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electronics fundamentals
circuits, devices, and applications
THOMAS L. FLOYD
DAVID M. BUCHLA
chapter 7
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Magnetic Quantities
Magnetic fields are described by drawing flux lines
that represent the magnetic field.
Where lines are close
together, the flux
density is higher.
Where lines are further
apart, the flux density
is lower.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
The Magnetic Field
Magnetic fields are
composed of
invisible lines of
force that radiate
from the north pole
to the south pole of a
magnetic material.
Field lines can be visualized with the aid of iron filings
sprinkled in a magnetic field.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
The Magnetic Field
The magnetic field lines surrounding a magnet actually
radiate in three dimensions. These magnetic field lines
are always associated with moving charge.
In permanent magnets, the moving
charge is due to the orbital motion
of electrons. Ferromagnetic
materials have minute magnetic
domains in their structure.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Magnetic Materials
In ferromagnetic materials such as iron, nickel, and cobalt,
the magnetic domains are randomly oriented when
unmagnetized. When placed in a magnetic field, the
domains become aligned, thus they effectively become
magnets.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Magnetic Quantities
The unit of flux is the weber. The unit of flux density is
the weber/square meter, which defines the unit tesla, (T),
a very large unit.
j

B=
Flux density is given by the equation
A
where
B = flux density (T)
Flux lines (j
j = flux (Wb)
A = area (m2)
2
Area (m)
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Magnetic Quantities
To measure magnetic fields, an instrument called a gaussmeter is
used. A typical gaussmeter is shown.
The gauss, (G) is a unit of flux
density and is a much smaller unit
than the tesla (104 G = 1 T).
Gaussmeters are commonly used for
testing motors, classifying magnets,
mapping magnetic fields, and quality
control by manufacturers of motors,
relays, solenoids, and other magnetic
devices.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Magnetic Quantities
What is the flux density in a rectangular
core that is 8.0 mm by 5.0 mm if the flux is 20 mWb?
j
B=
A
B

