Transcript Advanced Licence Course - Chelmsford Amateur Radio Society

Chelmsford Amateur Radio Society
Advanced Course
(3) Technical Aspects
Part-3 - Inductance, Transformers,
Power Supplies
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
1
Magnetism
• Magnetism is the property by which forces may be set up at a
distance to attract certain materials.
• Materials are said to be magnetised if they display the following
properties:– When suspended they lay North-South,
– Are able to impart magnetism to other materials,
– Capable of exerting a force on other magnetised materials.
• Materials which are attracted by the force of magnetism are called
FERROMAGNETIC
• Magnetic Forces can be generated by electric fields
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
2
Magnetic Materials
• Magnetic materials can be divide into classes
• SOFT magnetic materials LOSE the induced magnetism when
removed from the field
• HARD magnetic material will retain magnetism.
• NON FERROMAGNETIC materials have no influence on the field.
• Most magnets are based on Iron, Nickel or Cobalt
• The ability of a material to concentrate a magnetic field is known as
its PERMEABILITY
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
3
Electromagnetism
• Magnetism is created by the movement of electric charge.
• When a current flows in a conductor a magnetic field is produced.
• The Magnetic field exists, WITHIN AND AROUND THE CONDUCTOR
and weakens as the distance from the conductor increases.
• Loops or Coils of wire have a stronger magnetic field in their centre
• To produce an intense magnetic field very high currents are required.
• It is more usual to have relatively low currents, but many turns
• Caution: Unreel main extension coils which can have high currents in
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
4
Electromagnets
• A coil is wound on a SOFT magnetic core.
• With current flowing in the coil the core behaves as a
permanent magnet.
• The core loses its magnetism when current is switched OFF.
• This is called an ELECTROMAGNET.
• Ferromagnetic cores are used when strong magnetic fields
are required, as the material can concentrate the field lines.
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
5
Electromagnetic Induction
• Induction occurs when a conductor moves and cuts lines of
magnetic flux - the basis of alternators and dynamos
• A Voltmeter will give a momentary indication, and current will
flow in the circuit.
• This is called ELECTROMAGNETIC INDUCTION
• The polarity of the induced EMF reverses if either the direction of
field or the direction of motion reverses.
• The induced EMF is greatest when the conductor moves at
rightangles to the magnetic field, and is zero if the conductor
moves in line.
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
6
Inductance
• If a steady current flows there will a steady magnetic field around a
conductor and if there is a change in current there will be change in the
magnetic field.
• A change in the magnetic field will induce a Back EMF in the conductor
that will oppose the change being made. (Lenz's Law) and the effect is
known as Inductance which has the symbol L
• An Inductor is said to have a value of 1 Henry when the current through
the conductor changing at a rate of 1 ampere per second induces an
EMF of 1 volt.
• The Henry is the unit both for self inductance and mutual inductance.
• A Henry is rather large so pH, uH and mH are more common
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
7
Coils
• The self inductance of a wire is small so to increase the inductance
the wire is wound in the form of a coil.
• The inductance of a coil is dependant on the number of turns, coil
area, the permeability of the core of the coil, and spacing between the
coil turns.
• Coils are wound in various ways depending upon their application.
• The value of inductance will vary from several Henries for a power
supply filter to a microHenry for a RF circuit.
• In RF circuits the inductance can be increased by using non metallic
magnetic materials such as ferrites that have a high resistivity and
therefore low eddy current loss.
• An RF transformer can be adjusted by screwing in a ferrite slug
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
8
Mutual Inductance &
Back EMF
Mutual Inductance & Transformers
• When a change in current induces a change in current in another
circuit by flux linkage this is called mutual inductance.
• Coils that have nearly all lines of flux coupling are tightly coupled.
• Coils that are some distance apart and have less flux linkage
are loosely coupled.
Back EMF
• Because of the magnetic field generated by the current flow,
changes in this field will induce an EMF in the conductor that will
oppose the flow of current. This is known as the back EMF.
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
9
Series & Parallel Inductors
• Provided that there is no mutual coupling between inductors when
they are connected together . . .
• SERIES CONNECTION
L total = L1 + L2 + L3 etc.
• PARALLEL CONNECTION
1/ L total = 1/ L1 + 1/ L2 + 1/L3 etc.
