Fundamentals of high frequency currents priyank

2.01 - 2.10
FUNDAMENTALS OF HIGH
FREQUENCY CURRENTS

TEACHER : Dr. RINKU SHAH (PT)
Dr. KAJAL PATEL (PT)
VERIFIED BY : Dr. ASHOK CHAUDHARY (PT)
VALIDATED BY : Dr. POONGUNDRAN P. (PT)

MAGNETISM

2.01 to 2.10 Fundamentals of high frequency currents 2

• A magnet is an object which exhibits certain
properties.
• For example, when free to rotate, it will align
itself in the North—South direction.
• It also has the power to attract, and produce
magnetism in, certain other materials.
• So, A piece of substance, which possesses the
property of attracting small pieces of iron
towards, it is called a magnet.

• If the property of magnetism occurs naturally
the magnet is known as a Natural magnet.
• It is possible artificially to induce magnetism
by rubbing the given piece of substance with
magnets.
– The magnet thus produced is called an
Artificial Magnet.


The molecular theory of
magnetism
• No matter how many times a magnet is
divided, it will always present a North and a
South pole.
• This phenomenon could conceivably be
carried on down to molecular level, where it is
thought that the revolving electrons produce
a North and a South pole for each molecule,
giving so-called ‘Molecular Magnets’.


• In a non-magnetized state, these molecular
magnets are arranged in a haphazard way and
cancel out one another’s effects.
• In the magnetized state, the molecular
magnets are ordered so that one end of the
piece of metal exhibits a North pole and the
other a South.


• In magnetized materials such as steel, the
friction between the molecules is great and the
ordered magnetic effect is retained, giving a
permanent magnet.
• Heating or banging will, however, disrupt the
order and so the magnetism will be lost.


• Haphazard arrangement of molecular
magnets.


• Ordered arrangement of molecular magnets


• In a material such as soft iron there is little
friction between the molecules, so although they
can easily be influenced into an ordered pattern,
this pattern will also be lost very easily.
• Thus soft iron only forms temporary magnets
• The magnetic effect of a wire carrying an electric
current can be used to create an electromagnet,
which exists only for as long as current flows.

• Magnetization by contact.


• The magnetic field around a bar magnet.


Magnetic Poles

• The points inside the magnet, where
attraction is maximum are called poles.
• Every magnet has two poles: north pole and
south pole.
• A freely suspended magnet sets itself in the
direction of geographic north and south.


• North pole: The end of the magnet pointing
towards north is called north seeking pole or
north pole.
• South pole: The end of the magnet pointing
towards south is called south seeking pole or
south pole.


• In a magnet the unlike poles attract each
other and like poles repel each other.
• Two magnets one of which is suspended
comes closer to the other when their opposite
poles are kept nearer, whereas it moves apart
when their like poles are kept nearer.


Properties of a magnet

1. Setting in a North—South direction As the
Earth itself is a giant magnet, the Earth’s
magnetic field will influence a suspended
magnet so that one of its poles (ends) will
settle in the direction of the Earth’s North
Pole.


2. Like magnetic poles repel one another North
repels North and South repels South.
• Unlike magnetic poles attract one another,
i.e. North attracts South and South attracts
North.


3. Transmission of properties: A magnet can
produce properties of magnetism in suitable
materials.
• As one pole of a bar magnet is stroked along the
material; all the opposite poles of the molecular
magnets are attracted towards it so that the
object is magnetized.
• The end that the magnet leaves will have the
pole opposite to that used to induce the effect.

• A magnet may also produce a magnetic effect
in an object without contact between them
(magnetic induction).
• Once again, it is the influence of the magnet
over the molecular magnets of the susceptible
materials which produces the magnetic effect.


4. Attraction of suitable materials Magnets
attract certain materials.
• This effect is produced by magnetic
induction.


5. A magnetic field: This is the area or zone of
influence around a magnet in which its
magnetic forces are apparent.


• This field may be considered as being made up
of magnetic lines of force which have the
following properties:
a) They travel from North to South, which is the
path a free North Pole would take.
b) They attempt to take the shortest route possible
but repel one another so that they in fact become
curved.
c) They travel more easily through some materials,
e.g. metals, than through others.

