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

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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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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

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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...

...arithmetic mean of a set of readings of length of an object is 6.823m and standard deviation is 0.01m. Out
of 100 readings how many will fall within (a) ± 0.005m, (b) ± 0.02m, (c) ± 0.05m and (d) more than + 0.001m of
mean value. (Use normal probability density tables.) [(a)38, (b)95, (c)100, (d)46]
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SECTION 3: SERIES OR SHUNT OHMMETER.
Q7. It is desired to construct a series ohmmeter with a mid scale indication of 5000 ohms. The instrument used
requires 0.5 mA for full scale deflection and has a resistance of 50 ohms. The battery voltage is 3.00V.
(a) Find the values of resistors needed in the ohmmeter circuit. [4958.33 Ω ≈ 5 kΩ, 250 Ω]
(b) Find the range of R2 needed to zero the ohmmeter if the battery voltage may range from 2.70V to 3.10V. (Use
R1 as in (a)) [208.33 Ω to 625 Ω]
(c) If the instrument accuracy is 0.5% of full scale, what is the possible inaccuracy in resistance value indicated at
30% scale deflection due to the instrument alone? [11.6 kΩ ± 277.78 Ω]
(d) Find the point of minimum percentage inaccuracy due to the instrument. [50% of scale range]
R1
G
A
X
R2
B
Fig. Q7.
Q8. A shunt type ohmmeter uses a milli-ammeter with a full scale current of 2mA, a resistance of 25 ohms, and a
guaranteed accuracy of 1% of full scale. The battery has an emf of 1.5V.
(a) Find the value of R1 [725 Ω]
(b) At what percentage of full scale will 100 ohms be marked on...

...UNIVERSITI TEKNIKAL MALAYSIA MELAKA (UTeM)
FALKULTI KEJURUTERAAN ELEKTRONIK & KEJURUTERAAN KOMPUTER (FKEKK)
BACHELOR DEGREE OF ELECTRONIC ENGINEERING (TELECOMUNICATION ELECTRONIC) (BENT)
BENE 1183 ELECTRONIC INSTRUMENTATION
TITLE : SERIES TYPE OHMMETER
SUBJECT : BENE 1183 ELECTRONIC INSTRUMENTATION
1.0 Title:
Reproduce the series type Ohmmeter using PMMC of Ifsd = 30 μA for range of 0-10 kΩ.
2.0 Objectives :
1. To design a series type Ohmmeter using PMMC.
2. To understand the properties of series type Ohmmeter.
3. To design a series type ohmmeter when Ifsd=30μA is given.
4. To calculate the half scale deflection and full scale deflection of the series type ohmmeter from zero to infinity.
3.0 Equipment and component:
No. | Equipment | Quantity | No. | Component | Quantity |
1. | DMM CD771(Series no:IN4/04/1260 ) | 1 | 5. | Crocodile clip | 4 |
2. | DC Ammeter(Series no:81BA3893 ) | 1 | 6. | Decade Resistance Box(Series no:VR1/1030/INST ) | 1 |
3. | Power supply | 1 | 7. | Resistor 1.0kΩ | 2 |
4. | Project board | 1 | 8. | Resistor 10kΩ | 1 |
4.0 Theory
Moving Coil Instruments
Moving Coil Instruments (MC): The moving coil instruments can be of permanent magnet type (PMMC) or dynamometer type. The permanent magnet, polarized instruments are used for the...