20  106 Wb

8.0  103 m 5.0  103 m

 0.50 Wb/m2 = 0.50 T
Express this result in gauss.
 104 G 
0.5 T 
  5000 G
 T 
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Magnetic Quantities
• Magnetic flux lines surround a current carrying wire.
• The field lines are concentric circles.
• As in the case of bar magnets, the effects
of electrical current can be visualized with
iron filings around the wire – the current
must be large to see this effect.
Iron filings
Current-carrying wire
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Magnetic Quantities
• Permeability (m) defines the ease with which a
magnetic field can be established in a given material. It is
measured in units of the weber per ampere-turn meter.
• The permeability of a vacuum (m0) is 4p x 10-7 weber
per ampere-turn meter, which is used as a reference.
• Relative Permeability (mr) is the ratio of the absolute
permeability to the permeability of a vacuum.
mr 
Electronics Fundamentals 8th edition
Floyd/Buchla
m
m0
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Magnetic Quantities
• Reluctance (R) is the opposition to the establishment
of a magnetic field in a material.
R
l
mA
R= reluctance in A-t/Wb
l = length of the path
m = permeability (Wb/A-t m).
A = area in m2
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Magnetic Quantities
• Recall that magnetic flux lines surround a current-carrying
wire. A coil reinforces and intensifies these flux lines.
• The cause of magnetic flux is called magnetomotive
force (mmf), which is related to the current and number of
turns of the coil.
Fm = NI
Fm = magnetomotive force (A-t)
N = number of turns of wire in a coil
I = current (A)
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Magnetic Quantities
• Ohm’s law for magnetic circuits is
Fm
j
R
• flux (j) is analogous to current
• magnetomotive force (Fm) is analogous to voltage
• reluctance (R) is analogous to resistance.
What flux is in a core that is wrapped with a
300 turn coil with a current of 100 mA if the
reluctance of the core is 1.5 x 107 A-t/Wb ? 2.0 mWb
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Magnetic Quantities
The magnetomotive force (mmf) is not a true force in the
physics sense, but can be thought of as a cause of flux in
a core or other material.
Iron core
Current in the coil causes flux
in the iron core.
What is the mmf if a 250
turn coil has 3 A of current?
750 A-t
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
The Hall Effect
The Hall effect is occurrence of a very small voltage that is
generated on opposite sides of a thin current-carrying
conductor or semiconductor (the Hall element) that is in a
magnetic field.
The Hall effect is widely
employed by various sensors
for directly measuring
position or motion and can
be used indirectly for other
measurements.
Electronics Fundamentals 8th edition
Floyd/Buchla
Hall element
(semiconductor)
Current
N
+
Hall voltage _
Magnetic field
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Solenoids
A solenoid is a magnetic device that produces mechanical
motion from an electrical signal.
Stationary core
Sliding core
(plunger)
Spring
One application is valves that can
remotely control a fluid in a pipe,
such as in sprinkler systems.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Relays
A relay is an electrically controlled switch; a small control
voltage on the coil can control a large current through the
contacts.
Contac t points
StructureAlternate
schematic symbol
Schematic
Arm ature
Term inals
Spring
symbol
Fixed
contact
NC
contacts
CR1 -1
CR1
Movable contact
NO
contacts
Elec trom agnetic
c oil
Electronics Fundamentals 8th edition
Floyd/Buchla
CR1 -2
Fixed
Term inals
contact
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Magnetic field intensity is the magnetomotive force
per unit length of a magnetic path.
H=
Fm
l
or
H = NI
l
H= Magnetic field intensity (Wb/A-t m)
Fm = magnetomotive force (A-t)
l = average length of the path (m)
N = number of turns
I = current (A)
• Magnetic field intensity represents the effort that a
given current must put into establishing a certain flux
density in a material.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Magnetic Quantities
If a material is permeable, then a greater flux density
will occur for a given magnetic field intensity. The
relation between B (flux density) and H (the effort to
establish the field) is
B = mH
m = permeability (Wb/A-t m).
H= Magnetic field intensity (Wb/A-t m)
This relation between B and H is valid up to saturation,
when further increase in H has no affect on B.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
As the graph shows, the flux density depends on both
the material and the magnetic field intensity.
Magnetic material
Flux density, B, (Wb//m2)
Saturation begins
Non-magnetic material
Magnetic Field Intensity, H, (At/m)
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
As H is varied, the magnetic hysteresis curve is developed.
B
B
B
Saturation
Bsat
H
0
BR
Hsat
0
H
Hsat
H
H= 0
0
H
Bsat
Saturation
(a)
(b)
(c )
(d)
B
B
HC
Bsat
H
H = 0
0
0
BR
H
Hsat
H
H
HC
HC
B
(e)
Electronics Fundamentals 8th edition
Floyd/Buchla
B
B
(f )
(g)
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Magnetization Curve
A B-H curve is referred to as a magnetization curve for
the case where the material is initially unmagnetized.
The B-H curve differs for different materials; magnetic
materials have in common much larger flux density for a
given magnetic field intensity, such as the annealed iron
shown here.
Flux Density, B, (T)
2.0
1.5
Annealed iron
1.0
0.5
0
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Magnetic Field Intensity, H, (A-t/m)
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Relative motion
S
When a wire is moved across a magnetic field,
there is a relative motion between the wire and
the magnetic field.
N
S
When a magnetic field is moved past a
stationary wire, there is also relative motion.
In either case, the relative motion results in
an induced voltage in the wire.
N
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Induced voltage
The induced voltage due to the relative motion
between the conductor and the magnetic field when the
motion is perpendicular to the field is dependent on
three factors:
• the relative velocity (motion is perpendicular)
• the length of the conductor in the magnetic field
• the flux density
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Faraday’s law
Faraday experimented with generating current by
relative motion between a magnet and a coil of wire.
The amount of voltage induced across a coil is
determined by two factors:
S
1. The rate of change of the
N magnetic flux with respect
to the coil.
-V+
Voltage is indicated only
when magnet is moving.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Faraday’s law
Faraday also experimented generating current by
relative motion between a magnet and a coil of wire.
The amount of voltage induced across a coil is
determined by two factors:
S
-V+
1. The rate of change of the
