Ohmmeter

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  • Topic: Measuring instrument, Ohmmeter, Wheatstone bridge
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Resistance Measurement

ENE 240 Electrical and Electronic Measurement (2/2008) Class 8, January 14, 2009 Werapon Chiracharit, Ph.D., ENE, KMUTT werapon.chi@kmutt.ac.th

1

Analogue Ohmmeter
• Permanent-magnet moving-coil (Galvanometer, θ ∝ I ) with a total resistance Rg • Series type ohmmeter with battery E • Resistance R to be measured • Rz to be zero-ohm-adjusted Rz R E + – Rg
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Zero-Ohm Adjustment
• Short circuit at the terminals 0Ω • Resistance reading is zero, R = 0 • Adjust Rz for a full-scale current reading E = Ifsd (Rz + Rg) Ifsd = E / (Rz + Rg) • E and Rg are constant • Change Rz (change 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, θ 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 close together! 4

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Shunt Type 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
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I

Shunt Type Ohmmeter (Cont’d)
• 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 (Cont’d)
• 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.

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Series Ohmmeter

Shunt Ohmmeter
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Bridge Method
• Bridge methods are used for measurement of resistance, capacitance, inductance, etc. • e.g. the network will be balanced when the...
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