Types of Magnet
Magnets are of two types
• Natural magnets: The magnets found in
nature are called natural magnets. These
magnets are weak and shapeless.
• Artificial magnets: Man made magnets are
called artificial mans. These magnets are
strong and of different shapes.
• They may be bar shaped, horseshoe shaped,
magnetic needles, magnetic compass, etc.

Types of Artificial Magnets

1. Temporary magnets: Magnetism of these
magnets is temporary. It is made of soft iron.
2. Permanent magnet: Magnetism of these
magnets is permanent. It is made up of steel,
nickel and cobalt.


Electromagnetism


• Electromagnetism is one of the four
fundamental forces known to exist in the
universe.
• An electromagnet consists of a coil of wire
wound onto a soft iron bar.
• When a current passes through the wire it
magnetizes the bar by induction.


• As soon as the current is put off, the magnetic
effect is lost.
• Wires carrying an electric current produce
magnetic field around a straight wire in the
form of concentric circles with the wire at
their center.


• A coil of wire produces a field somewhat
similar to that of a bar magnet, with the main
difference being that in electromagnetism a
uniform field is produced inside it.
• This uniformity of field is used as an
advantage in SWD application.


• A coil of wire produces a field somewhat
similar to that of a bar magnet, with the main
difference being that in electromagnetism a
uniform field is produced inside it. This
uniformity of field is used as an advantage in
SWD application.


Electromagnetic induction
• Electromagnetic induction is the means by
which electricity is produced from magnetism
(and vice-versa).
• It is the result of interaction between a
conductor and magnetic lines of force.
• An EMF is produced in the conductor by the
magnetic lines of force surrounding a magnet,
without contact between the magnet and the
conductor.


• The factors essential to electromagnetic
induction are:
1. Conductor
2. Magnetic lines of force
3. Relative movement of 1 and 2


• If the conductor is part of a closed circuit, the
magnetic lines of force produce an EMF which
causes movement of the electrons in the
conductor.
• This can be shown with an ammeter connected
across a coil of wire When a magnet is moved
into the coil, the magnetic lines of force cut
across the conducting wire of the coil and cause
movement of electrons in the coil.


• These electrons repel adjacent electrons and
so on, and a current is set up in the circuit.
Movement of the ammeter needle, indicating
current flow, will be seen only when either
the magnet or the coil is moving. If the
magnetic lines of force are stationary relative
to the coil of wire, there is no induction.


• Electromagnetic induction also occurs if the
magnetic field used is that surrounding a coil
Of wire.
• The principles are the same. There must be
movement of the magnetic field relative to
the conductor.


• This may be achieved by using an alternative
current in the primary coil which causes the
magnetic field to build up, fall, then build up
in the opposite direction, then fall, etc.
• The current builds up to a maximum positive
value and then falls to zero. It then drops to a
maximum negative value before returning to
zero. This rise and fall of current produces
movement of the magnetic lines of force.

• In practice, the conductor in which the EMF is
induced is usually a coil of wire, while the
magnetic field used to induce the EMF is that
of a permanent magnet or a current-carrying
coil of wire.


• Movement of one of these relative to the
other is achieved either by spinning the
conductor in the magnetic field, as in a
dynamo, or by varying the intensity of current
in the coil of wire, as in a transformer


The direction of the
induced EMF

• The direction in which the magnetic lines of
force move relative to the conductor affects
the direction in which the induced current
flows. This can again be seen by using the bar
magnet and coil.


induced EMF
• As the magnet is moved into the coil, the
ammeter needle deflected in one direction.
• As it is withdrawn, deflection occurs in the
opposite direction, thus demonstrating that
the direction of current flow changes with a
reversal of movement of the magnetic field.


induced EMF
• The same is true when the inducing magnetic
field is that surrounding a current-carrying coil
of wire.
• As the current rises and the magnetic lines of
force move out, thus cutting the conductor,
deflection of the ammeter needle occurs in
one direction.


induced EMF
• As the current drops to zero, the magnetic
lines of force move back in towards the
primary coil. The direction of movement of
these lines of force is now reversed, and so is
the direction of the induced current indicated
by the ammeter.


induced EMF

• This result is often quoted as Lenz’s law,
which states that the direction of the induced
EMF is such that it tends to oppose the force
producing it.


