Ohmmeter

foeman Measurement ENE 240 Electrical and Electronic Measurement (2/2008) Class 8, January 14, 2009 Werapon Chiracharit, Ph. D. , ENE, KMUTT werapon. emailprotected ac. th 1 Analogue Ohmmeter Permanent-magnet moving-coil (Galvanometer, ? ? I ) with a total opponent Rg Series type ohmmeter with battery E Resistance R to be measured Rz to be zero-ohm- alter Rz R E + Rg 2 1 Zero-Ohm fitting suddenly circuit at the terminals 0? Resistance schooling is zero, R = 0 Adjust Rz for a complete current reading E = Ifsd (Rz + Rg) Ifsd = E / (Rz + Rg) E and Rg are constant Change Rz ( switch over 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 decreased, ? decreased blood between I and R is non-linear, it means a non-linear resistance exceed. Rz and Rg are sm each(prenominal), then for high resistances, the scale points are very close together 4 2Shunt Type Ohmmeter When R = ? (open circ uit), R1 is adjusted for a full-scale reading. E = Ifsd (R1 + Rg) Ifsd = E / (R1 + Rg) R1 R Ig IR Rg E 5 I Shunt Type Ohmmeter (Contd) When R is connected, the current passing through the meter is reduced by ringway 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 Type Ohmmeter (Contd) The current I is divided into 2 parts. IgRg = IRR Ig = I IR = I IgRg/R therefore 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)) Meter reading depends on the value of R, though R is a low resistance. 7 Series Ohmmeter Shunt Ohmmeter 8 4 Bridge Method Bridge methods are used for measurement of resistance, capacitance, inductance, etc. e. g. the network result be balanced when the detector reading becomes zero. Component Being Measured Bridge Ne twork sensing element 9 Wheatstone Bridge DC supply, Vs Output voltage, VoB R1 I1 A I2 R3 D + Vs R4 10 R2 Vo C 5 Wheatstone Bridge (Contd) When Vo = 0, the potential at B must equal to the potential at D I1R1 = I2R3 I1R2 = I2R4 consequently 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 can be adjusted to slide by the zero potential difference 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. Sensitivity of the zilch 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 VAD 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)) Then (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 avoid the leakage effect Rs2 may affect the R3 R4 detector sensitivity 24 12 Bridge Compensation The resistance of long leads will be affected by changes in temperatures To avoid this, 3 leads are required to connect to the coils They are all the same length and resistance 25 Bridge Compensation (Contd) Any changes in lead resistance will affect all 3 leads equally and occur in 2 arms of bridge and will cancel out. 3 R1 1 2 R3 Vs Vo R4 26 R2 13