Ohmmeter

Resistance standard ENE 240 galvanic and Electronic criterion (2/2008) Class 8, January 14, 2009 Werapon Chiracharit, Ph. D. , ENE, KMUTT werapon. emailprotected ac. th 1 par tout ensembleel of latitude Ohm touchstone Permanent-magnet moving-coil (Galvanometer, ? ? I ) with a total shelter Rg Series attribute ohmmeter with battery E Resistance R to be deliberate Rz to be energy-ohm- adjust Rz R E + Rg 2 1 Zero-Ohm adjustment Short lot at the terminals 0? Resistance see is zero, R = 0 Adjust Rz for a full-scale on-line(prenominal) reading E = Ifsd (Rz + Rg) Ifsd = E / (Rz + Rg) E and Rg atomic number 18 unceasing Change Rz (change Ifsd) for multirange 3 Zero-Ohm Adjustment (Contd) for the series type ohmmeter E = I (R + Rz + Rg) I = E / (R + Rz + Rg) R increased, I reduced, ? decreased Relationship mingled with I and R is non-linear, it means a non-linear opposite scale. Rz and Rg argon small, then for mettlesome subways, the scale points ar very croc ked together 4 2 bypass theatrical role Ohmmeter When R = ? (open circuit), R1 is adjusted for a full-scale reading. E = Ifsd (R1 + Rg) Ifsd = E / (R1 + Rg) R1 R Ig IR Rg E 5 I ring road Type Ohmmeter (Contd) When R is drawed, the current enactment through the meter is reduced by shunt resistor. 1/Rparallel = 1/R + 1/Rg Rparallel = RRg / (R + Rg) and E = I (R1 + Rparallel) = I (R1 + RRg/(R + Rg)) = I (R1R + R1Rg + RRg) / (R + Rg) = I (R1Rg + R(R1 + Rg)) / (R + Rg) 6 3 transfer Type Ohmmeter (Contd) The current I is divided into 2 parts. IgRg = IRR Ig = I IR = I IgRg/R thus Ig = E(R + Rg)/(R1Rg + R(R1 + Rg)) IgRg/R Ig(1+Rg/R) = E(R + Rg)/(R1Rg + R(R1 + Rg)) Ig(R+Rg)/R = E(R + Rg)/(R1Rg + R(R1 + Rg)) Ig = ER / (R1Rg + R(R1 + Rg)) measuring rod reading depends on the value of R, though R is a low resistance. 7 Series Ohmmeter Shunt Ohmmeter 8 4 duo order couple deck methods argon used for touchstone of resistance, capacitance, inductance, etc. e. g. the web will b e residuald when the detector reading becomes zero. particle Being thrifty link up web Detector 9 Wheatstone straddle DC supply, Vs Output voltage, VoB R1 I1 A I2 R3 D + Vs R4 10 R2 Vo C 5 Wheatstone connect (Contd) When Vo = 0, the strength at B moldiness equal to the potential at D I1R1 = I2R3 I1R2 = I2R4 Hence I1R1 = I2R3 = (I1R2/R4) R3 R1/R2 = R3/R4 The balance condition is independent of Vs 11 Wheatstone Bridge (Contd) R2 and R4 are known-fixed resistances. R3 tooshie be adjusted to give the zero potential divagation condition. R1 is the input resistance to be measured. A R1 Adjust R3 B Vo = 0 G B D Wheatstone Bridge 12 6 Wheatstone Bridge (Contd) Change in R1, change R3 precision about 1 ? to 1 M?Accuracy is up to the known resistors. aesthesia of the null detector Error comes from changes in resistances by changes in temperatures. 13 Wheatstone Bridge (Contd) If no galvanometer at the output, VAB = Vs R1/(R1+R2) VAD = Vs R3/(R3+R4) Thus, Vo = VAB V AD Vo = Vs ( R1/(R1+R2) R3/(R3+R4) ) The relationship between Vo and R1 is non-linear 14 7 Wheatstone Bridge (Contd) A change R1 to R1+? R1 gives a change Vo to Vo+? Vo Vo+? Vo=Vs((R1+? R1)/((R1+? R1)+R2) R3/(R3+R4)) thence (Vo+? Vo)Vo = Vs R1+? R1 R3 R1+? R1+R2 R3+R4 Vs R1 R3 R1+R2 R3+R4 = Vs R1+?R1 R1 R1+? R1+R2 R1+R2 15 Wheatstone Bridge (Contd) If small changes ? R1 R3 and Rs1//R3 to Rs1 ward off the leakage encumbrance Rs2 may concern the R3 R4 detector sensitivity 24 12 Bridge honorarium The resistance of pine leads will be cloaked by changes in temperatures To suspend this, 3 leads are required to connect to the coils They are all the same continuance and resistance 25 Bridge pay (Contd) Any changes in lead resistance will affect all 3 leads equally and egest in 2 arms of bridge and will come up out. 3 R1 1 2 R3 Vs Vo R4 26 R2 13