The strength of the induced
EMF

• This depends upon two factors:
1. the rate of change of the magnetic field
2. the inductance of the conductor


1 The rate of change of
the magnetic field

• The more rapid the movement of the
permanent magnet and the stronger the
magnet used, the greater is the rate at which
the magnetic lines of force cut the conductor
and the greater the induced EMF.


1 The rate of change of
the magnetic field

• In the case of a current-carrying coil of wire, if
the frequency of current is increased (and
hence the rate of rise and collapse of the
magnetic field), a stronger EMF is induced.


2 The inductance of the
conductor
• Inductance is the ability of a conductor to
have a current induced in it. Inductance is
measured in henries.
• Inductance is constant for any particular
conductor, but high inductance can be
designed into a conducting coil by
incorporating the following principles:


2 The inductance of the
conductor
1. Using many turns of wire in the coil
2. Placing the turns close together
3. Winding the coil onto a soft iron core
• This ensures that the magnetic lines of force
cut the maximum number of coils in the
conductor and thus induce a strong EMF into
it.


LAWS


Faraday’s Law of
Electromagnetic Induction

• Faraday’s laws deal with the induction of EMF
in an electrical circuit, when magnetic flux
linked with the circuit changes.


Faraday’s Law of
Electromagnetic Induction
• Whenever magnetic flux linked with a circuit
changes, an EMF is induced in it.
• An induced EMF exists in the circuit, so long as
the change in magnetic flux linked with it
continues.
• The induced EMF is directly proportional to
the negative rate of change of magnetic flux
linked with the circuit.


Lenz’s Law

• The law states that the direction of the
induced EMF (current) is such that it opposes
the very cause, which produces it.


Fleming’s Right Hand Rule

• It gives us the direction of the induced EMF
(current), in a conductor moving in a magnetic
field. It states that, if the thumb, index and
middle fingers of the right hand are stretched
mutually perpendicular, then thumb indicates
motion, index finger indicates the direction of
the field and middle finger indicates the
direction of induced current.


Mutual Induction

• Mutual induction is said to occur when an
EMF is induced in an adjacent conductor by
the magnetic field set-up around a coil of wire
carrying a varying current. In a transformer
and in physiotherapy this principle is very
much used in the electrotherapeutic
modalities, e.g. SWD.


Self Induction

• Self induction occurs with in a coil carrying a
varying current. A magnetic field is set-up
around each turn of wire.
• As the current increases, the magnetic lines of
force move out, cutting adjacent turns of wire
and thus inducing an EMF in them.


Self Induction

• Following Lenz’s law, the direction of the
induced EMF will be opposite to the force (or
current) producing it.
• Therefore, the induced EMF is in the opposite
direction to the main current and so opposes
its rise. Self induced EMF of this type is
therefore, called back EMF.


Self Induction
• A similar sequence of events occurs when the
primary current starts to fall. The magnetic
field now collapses and the lines of force
move back in, cutting adjacent turns of wire
but in the opposite direction from before.
• Consequently, the induced EMF is also in the
opposite direction and flows forward as the
forward EMF.


Self Induction

• The overall effect of back and forward EMF is
to retard the rate of rise of current and
prolong its fall.


Choke coil

• It is a device included in the circuit to produce
self induced EMF, maintaining a smooth flow
of current. It is of two types, i.e. low
frequency choke coil and high frequency
choke coil.


Low Frequency Choke Coil
Choke coil
• This consists of many
turns of insulated
wire, wound on a
laminated soft iron
frame, usually on the
central bar of a
rectangular frame


Choke coil
• When a current, which varies in intensity, is
passed through the coil, magnetic lines of
force are set-up, which cut the turns of wire
and induce EMF in them.
• There are many turns of wire, so the coil has
considerable inductance and self-induced EMF
is large.


Choke coil

• The core serves to concentrate the magnetic
field, it is made of soft iron, so that it is easily
magnetized and de-magnetized, and is
laminated to prevent eddy currents.


