Ohmmeter

Resistance quantity 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 resistor Rg Series fictitious character ohmmeter with battery E Resistance R to be measured Rz to be zero-ohm- set Rz R E + Rg 2 1 Zero-Ohm congealment Short locomote at the terminals 0? Resistance reading is zero, R = 0 Adjust Rz for a full-scale menstruation reading E = Ifsd (Rz + Rg) Ifsd = E / (Rz + Rg) E and Rg are constant Change Rz (change Ifsd) for multirange 3 Zero-Ohm Adjustment (Contd) for the series flake ohmmeter E = I (R + Rz + Rg) I = E / (R + Rz + Rg) R increased, I fall, ? decreased Relationship between I and R is non-linear, it means a non-linear resistance scale. Rz and Rg are small, then for high resistances, the scale points are very pissed together 4 2Shunt Type Ohmmeter When R = ? (ope n 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 Shunt Type Ohmmeter (Contd) When R is connected, the current passing 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 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)) pulse reading depends on the value of R, though R is a deplorable resistance. 7 Series Ohmmeter Shunt Ohmmeter 8 4 twosome mode span methods are used for measurement of resistance, capacitance, inductance, etc. e. g. the network pull up stakes be balanced when the detector reading becomes zero. Component Bein g calculated yoke Network Detector 9 Wheatstone brace DC supply, Vs end product voltage, VoB R1 I1 A I2 R3 D + Vs R4 10 R2 Vo C 5 Wheatstone link (Contd) When Vo = 0, the capableness at B must 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 can be adjusted to give 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 slide fastener 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 betwee n 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 capacious endures will be affected by changes in temperatures To avoid this, 3 leads are required to connect to the coils They are all the same duration and resistance 25 Bridge Compensation (Contd) Any changes in lead resistance will affect all 3 leads equally and make it in 2 arms of bridge and will cancel out. 3 R1 1 2 R3 Vs Vo R4 26 R2 13