Electric Circuits 8th Edition 96

Electric Circuits 8th Edition 96 - are difficrlt to measure...

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Simpte Resistive Circuitr R. ft,: *n:. (3.40) Now that we have voriticd rhe validity of Eq.3.33, several conments aboul lhe r'esull are in ordcr. First. note that ifihe ratio R /R1is unily. the unknown resistor R, cquals Rr. ln this case, the bridge resislorn3 must vary over a rangcthat includes the valueR.. For example, iI lhe unknown resistance lvere1000 O and Rl couldbevariedfrom0 to 100 (), the bridge could never be balanced. Th us to covera wide range of unknown resislors. we must be able to vary the ralio R2/Rr. In a commercial Wheatstone bridge, Rr aDd 112 consisl oI dccimal values of resistances that can be switched into the bridge circuil. Normall,v, the decimal values are 1, 10,100, and 1000 O so |hal the ratio R2/Rr can be varied fron 0.001 to 1000 in decimal steps.The variablc rcsistorRr is usuall]' adjustable iD iDle gral values ofresislance from :l to 11,000 O. Altlough Eq.3.33 inplies that R, can vary from zero to infinit),. the practical rarge ofR, is approxinalely I () to 1 MO. Lower resistances
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Unformatted text preview: are difficrlt to measure or a standard whealslone bridgc because of thermo-eleciric voltages generated at the juncliorls of dissimilar metals and bccause of thermal heating effects lhat is, i'R eIiecls. Highcr rcsjstances arc ditTicult to measure accurately because of leakage currcnts. In other words. if R, is large, the current ieakage in lhe eleclrical insulation may be conparable to the curfent in the branches oI lhe b dgc circuit. . obiective s-lJnderstand how a Wheatstone bridoe is used to measure resistance The bridge circuit shown is balanced wher R1 : 100 O. ./1, - 1000 O,and Rj - 150 O. Thc bridgc is energized from a 5V dc source. a) What is the value ofR,? b) Supposc each bridge resistor is capable of dissipating 250 mW Can the bridge be left in thc balanced state without exceeding the power dissipating capacity ofthe resistors, thereby damaging thc brjdgc l NOTE: AIso tr! Chupter Problem 3.4ti. Answer: (a) 1500 {); (b) yes....
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