High Frequency Choke
Coil
• A high frequency current varies very rapidly in
intensity so tend to produce a considerable
self-induced EMF.
• Consequently, it is unnecessary to have many
turns of wire, in a high frequency choke coil,
or to wind them on a soft iron core.
• The coil usually consists of several turns of
insulated wire wound on the bobbin of some
non-conducting material.

Uses of choke coil

• To even out the variations in the intensity of
the current, providing a smooth current flow:
The self-induced EMF, which is set-up when a
varying current is passed through a choke coil,
retards the rise of current to a maximum, and
prolongs the current flow, when the intensity
is falling, there by maintaining an even flow of
current.


Uses of choke coil

• To prevent the flow of a high frequency
current and allow the passage of the low
frequency one:
When a high frequency current is passed
through a choke coil, the inductive reactance
is considerable, there by retarding the flow of
such a current,


Uses of choke coil

when a low frequency current is passed, the
impedance to current flow is very less, due to
which the choke coil serves the above
function.


Eddy Current

• Any conductor lying in a
varying magnetic field has
an EMF induced in it. If
the conductor is solid, the
magnetic lines of force
passing through it set-up
circular currents called
eddy currents


Eddy Current

• In the figure shown below,
the solid conductor ‘B’ is
present in the varying
magnetic field which
produces the eddy currents
in it shown by the arrow
pointing up and down.


Eddy Current

• These eddy currents are perpendicular to the
magnetic lines of force and produce heating
effect in tissues in accordance with Joules


Transformer


• It is a device used for changing low alternating
voltage at high current. It changes the
alternating voltage without the loss of energy.


Types of Transformer

• Broadly the transformers are divided into
three types
1. static transformer
2. variable transformer
3. autotransformer


Static Transformer

• The static transformer is based on the
principles of electromagnetic induction, and is
used to alter voltage of an alternating current
and to render the current earth free.


Static Transformer-
construction
• It consists of two coils
of insulated wire, the
primary and the
secondary coils,
wound on a laminated
soft iron core.


Static Transformer-
construction
• The coils are completely insulated from each
other and one usually contains more turns of
wire than the other.
• The frame is often rectangular in shape and
the coils may be wound on opposite bars of
the frame or one on top of the other on a
central bar.


Static Transformer-
working
• An alternating current is passed through the
primary coil and sets up a varying magnetic
field, which cuts the secondary coil and
induces an EMF in it.
• It is essential that the primary current varies
in intensity, otherwise there is no movement
of the magnetic field relative to the conductor
and no EMF is induced in the secondary coil.


Static Transformer-
working
• There is no electrical conduction between the
primary and the secondary coils, the energy
being transmitted from one to the other by
electromagnetic induction.
• The core serves to concentrate the magnetic
field and is made of soft iron, as this material
is easily magnetized and de magnetized. It is
laminated to prevent eddy currents.


Static Transformer-
functions
1. Alters the voltage of an alternating current
• The EMF induced in the secondary coil
depends upon the number of turns of wire it
has, relative to the primary coil. Depending
on this number of turns, the transformers
can be classified as:
– Step up transformer
– Step down transformer
– Even ration transformer

Step up transformer

• If the number of turns in
the secondary are more
than that of the primary,
the voltage developed in
the secondary will be
increased or stepped up.
Such a device is called as
step up transformer.


Step down transformer
• If the secondary coil has
fewer turns than the
primary, then the EMF,
or voltage in the
secondary will be less
than in the primary, i.e.
it is stepped down. Such
an arrangement
produces a step down
transformer.

Even ratio transformer

• If the number of turns in the primary and
secondary coils are same, the voltage in the
primary is same as that of the secondary. Such
a device is called even ratio transformer.
• It is important to note that, the electrical
power in both the primary and the secondary
circuits are always the same.


Even ratio transformer

• Power is measured in watts (volts x ampere),
so the quantity watts x ampere must be same
for both the primary and secondary coils, i.e.
any change in voltage must be accompanied
by a change in current.


Static Transformer-
functions

2. Renders a current earth free
• The mains electricity is produced by the
dynamo, and the consumer is supplied with a
wire at high potential, called the live wire, and
a wire at zero potential connected to earth,
called the neutral wire.


