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2LB_Lab_7_DCElecCircuits11 - Physics 2LB DC Electrical...

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Physics 2LB DC Electrical Circuits (*** Includes Pre Lab Assignment) Materials: Two digital multimeters, 6 V battery, resistor-diode circuit board, potentiometer, Computer with Science Workshop 750 Interface and hook-up cables. Purpose: To become familiar with digital multimeters used for measurement of voltage, current, and resistance; to test Ohm's Law; to measure the I-V characteristics of non-linear circuit elements. INTRODUCTION An ideal device for measurement of voltage, current, or their ratio (resistance) in a circuit, would do so without in any way perturbing the circuit being measured. Real devices can approach this ideal in varying degrees. Thus a reasonably rugged and portable old style voltmeter, based upon a moving coil galvanometer, would typically draw one milliampere of current at full-scale deflection. If a range of 0–1 V was to be covered, a series resistance of 1 k (including coil resistance) would be connected to the coil to complete the device. The input impedance of this voltmeter would then be 1 k , low enough to lead to serious error in the measurement of voltage sources having output impedances of this order or higher. A modern voltmeter (Digital Multimeter) based upon solid-state circuitry will have an input impedance of 10 7 - 10 12 . The digital multimeter to be used here has high enough input impedance that for purposes of voltage measurement in this experiment , it may be considered as ideal. Current measurement can be accomplished by inserting a moving-coil galvanometer into the circuit. The galvanometer will include low value shunt resistors for higher current ranges. An alternative would be to measure the voltage drop across a low value resistor using a high impedance solid-state device. In either case an appreciable series resistance must be introduced into the circuit in which current is being measured. This resistance will be evaluated in a typical situation below. Resistance could be measured by a bridge technique or, as will be the case here, by passing a known current through the unknown resistor and measuring the voltage drop across it. Accuracy of this method requires both a high impedance current source and a high impedance voltmeter. For purposes of this experiment both conditions are satisfactorily met by the digital multimeters to be used. PROCEDURE A. Basic Measurements of Voltage, Resistance, and Current Using a Digital Multimeter (DMM) [1.5] 1. Set the DMM for DC voltage measurements by rotating the function switch until the black dot on the rotating bar is positioned at the 20 DCV range. Connect the positive (+) terminal of a 6 V battery to the red (V- -Hz) input connector and the negative ( - ) terminal to the black (COM) connector of the DMM.
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