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
10
LR Time Constant
• As with Capacitors, if an Inductance is connected in series with a
resistor then there is delay in obtaining the full current/voltage.
R
L
I = V/R
Final steady state current
V
I = V/R
Time
Time
t
t
Decay of Current in Inductor
Growth of Current in Inductor
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
11
LR Time Constant
• The LR time constant is defined as . . .
• The time in seconds for the current to reach 63.2% of
the full value.
T = L / R Seconds
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
12
Transformer Construction
• The property of mutual inductance is employed in transformers which
have two windings on a common soft magnetic core
• Changes in magnetic flux from an applied voltage in the primary
induce a voltage in the secondary windings, BUT they also induce an
EMF which can circulate in the magnetic core - EDDY CURRENTS.
• Two disadvantages with EDDY Currents are;
– Energy is expended in their production, creating losses.
– Heat is generated, and overheating can occur.
• EDDY CURRENTS are reduced by providing a high resistance path.
• Transformers are constructed from thin laminations which are
insulated from their neighbours by coats of varnish. At RF, Ferrite is
held in a non-conducting binder for the same reason
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
13
Transformer Calculations
Ip
• A simple transformer has primary
winding P and secondary winding S.
• Number of turns on the primary = Np
• Number of turns on the secondary = Ns
• Voltage across the primary = Vp
• Voltage across the secondary = Vs
Is
P
S
Vp
Zp
Np Turns
Zs
Vs
Ns Turns
• Since both windings are in the same AC magnetic field the induced
voltage will be in proportion to the turns on each coil.
• The ratio Ns/Np is called the turns ratio.
Vs = Vp . Ns / Np
Ip = Is . Ns / Np
• A Transformer has the property of being able to transform impedance.
Zp = Zs . ( Np / Ns ) 2
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
14
Peak Inverse Voltage
• Peak Inverse Voltage (PIV) is the voltage across the diode
when it is not conducting.
• This can be twice the peak supply voltage due to the charge
on the reservoir capacitor.
• Calculated by taking taking the secondary transformer RMS
voltage, multiplying by root 2, and then doubling
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
15
Power Supplies - 1
Half Wave Rectification
Best suited to low current applications
+V
• High Ripple Current and Low Efficiency.
• Diode PIV = 2.8 . Vac
Full Wave Rectification
+V
Circuit also known as Bi-Phase
rectification. Each diode conducts on one
half cycle. DC component of load flows
through the secondary in such a direction
to cancel magnetising current.
• Diode PIV = 2.8 . Vac
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
16
Power Supplies - 2
Bridge Rectification
+V
Preferred for high voltage, and
does not need a centre-tapped
transformer.
During each half cycle of the
input voltage two of the diodes
are conducting.
• PIV is half per diode.
• Diode PIV = 1.4 . Vac
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
17
Smoothing Circuits - 1
Simple Capacitor Filter
From
Rectifier
C1
Capacitor Input Filter
From
Rectifier
Chelmsford Amateur Radio Society
Advanced Licence Course
Load
L1
C1
C2
Carl Thomson G3PEM
Load
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
18
Smoothing Circuits - 2
Choke Input Filter
L1
From
Rectifier
C1
Load
Resistance Capacitance Filter
R1
From
Rectifier
Chelmsford Amateur Radio Society
Advanced Licence Course
C1
C2
Carl Thomson G3PEM
Load
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
19
Stabilising Circuits
Pass Transistor
Zener Diode
Input
Output
Input
Output
IC Regulator
Input
Chelmsford Amateur Radio Society
Advanced Licence Course
Output
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
20
Screening
• Screening is used to prevent coupling between circuits.
• Magnetic screens:
– A soft magnetic material is introduced to concentrate
the flux lines leaving the field weaker elsewhere.
Materials such as Mu-metal may be used for this.
• Electrostatic screening:
– Is when a conductor is surrounded completely by
another conductor which has the same potential along
its whole length - sometimes called a Faraday Cage
• Take care that screens are not too close and unduly
influence circuitry they are meant to protect.
Chelmsford Amateur Radio Society
Advanced Licence Course
Carl Thomson G3PEM
Slide Set 3: v1.1, 20-Aug-2006
(3) Technical Aspects - Inductors & PSUs
21