The meter measures the voltage difference (or electrical pressure difference) between the voltage at the “V " input and the voltage at the “COM” input. Generally, the red lead is inserted into the “V " input and the black lead into the “COM” input, although the color convention can be reversed if desired. Testing a battery with the “+” connected to the “V " lead and the “-” connected to the “COM” lead will result in a positive reading. Reversing the battery connections will produce a negative reading. Resistance: The meter measures resistance by imposing a small voltage between the “V " input and the “COM” input. By measuring the current that flows when an unknown resistance is connected to the test leads, the meter determines the unknown resistance value. Generally, the red lead is inserted into the “V " input and the black lead into the “COM” input, although the color convention can be reversed if desired. Reversing the connections while measuring resistance gives the same result unless you are testing a diode or a transistor. Current: The meter measures the current flowing into the A or mA input. The red lead is generally plugged into either the “A” input or the “mA” input, while the black lead is plugged into the “COM” input. To measure current in a particular lead or wire, the wire must be disconnected, and the meter leads are connected to the disconnected lead and the point of disconnection. In this way, the unknown current is forced to flow through the meter. This type of current measurement is called a direct measurement of current, as opposed to an indirect measurement of current where the current is calculated using Ohm’s Law from a voltage drop reading across a known resistance. The drawing to the right shows an example of the direct measurement of current flowing from terminal 77 to terminal 13. Selecting the proper range: For the greatest accuracy, one should always use the lowest range that does not give an “overrange” indication. Using a higher range still gives a valid reading, but there is a reduction in the number of significant figures in the answer. The overrange indication: If the quantity being measured is larger than the full-scale quantity, the reading will display the overrange indication. For most of the meters in Modern Electronic Technology, the overrange reading is simply a “one” followed by a “decimal” and “blanks” in each of the digit locations. For example, a 1.5 volt AA battery read on the 200 mV range will give the “overrange” indication. In another example, on any of the resistance ranges with the test leads 8 Laboratory 1 & 2 Combined: An introduction to the digital multimeter. not connected, the resistance between the test leads is effectively “infinity” which is larger than any of the test ranges. Hence, unconnected leads in the ohmmeter test always produce the “overrange” indicator. Examples of voltage readings of a 1.5 volt AA battery on different scales. DC Volts Range 1K 200 20 2 200m Reading 002. 1.6 1.56 1.559 1. Units volts volts volts volts (overrange) Examples of electrical quantities: Voltage: 1,000,000 volts 50,000 volts 25,000 volts 120 volts 12 volts 3.7 volts 3.3 volts 2.0 volts 1.5 volts 1.2 volts 0.001 volts 0.000001 volts Current: 600 amps 6 amps 0.83 amp 0.040 amp 0.000001 amp Resistance: 100,000,000 ohms 470,000 ohms 50,000 ohms 470 ohms 144 ohms 60 ohms 6 ohms 0.000001 ohms 1 MV 50 KV 25 KV 1 mV 1 V AC AC DC AC DC DC DC DC DC DC AC AC Power transmission lines Primary distribution lines Voltage on TV picture tube Household voltage (60 Hz) Automotive voltage Voltage from lithium-polymer battery Voltage from lithium 123 battery Voltage from lead-acid wet cell Voltage from carbon-zinc battery Voltage from nickel-cadmium battery Voltage from tape-deck pickup Voltage at radio receiver antenna 40 mA 1 A DC DC AC DC DC Current in car starter Current in car headlight Current in 100 watt light bulb Current in portable AM/FM receiver battery Current in quartz watch battery DC DC DC DC AC DC DC DC Resistance of reverse-biased diode Largest resistance in lab kit Resistance of control (R26, 28) in lab kit Moderate resistance in lab kit Resistance of lighted 100 watt light bulb Resistance of transformer (R3,5) in lab kit Resistance of radio coils in lab kit Resistance of 1 cm cube of metal 100 M 470 K 1 9 Laboratory 1 & 2 Combined: An introduction to the digital multimeter. On resistors and resistance. The first time I ever studied (a euphemism for torn up or destroyed) the inside of a radio (at about age 9), I asked my dad the name of some of the parts found inside. He didn’t know so he referred me to a local radio-TV repair shop. There, a friendly technician told me that some of the elements were resistors. "What do the resistors do?", I asked. (Probably one of a thousand questions that I pestered him with that day. He got even with me a few years later, but how he did will have to wait for our study of television.) The technician patiently answered, "the resistors hold back the flow of electricity." And I replied, with all the logic of a nine year old, "wouldn't this radio have worked better if the electricity wasn't held back?" That was a good comment, and the best way to think about the role of resistors in circuits is that they behave somewhat like the throttle in your automobile--you don't always want to go full speed ahead. ********** Notes on batteries: A battery consists of two different electrodes in contact with a liquid, paste, or gel electrolyte. The electrolyte chemically reacts with the electrodes to produce a voltage difference between the two electrodes. If the battery is connected into a circuit, the voltage produced by the chemical reaction results in the flow of current. The magnitude of the current can be calculated using Ohm’s Law. As current is produced, one electrode is gradually consumed. Gasses may also be given off as the electrodes react with the electrolyte. All batteries deliver DC (direct current). I am not aware of any battery that delivers AC by itself, although there are methods for converting the DC to AC. Your standard AAA, AA, C, or D battery is a zinc-carbon dry cell, and it delivers about 1.5 volts DC. The zinc-carbon refers to the materials that make up the electrodes of the battery. Zinc-carbon batteries are not rechargeable. Alkaline batteries are similar to zinc-carbon batteries in composition of the electrodes and the output voltage, but a different electrolyte gives a lower internal resistance and a greater output current. Alkaline batteries are not rechargeable. Silver oxide and lithium batteries are also examples of dry batteries that are not rechargeable. 10 Laboratory 1 & 2 Combined: An introduction to the digital multimeter. Nickel-cadmium or nickel-metal-hydride batteries deliver about 1.2 volts per cell, and they are rechargeable. Lead-acid batteries also are rechargeable, and they deliver about 2.0 volts per cell. An automobile battery is made up of six lead-acid cells connected in series for a nominal 12 volt battery. Lithium 123 batteries deliver about 3.3 volts per cell, and they are rechargeable. Lithium polymer batteries deliver about 3.7 volts per cell, and they are rechargeable. Fresh batteries generally produce a voltage from 0.1 to 0.2 volts greater than the nominal voltage for that battery type. As the stored energy is depleted, the voltage gradually falls and internal resistance increases until there is insufficient voltage at the terminals to operate the intended application. At that time, dry batteries must be replaced, or rechargeable batteries must be charged. Series and Parallel Your multimeter can be used to measure (or check) the voltage of a battery. Set your multimeter to read DC volts on the 20 volt scale. To measure the voltage of a single AA battery as shown to the left below, connect the V lead to the “+” terminal and the COM to the “-” terminal. Single battery. Batteries in series. Batteries in parallel. The voltage of batteries in series is the sum of the individual voltages! Series combinations of batteries are used when a larger voltage is needed. The middle drawing shows the multimeter measuring the voltage of batteries in series. The voltage of identical batteries connected in parallel is the same as the individual batteries! Parallel combinations of batteries are needed when additional current capacity (or longer battery life) is needed. The right-hand drawing shows the voltage measurement of batteries in parallel. 11 Laboratory 1 & 2 Combined: An introduction to the digital multimeter. 12