Transcript File

Inductance
 The property of inductance might be
described as
 "when any piece of wire is wound into a coil
form it forms an inductance which is the
property of opposing any change in current".
Inductance
 Alternatively it could be said
 "inductance is the property of a circuit
by which energy is stored in the form of
an electromagnetic field".
Inductance
 We said a piece of wire wound into a
coil form has the ability to produce a
counter emf (opposing current flow)
and therefore has a value of
inductance.
Inductance
 The standard value of inductance is the
Henry, a large value which like the Farad
for capacitance is rarely encountered in
electronics today
 Typical values of units encountered are
milli-henries mH, one thousandth of a
henry or the micro-henry uH, one
millionth of a henry.
Inductance
 A small straight piece of wire exhibits
inductance (probably a fraction of a
uH) although not of any major
significance until we reach UHF
frequencies.
 The value of an inductance varies in
proportion to the number of turns
squared.
Inductance
 If a coil was of one turn its value might
be one unit.
 Having two turns the value would be
four units while three turns would
produce nine units although the length
of the coil also enters into the equation.
Inductance formula
 The standard inductance formula for
close approximation - imperial and
metric is:
Imperial measurements
 L = r2 X N2 / ( 9r + 10len )
where:
L = inductance in uH
r = coil radius in inches
N = number of turns
len = length of the coil in inches
Metric measurements
 L = 0.394r2 X N2 / ( 9r + 10len )
where:
L = inductance in uH
r = coil radius in centimetres
N = number of turns
len = length of the coil in centimetres
Reactance
 Reactance is the property of resisting or
impeding the flow of ac current or ac
voltage in inductors and capacitors.
 Note particularly we speak of
alternating current only ac, which
expression includes audio af and radio
frequencies rf.
Reactance
 NOT direct current dc.
 This leads to inductive reactance and capacitive
reactance.
Inductive Reactance
 When ac current flows through an
inductance a back emf or voltage
develops opposing any change in the
initial current.
 This opposition or impedance to a
change in current flow is measured in
terms of inductive reactance.
Inductive Reactance
 Inductive reactance is determined by
the formula:
 2 * pi * f * L
 where: 2 * pi = 6.2832; f = frequency in
hertz and L = inductance in Henries
Capacitive Reactance
 When ac voltage flows through a
capacitance an opposing change in the
initial voltage occurs,
 this opposition or impedance to a
change in voltage is measured in terms
of capacitive reactance.
Capacitive Reactance
 Capacitive reactance is determined by
the formula:
 1 / (2 * pi * f * C)
 where: 2 * pi = 6.2832; f = frequency in
hertz and C = capacitance in Farads
Some examples of Reactance
 What reactance does a 6.8 uH inductor
present at 7 Mhz? Using the formula
above we get:
 2 * pi * f * L
 where: 2 * pi = 6.2832; f = 7,000,000 Hz
and L = .0000068 Henries
 Answer: = 299 ohms
Some examples of Reactance
 What reactance does a 33 pF capacitor
present at 7 Mhz? Using the formula
above we get:
 1 / (2 * pi * f * C)
 where: 2 * pi = 6.2832; f = 7,000,000 Hz
and C = .0000000000033 Farads
 Answer: = 689 ohms
Resonance
 Resonance occurs when the reactance of an
inductor balances the reactance of a
capacitor at some given frequency.
 In such a resonant circuit where it is in
series resonance, the current will be
maximum and offering minimum
impedance.
Resonance
 In parallel resonant circuits the
opposite is true.
