DC bridges and AC Bridges

1. By
Dr. G.N. Kodanda Ramaiah /
P. Siva Nagendra Reddy

2. Introduction
DC & AC Bridge are used to measure
resistance, inductance, capacitance and
impedance.
Operate on a null indication principle. This
means the indication is independent of the
calibration of the indicating device or any
characteristics of it.
Very high degrees of accuracy can be
achieved using the bridges

3. Types of bridges
Two types of bridge are used in measurement:
1) DC bridge:
a) Wheatstone Bridge
b) Kelvin Bridge
2) AC bridge:
a) Similar Angle Bridge
b) Opposite Angle Bridge/Hay Bridge
c) Maxwell Bridge
d) Wein Bridge
e) Radio Frequency Bridge
f) Schering Bridge

4. Wheatstone Bridge
A Wheatstone bridge is a measuring instrument
invented by Samuel Hunter Christie (British
scientist & mathematician) in 1833 and improved
and popularized by Sir Charles Wheatstone in
1843. It is used to measure an unknown electrical
resistance by balancing two legs of a bridge
circuit, one leg of which includes the unknown
component. Its operation is similar to the original
potentiometer except that in potentiometer circuits
the meter used is a sensitive galvanometer.
Sir Charles Wheatstone (1802 – 1875)

5. Thévenin’s Theorem
An analytical tool used to extensively analyze an unbalance bridge.
Thévenin's theorem for electrical networks states that any combination of voltage
sources and resistors with two terminals is electrically equivalent to a single voltage
source V and a single series resistor R. For single frequency AC systems the theorem
can also be applied to general impedances, not just resistors. The theorem was
first discovered by German physicist Hermann von Helmholtz in 1853, but
was then rediscovered in 1883 by French telegraph engineer Léon Charles
Thévenin (1857-1926).
Hermann von Helmholtz (1821 – 1894)
Léon Charles Thévenin (1857-1926)German Physicist
French Engineer

6. Wheatstone Bridge

62. Hay’s Bridge
It is also a modification of the Maxwell’s
Wien Bridge and is particularly useful if
the phase angle of the inductive
impedance is large.
In this case a comparatively smaller series
resistance R1 is used instead of a parallel
résistance.( which has to be of a very
large value) as shown in fig.

63. Fig
L3= C1 R2 R4 R3= w C1 R1 R2 R4
1+w R1 C1 1+w R1 C1

64. Wien Series Bridge
It is a simple ratio bridge and is used for
audio-frequency measurement of
capacitance over a wide range. As shown in
fig.

65. Fig.
R1=R2R4/R3 C1=C4(R3/R2)

66. Wien Parallel Bridge
It is also a ratio bridge used mainly as the
feedback network in the wide range audio-
frequency R-C oscillators.
It is may be used for the measurement of
the audio-frequency but it is not as
accurate as the modern digital frequency
meters. As shown in fig.

67. Fig.
C2 = R2 = R3
C1 R1 R4

69. EMI AMD EMC
 EM interference (EMI): The unwanted effect of EM noise
interfering with our signals
 The absence of Electromagnetic Interference (EMI) in a
system is called Electromagnetic Compatibility (EMC).
(or)
 EM compatibility means the ability of equipment to
function satisfactorily in its EM environment without
introducing intolerable EM disturbances to other equipment
in that environment.

70. INTRODUCTION:
 Electromagnetics (EM) is a branch of physics or electrical engineering in
which electric and magnetic phenomena are studied.
 Electromagnetic interference exist in every communication link. it
manifests itself as noise.
 Electromagnetic interference (EMI) is electromagnetic energy that
adversely affects the performance of electrical/electronic equipment by
creating undesirable responses or complete operational failure.
 Electromagnetic compatibility (EMC) is the ability of electrical or
electronic equipment/systems to function in the intended operating
environment without causing or experiencing performance degradation due
to intentional EMI.
 The most common methods of noise reduction include proper equipment
circuit design, shielding , grounding, filtering, isolation, separation and
orientation and noise cancellation techniques.
70

71. DEFINITION OF EMI & EMC:
EMI???
 Electromagnetic interference is the degradation in the performance of a
device due to the fields making up the electromagnetic environment.
EMC???
 Electromagnetic compatibility is achieved when a device functions
satisfactorily without introducing intolerable disturbances to the
electromagnetic environment.
71

72. ELECTRO MAGNETIC
INTERFERENCE
72

73. 73
Example:
Electromagnetic
interference in
TV signal

74. CAUSES OF EMI/EMC
 SOURCES
 Refrigerators, washing machine, electric motors.
 Arc welding machine.
 Electric shavers, AC, Computers.
 Fast switching digital devices, ICs etc
 Power chords of computers, UPS etc.
 Aircraft navigation and Military equipments.
 VICTIMS
 Communication Receivers.
 Microprocessors, Computers.
 Industrial Controls.
 Medical Devices.
 Household Appliances.
 Living Beings.