magnetic flux with respect
N
to the coil.
2. The number of turns of
wire in the coil.
Voltage is indicated only
when magnet is moving.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Magnetic field around a coil
Just as a moving magnetic field induces a voltage,
current in a coil causes a magnetic field. The coil acts as
an electromagnet, with a north and south pole as in the
case of a permanent magnet.
South
Electronics Fundamentals 8th edition
Floyd/Buchla
North
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
DC Generator
A dc generator includes a rotating coil,
which is driven by an external
mechanical force (the coil is
shown as a loop in this
simplified view). As the coil
rotates in a magnetic field, a
pulsating voltage is generated.
Mechanical drive
turns the shaft
Brushes
Commutator
To external circuit
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
DC Motor
A dc motor converts electrical energy to mechanical
motion by action of a magnetic field set up by the rotor.
Mechanical
The rotor field interacts with the stator
output
field, producing torque, which
causes the output shaft to rotate.
The commutator serves as a
mechanical switch to reverse the
current to the rotor at just the
right time to continue the
+
rotation.
–
I
Commutator
Brushes
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Brushless DC Motor
A brushless dc motor has rotating field and a permanent
magnet rotor. An electronic controller periodically
reverses the current in the field coils. This causes the
stator field to rotate, and the permanent magnet rotor
moves to keep up with the rotating field.
Hall sensor
Permanent magnet rotor
(Courtesy of Bodine Electric Company)
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Brushless
Series
wound
DC dc
Motor
motor
•A series wound dc motor
has the field coil(s) in
series with the armature.
•The series wound motor
has very high starting
torque.
•It can run too fast if a load
is not connected; therefore
it is always used with a
load.
Electronics Fundamentals 8th edition
Floyd/Buchla
Internal resistance
DC Voltage
Field coil
Armature
Speed control
Interpole windings
Speed
Starting torque
Torque
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Shunt wound dc motor
•A shunt wound dc motor
has the field coil(s) in
parallel with the armature.
•The magnetic flux is
constant because of the
parallel arrangement.
•Unlike the series wound
motor, torque tends to be
nearly constant for
different loads.
Electronics Fundamentals 8th edition
Floyd/Buchla
RF
RA
DC Voltage
Armature
Field
coil
Full load torque
Speed
Torque
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Magnetic units
It is useful to review the key magnetic units from this
chapter:
Quantity
SI Unit
Magnetic flux density
Flux
Permeability
Reluctance
Magnetomotive force
Magnetizing force
Tesla
Weber
Weber/ampere-turn-meter
Ampere-turn/Weber
Ampere-turn
Ampere-turn/meter
Electronics Fundamentals 8th edition
Floyd/Buchla
Symbol
B
f
m
R
Fm
H
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Selected Key Terms
Magnetic field A force field radiating from the north pole to
the south pole of a magnet.
Magnetic flux The lines of force between the north pole and
south pole of a permanent magnet or an
electromagnet.
Weber (Wb) The SI unit of magnetic flux, which represents
108 lines.
Permeability The measure of ease with which a magnetic
field can be established in a material.
Reluctance The opposition to the establishment of a
magnetic field in a material.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Selected Key Terms
Magnetomotive The cause of a magnetic field, measured in
force (mmf) ampere-turns.
Solenoid An electromagnetically controlled device in
which the mechanical movement of a shaft or
plunger is activated by a magnetizing current.
Hysteresis A characteristic of a magnetic material
whereby a change in magnetism lags the
application of the magnetic field intensity.
Retentivity The ability of a material, once magnetized, to
maintain a magnetized state without the
presence of a magnetizing current.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Selected Key Terms
Induced voltage Voltage produced as a result of a changing
(vind) magnetic field.
Faraday’s law A law stating that the voltage induced across a
coil of wire equals the number of turns in the
coil times the rate of change of the magnetic
flux.
Lenz’s law A law stating that when the current through a
coil changes, the polarity of the induced
voltage created by the changing magnetic
field is such that it always opposes the change
in the current that caused it. The current
cannot change instantaneously.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Quiz
1. A unit of flux density that is the same as a Wb/m2 is the
a. ampere-turn
b. ampere-turn/weber
c. ampere-turn/meter
d. tesla
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Quiz
2. If one magnetic circuit has a larger flux than a second
magnetic circuit, then the first circuit has
a. a higher flux density
b. the same flux density
c. a lower flux density
d. answer depends on the particular circuit.
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Quiz
3. The cause of magnetic flux is
a. magnetomotive force
b. induced voltage
c. induced current
d. hysteresis
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Quiz
4. The measurement unit for permeability is
a. weber/ampere-turn
b. ampere-turn/weber
c. weber/ampere-turn-meter
d. dimensionless
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Quiz
5. The measurement unit for relative permeability is
a. weber/ampere-turn
b. ampere-turn/weber
c. weber/ampere-turn meter
d. dimensionless
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Quiz
6. The property of a magnetic material to behave as if it
had a memory is called
a. remembrance
b. hysteresis
c. reluctance
d. permittivity
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Quiz
7. Ohm’s law for a magnetic circuit is
a. Fm = NI
b. B = mH
Fm
j

c.
R
d. R 
l
mA
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Quiz
8. The control voltage for a relay is applied to the
a. normally-open contacts
b. normally-closed contacts
c. coil
d. armature
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Quiz
9. A partial hysteresis curve is shown. At the point
indicated, magnetic flux
B
a. is zero
b. exists with no magnetizing force
c. is maximum
BR
H
d. is proportional to the current
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Quiz
10. When the current through a coil changes, the induced
voltage across the coil will
a. oppose the change in the current that caused it
b. add to the change in the current that caused it
c. be zero
d. be equal to the source voltage
Electronics Fundamentals 8th edition
Floyd/Buchla
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.
Chapter 7
Quiz
Answers:
Electronics Fundamentals 8th edition
Floyd/Buchla
1. d
6. b
2. d
7. c
3. a
8. c
4. c
9. b
5. d
10. a
© 2010 Pearson Education, Upper Saddle
River, NJ 07458. All Rights Reserved.