EE-383 Instrumentation and Measurements
Course Information
Course Number and Title: EE-383 Instrumentation and Measurements
Credits: 4 (3+1)
Instructor(s)-in-charge: Dr Mazhar Abbas (Asst Prof)
Course type: Lecture + Lab
Required or Elective: Required
Course pre-requisites EE-215 Electronic Devices and Circuits
Degree and Semester DE-42 Department of Electrical Engineering, Semester 5
Month and Year Fall 2022
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Contents
Chapter 5: DC Ammeter
Introduction
Working Principle
Types
Loading effect
Describe and Explain working of Analog and Digital
CLO 2 Electrical-Measuring Instruments, Transducers, Transducer PLO 2 C2
interfacing, and Instrument Communication Protocols
C2 : Focus on the understanding of construction and working principles
Course book and Related Course Material
Textbook: 1. Principles of Electronic Instrumentation and Measurement by Berlin & Getz
1. Electronic Instrumentation and Measurement by David A Bell, 1994
Reference Books: 2. Electrical and Electronic Measurement & Testing by W Bolton, 1992
3. Process Control Instrumentation Technology, 8th Ed. by Curtis D. Johnson
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Chapter 5: DC indicating meters
DC Ammeter
The PMMC instrument could be used as
an ammeter to measure DC current (from
few microamperes to mill amperes).
However, the PMMC wire could be quickly
destroyed by large currents.
For large currents (several hundreds of am
peres), a PMMC could be modified by
adding a parallel (shunt) resistance Rs.
Most of the measured current will pass
through Rs and a small portion of it will
pass through the moving coil.
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Chapter 5: DC indicating meters
DC Ammeter
𝑽𝑺𝒉𝒖𝒏𝒕 = 𝑽𝒎𝒐𝒗𝒆𝒎𝒆𝒏𝒕
𝑰 𝑴 𝑹𝑴
𝑰𝑺 𝑹𝑺 = 𝑰𝑴 𝑹𝑴 𝑹𝑺 =
𝑰𝑺
𝑰𝑺 = 𝑰𝑻 − 𝑰𝑴
𝑰𝑭𝑺𝑫 𝑹𝑴
𝑹𝑺 =
𝑰𝑻 −𝑰𝑭𝑺𝑫
𝑰𝑻
𝑵=
𝑰𝑭𝑺𝑫
𝑹𝑴
𝑹𝑺 =
𝑵−𝟏
• 𝑹𝑴 : Meter movement coil resistance
• 𝑹𝑺 : Shunt resistance
• 𝑰𝑺 : Current through shunt resistance
• 𝑰𝑻 : Full scale current through ammeter
• 𝑰𝑭𝑺𝑫 : Full scale current through meter movement coil
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Chapter 5: Example 5-1
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Chapter 5: DC indicating meters
Multiple Range DC Ammeters
Make-before-break Ayrton shunt-type
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Chapter 5: DC indicating meters
DC Ammeter loading effect
Without Ammeter
Sensitivity of Ammeter
Ammeter in the circuit 𝜽 𝑵𝑩𝑨𝒄
k= 𝑰𝒎
=
𝑪
• N : Number of turns in coil
• B : Magnetic field strength
• Ac : Area of coil
• C : Spring torque constant
Resistance of conductor
• A : Cross sectional area of
conductor
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Chapter 5: Example 5-2
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Chapter 5: DC indicating meters
DC Voltmeter
Used for measuring steady voltages from one point to another.
A PMMC movement DC ammeter connected with a series resistor.
Capable of measuring voltages from micro-volts to kilo-volts.
A basic circuit consists of a series resistor, Rs , called multiplier resistor.
With full-scale voltage ,VFSD, applied to the meter circuit, the total
resistance of the circuit must limit the current through the meter to
exactly the value needed for full-scale deflection, IFSD.
𝑹𝑻𝒐𝒕𝒂𝒍 = 𝑹𝒔 + 𝑹𝑴
𝑽𝑭𝒔𝑫
=
𝑰𝑭𝒔𝑫
𝑽𝑭𝒔𝑫
𝑹𝒔 = − 𝑹𝑴
𝑰𝑭𝒔𝑫
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Example
A PMMC instrument with FSD of 100 uA and a coil resistance of 1 k is to
be converted into a voltmeter. Determine the required multiplier resistance if
the voltmeter is to measure 50 V at full scale. Also, calculate the applied
voltage when the instrument indicates 0.2 of FSD.
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Chapter 5: DC indicating meters
Multiple Range DC Voltmeters
The range of this voltmeter is Configuration 2
𝑉 = 𝐼𝑚 (𝑅𝑚 + 𝑅) 𝑹 = 𝑅1 or 𝑅1 + 𝑅2 or 𝑅1 + 𝑅2 + 𝑅3
Configuration 1
𝑹 = 𝑅1 or 𝑅2 or 𝑅3
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Chapter 5: DC indicating meters
Sensitivity factor (S) of DC voltmeter
Sensitivity (ohm-per-volt) of a voltmeter is equal to the resistance per volt.
𝑹 +𝑹 𝟏
𝑺𝒗 = 𝑴 𝒔 = =𝜴/𝑽
𝑽𝑭𝑺𝑫 𝑰𝑭𝑺𝑫
The voltmeter sensitivity is always specified by the manufacturer.
If the sensitivity is known, the total voltmeter resistance is easily calculated
as (Resistance=sensitivity x range).
For example, a voltmeter with a rating of 20 kΩ /V on its 100-V range
would offer a resistance of
Resistance = (20 kΩ /V) x (100 V) = 2 MΩ
The ohms-per-volt rating of a voltmeter is also useful in calculating the size
of multiplier resistor used with a given meter movement, so that
𝑹𝒔 = multiplier resistor
𝑹𝒔 = 𝑺 × 𝑽𝑭𝒔𝑫 − 𝑹𝑴
S = ohms-per-volt rating of the meter
𝑽𝑭𝒔𝑫 = full-scale voltage
𝑹𝑴 = meter movement resistance
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Chapter 5: DC indicating meters
Voltmeter loading effect
An analog voltmeter will draw some amount of deflection current from the
circuit under test.
Thus, the meter acts as an electrical load on the portion of the circuit to
which it is connected.
In addition, this has the same effect as connecting a resistor equal to the
resistance of the voltmeter across that portion of the circuit.
𝑽𝒓𝒆𝒂𝒅
%𝑬𝒓𝒓𝒐𝒓 = 𝟏 − × 𝟏𝟎𝟎%
𝑽𝒂𝒄𝒕𝒖𝒂𝒍
Ideally, a voltmeter should have an extremely high resistance (the higher
the ohms-per-volt rating, the better).
Always be aware of the effects of meter loading and select the proper
meter and range for the application.
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Quiz (Syndicate C)
Q.1(5 marks)
Multiply 10.1 ± 0.1 by 5.3 ± 0.2.
Q.2 ( 5 marks)
A 4-digit (digital) voltmeter is
specified as having an accuracy of
±2% of the reading, plus two counts.
Determine the maximum error and
the percentage error when the meter
reads 100.0V.
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Thank you
Questions/Comments??
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