• Most electrical
apparatus works on a
current, which flows
from the live wire,
through the
apparatus, to the
neutral wire and
earth.


• If an accidental connection is made between
live wire and earth, current will flow along it.
• If this connection were made through a
person, they would then receive an earth
shock, as the current flows through them to
earth.


• The static transformer
reduces this danger by
using electromagnetic
induction, to transfer
the electrical energy
into the secondary coil
where earth plays no
part in the circuit.


• The effect on the secondary coil of the
magnetic field around the primary is to cause
electrons to move around the secondary
circuit, but not to leave it. Earth plays no part
in the secondary circuit, because, even if an
earth connection is made with it, electrons
will not leave the circuit, but will continue to
flow around it


• This is an important safety factor, and that all
currents applied to patients are rendered
earth free by using a static transformer.


Variable Transformer

• It consists of a
primary and
secondary coil, but is
constructed so that
one of them can be
altered in length.


Variable Transformer

• The primary coil has a number of tapings
taken from it and a movable contact can be
made on any one of these by turning a knob.
• The effect of decreasing the number of turns
in the primary coil relative to the secondary is
to cause a step up voltage in the secondary
coil. In his way a very crude control of voltage
is obtained.


Autotransformer

• It consists of a single coil of wire with four
contact points coming from it.


Autotransformer

• It can be used as a step up, or a step down
transformer. When used as the step up, CD is
the primary coil and AB is the secondary coil.
When used as the step down, AB is the
primary and CD is the secondary coil.
Although the autotransformer works on the


Autotransformer

• Although the autotransformer works on the
principles of electromagnetic inductions, it
has the disadvantage that, it allows only a
small step up, and does not render the
current earth free.
• It is used in the starter circuit of ultraviolet
lamp to strike the arc in the lamp.


Diode & triode valves


Valve

• Valve is a device, which transmits the flow in
one direction only, common examples being
that of the valves of heart, or vein.
• In electronics a thermionic valve is defined as
a device allowing unidirectional flow of
current.


Valve

• There are various types of thermionic valves,
which are named according to the number of
electrodes they contain. They are:
1. Diode valve
2. Triode valve


Diode valve

• This is the simplest form of thermionic valve,
containing a cathode with a filament and an
anode, enclosed in an evacuated glass tube.
• The valve may either be evacuated or may
contain an inert gas at low pressure.


Diode valve

• For the current to pass through the valve, the
filament must be heated, causing emission of
electrons by the process of thermionic
emission & a PD when applied makes the
plate (anode) positive in relation to the
cathode.


Diode valve

• The filament used can be directly or indirectly
heating type and the anode plate is made
from some metal, which does not allow
thermionic emission readily and is in the form
of a cylinder surrounding the cathode.


Diode valve
• The directly heating filament is a loop of fine
wire of thoriated tungsten (tungsten can
tolerate repeated and cooling, allowing
emission of electrons at low temperature)
• The indirectly heating filament is a fine loop of
wire embedded in some insulated material
and the whole device is surrounded by a
metal cylinder from which thermionic
emission takes place.

Diode valve
• The electrons so emitted will be attracted by
the positive anode constituting an electrical
current across the device.
• When the applied PD is reversed, so that the
plate (anode) is negative with respect to the
cathode, no current flows through the device,
indicating that the electrons can pass from
cathode to plate, not in the reverse direction,
i.e. the current can flow only in one direction.

Diode valve
• The intensity of current that flows across the
valve depends on the heating of the filament
and on the PD between the filament and the
plate.
• If more current is applied to the filament
causing increased heating of the same, it will
emit more number of electrons and this when
combined with an increase in PD, makes
available greater force to attract the
electrons.

Diode valve

• And there by increasing
the current flow across the
valve. In a diode there are
the filament circuit and
the anode circuit.
• The diode is symbolically
represented and three
dimensionally as in Figure


Triode valve

• The triode valve is a device that contains three
electrodes viz. cathode, grid, and the anode.
• The grid, whose potential can be altered, is
placed between the cathode and the anode.
• The grid, which surrounds the filament, may
consist of a metal cylinder, perforated to
allow the electrons to pass through, or may be
a spiral of metal wire.


