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.