75. BASIC ELEMENTS OF EMI
SITUATION
75
Interference occurs if the received energy causes the receptor to
function in unwanted manner.
Whether the receiver is functioning in wanted or unwanted
manner, depends on the coupling path as well as the source and
victim.
The medium is to be made as inefficient as possible.

76. CLASSIFICATION OF EMI:
76

77. RADIATED INTERFERENCE
77
Narrow band interference usually arises from intentional transmissions such as radio
and TV stations, pager transmitters , cell phones etc. It is a high frequency operation .
Example: proximity effect
Broad band interference usually comes from incidental radio frequency emitters.
These includes electric power transmission lines, electric motors etc. It is a low
frequency operation
Example : skin effect

78. CONDUCTED INTERFERENCE
 Conducted electromagnetic interference is caused by the physical contact of the
conductors as opposed to radiated EMI, which is caused by induction (without
physical contact of the conductors).
 Electromagnetic disturbances in the EM field of a conductor will no longer be
confined to the surface of the conductor and will radiate away from it.
 This persists in all conductors and mutual inductance between two radiated
electromagnetic fields will result in EMI
78

79. Differences between conducted and
radiated interference
79

80. 80

81.  Intra system EMI causes  Inter system EMI
causes
81
Effects of EMI
Momentary disturbance in TV and radio reception due to operation of mixer-
grinder/electric shavers/a passing vehicles etc
Reset of computers and loss of data.
Change of setting of status of control equipments.
Failure of pace maker implemented in a patient due to a ‘walkie talkie’
Malfunctioning of flight controlling system due to use of laptop by passenger.
Biological hazards.

82. Sources of EMI:
The sources of EMI can be broadly classified into two groups
 Natural sources of EMI
example: lightning
 Manmade sources of EMI
example: commercial radio and telephone communications
In specific we can classify as
 Functional: EMI can originate from any source designed to generate
electromagnetic energy and which may create interference as a normal part of its
operation
 Incidental: EMI can originate from man made sources .These sources are not
designed specifically to generate electromagnetic energy but which do infact cause
interference.
 Natural: EMI can be caused by natural phenomena, such as electrical storms , rain
particles , solar and interstellar radiation.
82

83. 83
Fig:1
Fig: 2 Fig1-lightning hitting
between the
Buildings.
Fig2-lightning
hitting a tree

84. EMI CONTROL
TECHNIQUES:
To control or suppress EMI, the three
common means employed in the
design process are
Grounding
Shielding
Filtering
84

85. Grounding:
85
Grounding is the establishment of an
electrically conductive path between two points
to connect electrical and electronic elements of
a system to one another or to some reference
point, which may be designated as the ground.
.An ideal ground plane is a zero-potential
,zero-impedance body that can be used as a
reference for all signals in associated circuitry
and to which any undesired current can be
transferred for the elimination of its effects.
Bonds provide protection from electrical shock, power circuit current return paths, and antenna
ground plane connections, and also minimize the potential difference between the devices. They
have the ability to carry large fault current.
Bonding is the establishment of a low-impedance
path between two metal surfaces. Grounding is a
circuit concept, while bonding denotes the physical
implementation of that concept.

86. Shielding:
 The purpose of shielding is to confine radiated energy to a specific region or to prevent radiated
energy from entering a specific region.
 Shields may be in the form of partitions and boxes as well as in the form of cable and connector
shields.
 Shield types include solid, nonsolid (e.g., screen), and braid, as is used on cables. In all cases, a
shield can be characterized by its shielding effectiveness.
 The shielding effectiveness is defined as
SE=10 log incident power density
transmitted power density
 where the incident power density is the power density at a measuring point before a shield is installed
and the transmitted power is the power density at the same point after the shield is in place.
86
10

87. Filtering:
 An electrical filter is a network of lumped or
distributed constant resistors, inductors, and
capacitors that offers comparatively little
opposition to certain frequencies, while
blocking the passage of other frequencies.
 Filter provides the means whereby levels of
conducted interference are substantially
reduced.
 The most significant characteristic of a filter
is the insertion loss it provides as a function
of frequency.
 Insertion loss is defined as
IL=20 log V2/V1
 Where V1 is the output voltage of a signal
source with the filter in the circuit, and V2 is
the output voltage of the signal source
without the use of the filter.
87
10