Triode valve

• A lead from the grid is brought to a pin
outside the base of the valve, necessitating
four pins, i.e. two for the filaments, one for
the grid and one for the anode.
• When the filament will be heated as like the
diode, current passes from the valve in one
direction only, i.e. from plate to cathode.


Triode valve
• If the grid is uncharged, it has no effect on the
current flow.
• If the grid is given with a negative charge from
the outside source, it repels electrons, either
causing a reduction of current flow, or
resulting in complete cessation of current
flow.
• If however, the grid is given a positive charge,
the electrons can pass and the current flows.

Triode valve
• The charges applied to the grid from the
external source are called as grid bias.
• As the grid lies close to the cathode, than the
anode, the charges on the grid has a greater
influence, on the flow of current than a similar
charge on the anode.
• The flow of current across the triode valve can
be regulated by adjusting the bias of the grid.


Triode valve
• The triode valve is represented symbolically
and three dimensionally as in Figure


Uses of a triode valve
• Used for the production of interrupted
current and other muscle stimulating
currents.
• Used for the production of high frequency
currents in conjunction with a condenser and
inductance.
• It is not used as a rectifier, but rectifies the
current that passes through it.
• It is used as a switch.

Semiconductors


• Semiconductors are usually metals, which
because of thermal agitations, or addition of
impurities, have electrons free to conduct
current.
• A semiconductor can either be of n-type, or p-
type.


• In a n-type semiconductor, there is an excess
of electron, which carries current, where as in
a p-type, the deficiency of electron give rise to
positive hole, due to which current flow
occurs.
• If a n-type and a p-type semiconductors are
fused together, electrons can only pass in the
n—>p direction, and the semiconductor
therefore acts as a valve.

N-type, Semiconductor
• An atom of silicon with atomic number 14, has
4 electrons in the outer shell, and in a crystal
of silicon these are held in forming bonds with
neighboring atoms, so that there are no free
electrons to transmit an electric current.
• When certain other materials such as
phosphorous (atomic number 15, with 5
electrons outside) are added to silicon it
transmits current.

• When silicon and phosphorous form covalent
bonds, four electrons of phosphorus make
bond with four electrons of silicon, leaving
behind one free electron in the phosphorous
which are not held in bond with other atoms,
therefore carrying current, when connected
with a source in the same way like the
conductors.
• Such a material is called n-type
semiconductor.

P-type Semiconductor
• When silicon is added with certain other
substances such as aluminium with an atomic
number 13, the three outer electrons in the
aluminium atom, makes bond with three
electrons in the outer orbit of silicon, whereas
for the 4th electron of silicon, there is no
electron available on the outer orbit of
aluminium, creating an electron deficiency
called hole.

• When a PD is applied to such a material,
electrons move from some of the atoms into
these unoccupied bonds or holes nearer to
the positive poles, so that as the electrons
move away from the negative towards the
positive, the holes move from the positive
towards the negative, constituting a flow of
current.


• The movement of positive holes from positive
towards negative is equivalent to the
movement of electrons from negative to
positive.
• The material that transmits current in this
manner is called a p-type semiconductor.


Semiconductor Diode
• When an n-type semiconductor, which has
free electrons, is placed in contact with a p-
type semiconductor, which has positive holes,
electron move from the n-type to occupy the
holes in the p-type, while positive holes move
in the reverse direction.
• In this device the current can only pass in one
direction, i.e. from p—>n, and such a device is
called a semiconductor diode.

Construction of The
Semiconductor Diode
• When the semiconductors n-type and p-type are
connected to a source of EMF, the PD at their
junction affects the current flow.
• If the n-type semiconductor is made more
negative, than the equilibrium value, and the p-
type more positive, electrons lost from the n-
type are replaced from the supply, while excess
electrons are withdrawn from the p-type to the
supply.

• The PD at the junction is reduced, electrons
are able to pass from the n-type to the p-type
and current flows across the circuit.
• If however, the p-type semiconductor is
negative, relative to the n-type, the PD at the
junction opposes the electron movement, and
no current flows until the applied PD reaches
a certain critical value.


