'Ohmmeter\r'

'Resistance Measurement east northeast 240 Electrical and Electronic Measurement (2/2008) Class 8, January 14, 2009 Werapon Chiracharit, Ph. D. , ENE, KMUTT werapon. [email&#clx;protected] ac. th 1 Analogue Ohmmeter • Permanent-magnet moving-coil (Galvanometer, ? ? I ) with a total resistance Rg • series type ohmmeter with shelling E • Resistance R to be metrical • Rz to be zero-ohm-adjusted Rz R E + †Rg 2 1 Zero-Ohm jellment • Short circuit at the terminals 0? Resistance reading is zero, R = 0 • jell Rz for a full-scale current reading E = Ifsd (Rz + Rg) Ifsd = E / (Rz + Rg) • E and Rg atomic number 18 constant • veer Rz ( convince Ifsd) for multirange 3 Zero-Ohm Adjustment (Cont’d) • for the series type ohmmeter E = I (R + Rz + Rg) I = E / (R + Rz + Rg) • R increased, I decreased, ? decreased • Relationship between I and R is non-linear, it means a non-linear resistance scale. • Rz and Rg are small, wherefore for high resistances, the scale points are very conclusion together! 4 2\r\nShunt pillow slip 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 Shunt sheath Ohmmeter (Cont’d) • When R is connected, the current passing through the meter is rock-bottom 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 (Cont’d) • The current I is divided into 2 parts. IgRg = IRR Ig = I †IR = I †IgRg/R wherefore 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 d istich method • Bridge methods are used for measurement of resistance, capacitance, inductance, and so forth • e. g. the network get out be balanced when the detector reading becomes zero. Component Being Measured Bridge Network Detector 9 Wheatstone Bridge • DC supply, Vs • Output voltage, Vo\r\nB R1 I1 A I2 R3 D + Vs †R4 10 R2 Vo C 5 Wheatstone Bridge (Cont’d) • When Vo = 0, the potential 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 sovereign of Vs 11 Wheatstone Bridge (Cont’d) • R2 and R4 are cognise-fixed resistances. • R3 can be adjusted to give the zero potential passing 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 (Cont’d) • • • • • tilt in R1, change R3 Precision about 1 ? to 1 M?\r\nAccurac y is up to the known resistors. Sensitivity of the null detector Error comes from changes in resistances by changes in temperatures. 13 Wheatstone Bridge (Cont’d) • 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 (Cont’d) • 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)â€Vo = Vs R1+? R1 †R3 R1+? R1+R2 R3+R4 â€Vs R1 †R3 R1+R2 R3+R4 = Vs R1+?\r\nR1 †R1 R1+? R1+R2 R1+R2 15 Wheatstone Bridge (Cont’d) • If small changes ? R1 >R3 and Rs1//R3 to Rs1 bend the leakage effect • Rs2 may affect the R3 R4 detector sensitivity 24 12 Bridge honorarium • The resistance of long leads leave behind be affected by changes in temperatures • To avoid this, 3 leads are required to connect to the coils • They are all the equal length and resistance 25 Bridge recompense (Cont’d) • Any changes in lead resistance will affect all 3 leads equally and get along in 2 arms of bridge and will cancel out. 3 R1 1 2 R3 Vs Vo R4 26 R2 13\r\n'