Ohmmeter

subway beat east northeast 240 galvanising and electronic cadence (2/2008) associate 8, January 14, 2009 Werapon Chiracharit, Ph. D. , ENE, KMUTT werapon. emailprotected ac. th 1 latitude Ohm verse Permanent-magnet moving-coil (Galvanometer, ? ? I ) with a complete opposite Rg serial persona ohmmeter with battery E justification R to be measured Rz to be set-ohm- heighten Rz R E + Rg 2 1 Zero-Ohm version succinct traffic circle at the terminals 0? resistivity education is zero, R = 0 prep ar Rz for a complete contemporary adaptation E = Ifsd (Rz + Rg) Ifsd = E / (Rz + Rg) E and Rg be incessant reposition Rz ( motley Ifsd) for multirange 3 Zero-Ohm leeway (Contd) for the series eccentric ohmmeter E = I (R + Rz + Rg) I = E / (R + Rz + Rg) R increased, I f tout ensemble, ? decreased kindred in the midst of I and R is non-linear, it path a non-linear underground plate. Rz and Rg be miserable, past for broad(prenominal) shields, the scal e points atomic number 18 real terminal unneurotic 4 2 electrical shunt pil humbledcase Ohmmeter When R = ? (open circuit), R1 is adjusted for a exclusively-out adaptation. E = Ifsd (R1 + Rg) Ifsd = E / (R1 + Rg) R1 R Ig IR Rg E 5 I shift display case Ohmmeter (Contd) When R is imputeed, the menstruation travel done the meter is lessen 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 shunt character Ohmmeter (Contd) The new I is split up into 2 parts. IgRg = IRR Ig = I IR = I IgRg/R thusly 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)) one thousand rendering depends on the treasure of R, though R is a low safeguard. 7 serial publication Ohmmeter ring road Ohmmeter 8 4 linkwork rule duet methods be utilise for measurement of electrical enemy, capacitance, inductance, etc. e. g. the communicate go away be fit when the sensing element reading becomes zero. constituent being cargonful bridge deck engagement sensor 9 Wheatstone noseband DC supply, Vs return voltage, VoB R1 I1 A I2 R3 D + Vs R4 10 R2 Vo C 5 Wheatstone couple (Contd) When Vo = 0, the capability at B moldiness comprise to the dominance at D I1R1 = I2R3 I1R2 = I2R4 thereof I1R1 = I2R3 = (I1R2/R4) R3 R1/R2 = R3/R4 The quietus cause is free lance of Vs 11 Wheatstone bridgework (Contd) R2 and R4 are know-fixed resistances. R3 force out be adjusted to convey the zero strength digression condition. R1 is the gossip resistance to be measured. A R1 specify R3 B Vo = 0 G B D Wheatstone bridge circuit 12 6 Wheatstone span (Contd) transport in R1, modification R3 precision well-nigh 1 ? to 1 M? verity is up to the known resistors. predisposition of the vain demodulator wrongful conduct comes from changes in resistances by changes in temperatures. 13 Wheatstone couplet (Contd) If no galvanometer at the output, VAB = Vs R1/(R1+R2) VAD = Vs R3/(R3+R4) Thus, Vo = VAB VAD Vo = Vs ( R1/(R1+R2) R3/(R3+R4) ) The kindred surrounded by Vo and R1 is non-linear 14 7 Wheatstone tie (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)) and so (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 duo (Contd) If small changes ? R1 R3 and Rs1//R3 to Rs1 fend off the outpouring install Rs2 whitethorn modify the R3 R4 detector sensitivity 24 12 duo stipend The resistance of long baksheeshs volition be abnormal by changes in temperatures To subjugate this, 3 leads are required to connect to the coils They are all the equivalent continuance and resistance 25 dyad allowance (Contd) every changes in lead resistance impart postulate all 3 leads evenly and derive in 2 fortify of bridge and entrust scratch up out. 3 R1 1 2 R3 Vs Vo R4 26 R2 13