88. ELECTRO MAGNETIC
COMPATIBILITY
 Electromagnetic compatibility (EMC) is the branch of electrical science
which studies the unintentional generation, propagation and reception of
electromagnetic energy with reference to the unwanted effects
(Electromagnetic interference, or EMI) that such energy may induce.
 The goal of EMC is the correct operation, in the same electromagnetic
environment, of different equipment which use electromagnetic
phenomena, and the avoidance of any interference effects.
 A system is said to be electro magnetically compatible if :-
• It doesn't cause interference with other system .
• It is not susceptible to emissions from other systems.
• It doesn’t cause interference with itself.
 EMI is a phenomenon while EMC is an equipment characteristic or
a property not to generate EMI above a certain limit and not to
be affected or disturbed by EMI. The statement "Live and let live" is the
best way to describe EMC. 88

89. The methodologies
used to prevent
EMI are:-
 Suppress the emissions at source point
, best method to control EMI.
 Make the coupling path as inefficient
as possible.
 Make the receiver less susceptible to
emission.
89

90. COUPLING MECHANISM:
90
The basic arrangement of noise source, coupling path and victim, receptor or sink is
shown in the figure below. Source and victim are usually electronic hardware devices,
though the source may be a natural phenomenon such as a lightning strike, electrostatic
discharge(ESD) or, in one famous case, the Big Bang at the origin of the Universe.

91. There are four basic coupling mechanism :
1. Conductive
2. Capacitive
3. Magnetic/Inductive
4. Radiative
Conductive coupling:
Conductive coupling occurs when the coupling path between the source and the
receptor is formed by direct contact with a conducting body, for example a
transmission line, wire, cable, PCB trace or metal enclosure.
Conduction modes:
Conducted noise is also characterized by the way it appears on different conductors:
 Common mode or common impedance coupling:
Noise appears in phase(in the same direction) on two conductors.
 Differential mode coupling:
Noise appears out of phase(in the opposite direction)on two conductors.
91

92. Capacitive coupling:
 Capacitive coupling occurs when a varying electrical field exists between
two adjacent conductors typically less than a wavelength apart, inducing a
change in voltage across the gap.
Inductive coupling:
 Inductive coupling occurs where the source and receiver are separated by
a short distance (typically less than a wavelength).
 Strictly, "Inductive coupling" can be of two kinds, electrical induction and
magnetic induction.
 It is common to refer to electrical induction as capacitive coupling, and to
magnetic induction as inductive coupling.
Magnetic coupling:
 Magnetic coupling (MC) occurs when a varying magnetic field exists
between two parallel conductors typically less than a wavelength apart,
inducing a change in voltage along the receiving conductor.
92

93. Radiative coupling:
 Radiative coupling or electromagnetic coupling occurs when source and
victim are separated by a large distance, typically more than a wavelength.
 Source and victim act as radio antennas: the source emits or radiates
an electromagnetic wave which propagates across the open space in
between and is picked up or received by the victim.
93

94. NEED FOR EMC STANDARDS:
 The EMC standards are required for trouble free co-existence
and to ensure satisfactory operation.
 They are also required to provide compatibility between
electrical, electronic, computer, control and other systems.
 Standards are required as manufacturer-user interaction and
user’s knowledge on EMI are limited.
 They are also required for establishing harmonized standards
to reduce international trade barriers and to improve product
reliability and life of the product.

95. EMC STANDARDS:
These are of two types
 Military Standards :
Military EMC standards are made in order to ensure system-to-system
compatibility in the real time military environment. Military standards are
more stringent than civilian standards. Most of the military standards are
broadly based on MIL-STD 461 and 462.
 Civilian Standards:
The civilian EMC standards are applicable for equipments used for
commercial, industrial and domestic applications. The emission standards
are specified to protect the broadcast services from interference..
95

96. ADVANTAGES OF EMC
STANDARDS
The advantages are:
Compatibility, reliability and maintainability
are increased.
Design safety margin is provided.
 The equipment operates in EMI scenario
satisfactorily.
 Product life and profits are increased.
96

97. DEPARTMENT OF ECE,
KUPPAM ENGINEERING COLLEGE,KUPPAM.

98. DEPARTMENT OF ECE,
KUPPAM ENGINEERING COLLEGE,KUPPAM.