1302_LabManual_sp13

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Unformatted text preview: ge, the DMM must be placed in the circuit so that the potential difference across the circuit element you want to measure is across the DMM. 3. If no number appears, try a different measurement scale. Start at the highest voltage scale and work your way down the scales until you get a satisfactory reading. Measuring Resistance: The element whose resistance you are measuring must be free from all other currents (due to other batteries, power supplies, etc.) for the DMM to work. That means you must remove it from a circuit. To measure resistance: 1. Set the DMM selection dial to measure ohms (). Insert one wire into the socket labeled 'V' and a second wire into the socket labeled 'COM'. 2. Make sure that the circuit element whose resistance you wish to measure is free of any currents. Attach the wires across the circuit element, as shown in the example below. 4. If no number appears, try a different measurement scale. Use a logical method that covers all scales, such as beginning at the largest scale (20 M) and working your way down. 3. 201 APPENDIX: EQUIPMENT A Brief Introduction to RMS Measurements: A problem arises when one wishes to measure an alternating current or potential. All measuring instruments sample a signal over some period of time. A device that samples over a time longer than one period of the signal (such as the DMM) essentially measures the average signal. For sine or cosine functions, the average is zero, which doesn't tell you much about the signal strength. The solution to this difficulty is to use root-mean-square (RMS) averaging. To eliminate the cancellation of the positive and negative parts of the sine function, it is squared, then the average is taken1, and the square root of this average yields the RMS value. For example, to find the RMS value of an AC current that has a maximum value of Io: I t I 0 sin t I 2 t I 02 sin 2 t I 2 1 2 2 I sin 2 t d t 0 2 2 0 I 2 2 0 1 sin t d t 2 I 2 2 0 0 I RMS I2 1 I0 2 When in AC mode, your DMM displays the RMS values of current and voltage. 1 When a quantity that varies with time is averaged, as in this case, the average value is often designated by putting angle brackets around the quantity. For example, the time average of a sinusoidally varying current is: I I 0 2 2 sint dt 0 0 202 APPENDIX: EQUIPMENT CATHODE RAY TUBE (CRT) AND ACCESSORIES: Use of the cathode-ray tube and its relatives is widespread. It is the heart of many familiar devices, from your computer monitor to your television. The following is a sketch of the tube you will be using and its connections. D tot V acc Figure 3: Cathode Ray Tube. D Deflection plates D = 7.4 cm L = 2.0 cm S = 0.30 cm Dtot = 9.6 cm 6.3 V AC L electron beam S Electron gun Vy Vx How the CRT works: Within the electron gun: • A thin filament (represented above as a coil of wire), similar to a light-bulb filament, is heated by a current. When the CRT is operating, this filament can be seen as an orange, glowing wire. This hot filament ejects slow-moving electrons. • Some slow electrons drift toward the high-voltage “acceleration plates.” These plates are labeled as Vacc in Figure 3. The electric field between the charged plates accelerates the electrons to high velocities in the direction of the fluorescent screen. The final velocity of an accelerated electron is much greater than its initial “drift” velocity, so the initial electron velocity can be ignored in calculations. After the electron gun: • Before hitting the screen, the high-velocity electrons may be deflected by charged plates along the length of the CRT. These charged plates are usually called the “x-deflection” and “y-deflection” plates. • When the electrons reach the end of the tube, their energy causes the material that coats the end of the tube to glow. This material is similar to the material inside fluorescent light bulbs. The end of the CRT is called the fluorescent screen. To supply the necessary electric potentials to the CRT you will use a power supply. The power supply provided has the proper potential differences to heat the CRT filament and to accelerate the electrons. The power supplies we use also have built-in circuit breakers. Should you attempt to draw too much current from your power supply, it will shut itself off with an audible “click.” If this happens, check to make sure all of your wires are connected properly, then press in the small white button on the side of the power supply. Note that the CRT and power supply come as a set, and many of the connections are color-coordinated to avoid potentially damaging misconnections. You will also have an assortment of batteries, which will be used to control the electric field between the CRT x- and y-deflection plates. WARNING: You will be working with equipment that generates large electric voltages. Improper use can cause painful burns. To avoid danger, the power should be turned OFF and you should WAIT at least one minute before any wires are disconnected from or connected...
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This document was uploaded on 02/23/2014 for the course MANAGMENT 2201 at University of Michigan.

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