 Resonance formula
 2 * pi * f * L = 1 / (2 * pi * f * C)
 where: 2 * pi = 6.2832; f = frequency in
hertz L = inductance in Henries and C
= capacitance in Farads
Resonance
 Which leads us on to:
 f = 1 / [2 * pi (sqrt LC)]
 where: 2 * pi = 6.2832; f = frequency in
hertz L = inductance in Henries and C
= capacitance in Farads
Resonance
 A particularly simpler formula for radio
frequencies (make sure you learn it) is:
 LC = 25330.3 / f 2
 where: f = frequency in Megahertz
(Mhz) L = inductance in microhenries
(uH) and C = capacitance in picofarads
(pF)
Resonance
 Following on from that by using simple
algebra we can determine:
 LC = 25330.3 / f 2 and L = 25330.3 / f 2
C and C = 25330.3 / f 2 L
Impedance at Resonance
 In a series resonant circuit the
impedance is at its lowest for the
resonant frequency
 whereas in a parallel resonant circuit
the impedance is at its greatest for the
resonant frequency.
 See figure.
Resonance in series and parallel
circuits
Impedance
 Electrical impedance describes a
measure of opposition to alternating
current (AC).
 Electrical impedance extends the
concept of resistance to AC circuits,
Impedance
 describing not only the relative
amplitudes of the voltage and current,
but also the relative phases.
 When the circuit is driven with direct
current (DC) there is no distinction
between impedance and resistance;
 the latter can be thought of as impedance
with zero phase angle.
Impedance
 The symbol for impedance is usually Z
and it may be represented by writing
its magnitude and phase in the form
|Z|< θ
Combining impedances
 The total impedance of many simple
networks of components can be
calculated using the rules for
combining impedances in series and
parallel.
Combining impedances
 The rules are identical to those used for
combining resistances,
 except that the numbers in general will
be complex numbers.
 In the general case however, equivalent
impedance transforms in addition to
series and parallel will be required
Series combination
 For components connected in series,
the current through each circuit
element is the same;
 the total impedance is the sum of the
component impedances
Impedance
Parallel combination
 For components connected in parallel,
 the voltage across each circuit element
is the same;
 the ratio of currents through any two
elements is the inverse ratio of their
impedances
Parallel combination
Parallel combination
 Hence the inverse total impedance is
the sum of the inverses of the
component impedances
Diodes
 Diodes are semiconductor devices
which might be described as passing
current in one direction only.
 The latter part of that statement
applies equally to vacuum tube diodes.
Diodes
 Diodes can be used as voltage regulators,
 tuning devices in rf tuned circuits,
 frequency multiplying devices in rf circuits,
 mixing devices in rf circuits,
 switching applications or can be used to
make logic decisions in digital circuits.
Diodes
 There are also diodes which emit
"light", of course these are known as
light-emitting-diodes or LED's.
Schematic symbols for Diodes
Types of Diodes
 The first diode in figure is a
semiconductor diode
 Commonly used in switching
applications
 You will notice the straight bar end has
the letter "k", this denotes the
"cathode" while the "a" denotes anode.
Types of Diodes
 Current can only flow from anode to
cathode and not in the reverse
direction, hence the "arrow"
appearance.
 This is one very important property of
diodes
Types of Diodes
 The second of the diodes is a zener
diode which are fairly popular for the
voltage regulation of low current power
supplies.
Types of Diodes
 The next is a varactor or tuning diode.
 Depicted here is actually two varactor
diodes mounted back to back with the
DC control voltage applied at the
common junction of the cathodes.
 These cathodes have the double bar
appearance of capacitors to indicate a
varactor diode.
Types of Diodes
 When a DC control voltage is applied
to the common junction of the
cathodes,
 the capacitance exhibited by the diodes
(all diodes and transistors exhibit some
degree of capacitance) will vary in
accordance with the applied voltage.
Types of Diodes
 The next diode is the simplest form of
vacuum tube or valve.
 It simply has the old cathode and anode.
 These terms were passed on to modern
solid state devices.
 Vacuum tube diodes are mainly only of
interest to restorers and tube enthusiasts
Types of Diodes
 The last diode depicted is a light
emitting diode or LED.
 A led actually doesn't emit as much
light as it first appears,
 a single LED has a plastic lens installed
over it and this concentrates the
amount of light.
Types of Diodes
 Seven LED's can be arranged in a bar
fashion called a seven segment LED
display and when decoded properly can
display the numbers 0 - 9 as well as the
letters A to F.