• The current can flow only when ‘n’ is negative
and ‘p’ is positive, constituting a
unidirectional flow like a valve, so called as
semiconductor diode.


Transistor


• Transistors are electrical device, which utilize
a sandwich, of p and n-type semiconductor
materials.
• It can be NPN, or PNP types.
• In a NPN transistor the two thick layers of n-
type semiconductors are separated by a thin
layer of p-type.
• The semiconductor has got three parts:
Emitter, Base, Collector.

• One of the n-type at the left is the emitter,
the other at the right is the collector, and the
central p-type is the base.
• On contact being made between materials,
say n-p-n semiconductors in this case, PD
develops at their junctions, the emitter and
the collector, being positive relative to the
base.


• When the device is connected to a source of
EMF, with the emitter negative and the
collector positive, no current flows unless the
EMF exceeds the critical value, as the
electrons are unable to pass from the
negative p-type to the positive n-type
semiconductor, so cannot cross the base
collector junction.


• A second source of EMF is connected to the
base and the emitter, the base being positive
relative to the emitter.
• The electrons can pass from the negative n-
type to the positive p-type semiconductor.
• So the current flows across the base collector
junction.


• In the circuit described, there is a thick layer
of n-type semiconductor, a thin layer of p-
type semiconductor, so the current consist
largely of the movement of electrons and the
electrons from the emitter soon pass into the
base.


• The base has now an adequate supply of
electrons, and as it is very thin these come
close to the base collector junction, and are
attracted into the collector, to replace those
that had migrated into the base.
• This reduces the barrier effect across the
base-collector junction, and current flows
across the transistor.


• Thus a current fed into the base, renders the
transistor capable of conducting current, and
small variations in this base current, causes
greater variation of current flowing across the
transistor.
• In this respect the current fed into the base of
the transistor has an effect, comparable to
that of a positive charge applied to the grid of
a triode valve.

Uses of Transistor

• Transistors are used in preference to the
valves, in most modern electrical equipment,
as they are durable, have a long life, consume
less power and need no heating device.
• As the power output is limited they are
suitable for use in the production of low
frequency but fail to produce high frequency
currents. e.g. SWD.


Oscillator circuit


• It is also called as the generator or the
machine circuit.
• The frequency current is generated by this
circuit, which consists of a capacitance and
inductance whose dimensions are arranged to
allow electron oscillation at a precise
frequency.


• The frequency (F) at which the circuit will
oscillate depends only on its electrical size,
which is the product of capacitance (C) and
inductance (L):

1
F=
2Π√LC


• The main function is to give an amplified AC,
that has a high frequency.
• It consists of,
I. Main supply
II. Triode valve
III. Grid leak resistance
IV. Oscillator circuit


1. Main supply: It is connected with AC mains
that gives 220 or 240 volts and frequency of
50 cycles/second.
2. Transformer: There are two types of
transformer which are used in the
construction, such as:


a. Step down transformer: The secondary coil
of which is connected with the filament of
the triode valve and produces a potential of
20 volts, which causes emission of electrons
from the cathode through thermionic
emission.
b. Step up transformer: The secondary coil of
this transformer is connected with the
oscillator circuit, which in turn is connected
with the triode valve.

3. Triode valve: This is the thermionic valve,
which allows electrons to flow in one direction.
• When the current flows through the filament
electrons are emitted by thermionic emission
from the cathode.
• The electrons emitted move towards the
anode provided the grid does not have any
charge.


• The grid of the triode valve is connected with
the grid leak resistance.
• The grid of the triode valve acts as a
regulator to the flow of the current, i.e. when
positive allows flow of current and when
negative stops the current flow.


4. Grid leak resistance: It consists of a
resistance coil connected to the grid of the
triode valve at one end and the filament of
the cathode at the other.
5. Oscillator: It consists of a stable condenser
and an oscillator coil, which gives high
magnitude, high frequency oscillating
currents to the resonator circuit.


Reference

1. Electrotherapy Simplified – Nanda

2. Clayton’s Electrotherapy

3. Electrotherapy Evidence-based Practice –
Sheila Kitchen


Fundamentals of high frequency currents priyank

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