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. Conductor2. Magnetic lines of force3. 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.



• 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.



• 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.



• 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.



• 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 coil2. Placing the turns close together3. 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 CoilChoke 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



• 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.



• 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 transformer2. variable transformer3. 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 valve2. 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):


2Π√LC

1F =

• The main function is to give an amplified AC, that has a high frequency.

• It consists of,I. Main supplyII. Triode valveIII. Grid leak resistanceIV. 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


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