The document discusses various components used in rectification circuits, including thermionic valves, semiconductors, transistors, rectifiers, transformers, and choke coils. It provides details on their construction and working principles. The key components and processes discussed are diode valves and their use in half-wave rectification of alternating current to direct current.

1. CURRENT
RECTIFICATION

2. CONTENTS
 THERMIONIC VALVES
 SEMICONDUCTORS
 TRANSISTORS
 RECTIFIER
 TRANSFORMER
 CHOKE COIL
 RECTIFICATION OF ALTERNATING CURRENT
 SMOOTHING CIRCUIT

3. THERMIONIC VALVES
 These are the devices which allow electron flow in one
direction only and work using heat
 There are various types of thermionic valves, which are
named according to the number of electrodes they contain
 They are as follows
• Diode valves
• Triode valves

4.  DIODE VALVES
- 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
- For the current to pass through the valve, the
filament must be heated, causing emission of
electrons by the process of thermionic emission, and
a PD when applied makes the plate ( anode) positive
in relation to the cathode
- The filament used can be directly or indirectly heating
type and the anode plate is made from some metal
and is in the form of a cylinder surrounding the
cathode

5.  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

6.  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 and there by increasing the
current flow across the valve
 In a diode there are the filament circuit and the
anode circuit
 Function of diode valve:
- Rectification of AC to DC

8.  TRIODE VALVES
- The triode valve is a device that contains three electrodes,
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 or metal wire
- 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

9.  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.
 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 the grid is given positive charge, the electrons can pass and
the current flows
 The flow of current across the triode valve can be regulated
by adjusting the bias of the grid

10.  Uses of 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

11.  fig

12. 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 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 gives rise to positive hole, due
to which current flow occurs
 If a n type and p type semiconductor are fused together,
electrons can only pass in the n to p direction and the
semiconductor therefore acts as a valve.

14. 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 neighbouring atoms, so that there are no
free electrons to transmit an electric current
 When certain other materials such as phosphorus ( with 5
electrons outside) are added to silicon it transmits current
 When silicon and phosphorus form covalent bonds, 4 electrons
of phosphorus make bond with 4 electrons of silicon, leaving
behind one free electron in phosphorus which are not held in
bond with other atoms, therefore carrying current
 Such a material is called n type semiconductor

16. 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, where as 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

17.  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

19. 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, i,.e from p to n, and such a device is called
a semiconductor diode

20. 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 and
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.

22.  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, ex. SWD

23. RECTIFIERS
 Rectification is the process of conversion of AC current
into DC current
 The device that produces the process is called as rectifiers
 Rectifiers can be metal rectifiers or the diode valve

24. METAL RECTIFIER
 A metal rectifier works on the principle of semiconductor
diode
 One type of rectifier consists of a copper disk, coated on the
surface with copper oxide
 Copper oxide is a p type of semiconductor and copper being a
metal has free electrons, so acts like an n type semiconductor
 So when the two materials are in contact, a PD develops at
their junction
 When the rectifier is connected into a circuit with the copper
negative relative to the copper oxide, current passes more
easily than when the polarity is reversed
 A series of disks can be used to rectify larger voltages but
must be separated from each other by suitable materials;
otherwise the PD developed at the contacts would cancel each
other out.

25.  fig

26. TRANSFORMER
 Transformer is a device used for changing low alternating
voltage at high current.
 It changes the alternating voltage without the loss of energy.
 Types of transformers:
- Static transformer
- Variable transformer
- Autotransformer

27. STATIC TRANFORMER
 An electrical transformer works on the principles of
electromagnetic induction and is used to alter voltage or to
render a current earth free
 CONSTRUCTION
- The transformer consists of two coils of insulated wire wound
onto a laminated soft iron frame
- 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.
- The two coils may be wound on top of one another, or on
opposite sides of the frame

29.  WORKING
- An alternating current is passed through the primary coil and
this sets up a varying magnetic field which cuts the secondary
coil.
- By electromagnetic induction an EMF is induced into the
secondary circuit.
- 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.
- There is no electrical conduction between the primary and the
secondary, 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
demagnetized.
- It is laminated to prevent eddy currents.

30. FUNCTIONS OF THE TRANSFORMER
 TO ALTER 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.
1. If both primary and secondary coils have the same number of
turns, then the voltage in each will be the same - Even ratio
transformer
2. If the secondary coil has fewer turns than the primary then the
EMF or voltage in the secondary will be less than the primary, i.e.
it is stepped down- step down transformer
for ex. If the primary coil has 120 turns and an applied voltage
of 100 volts, and the secondary has 60 turns, then the voltage in the
secondary will be stepped down to 50 volts

31. 3. If the secondary coil has more turns than the primary , the
voltage developed in the secondary will be increased or stepped
up : step up transformer
For ex. If the primary coil has 120 turns and an EMF of 100
volts and the secondary has 240 turns, then the EMF developed
in the secondary coil will be 200 volts
 It is important to note that the electrical power in both
primary and secondary coils is the same.
 Power is measured in watts ( watts= volts*amps), so the
quantity watts*amps must be the same for the primary and
the secondary coils, i.e any change in voltage must be
accompanied by a change in current

32.  For example in fig, which shows a step down transformer, if
the voltage is halved in the secondary coil, the current must
be doubled

33.  In fig, for the step up transformer, where the voltage in the
secondary coil is doubled, the current is halved

34.  TO RENDER A CURRENT EARTH FREE
- Mains electricity is produced by a 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 the live wire and
the earth, current will flow along it: if this connection were
made by a person they would then receive an ‘earth shock’ as
the current flowed 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.

35. - 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 all currents applied to
patients are rendered earth free by using a static transformer

36. VARIABLE TRANSFORMER
 This consists of a primary and a secondary coil, but
is constructed so that one of them can be altered in
length
 The primary coil has a number of tappings taken
from it and a movable contact can be placed 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 of voltage in the secondary coil
 In this way a very crude control of voltage obtained

37. AUTOTRANSFORMER
 An autotransformer consists of a single coil of wire
with four contact points coming from it
 When it is used as a step up transformer, CD is the
primary coil and AB the secondary.
 Although the autotransformer works on the
principles of electromagnetic induction, it has the
disadvantage that it allows only a small step up and
does not render the current earth free

39. 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:
- Low frequency Choke coil
- High frequency choke coil

40. LOW FREQUENCY 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
 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
demagnetized, and is laminated to prevent eddy
currents.

41. 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.

42. 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 the 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.

43.  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 considered,
there by retarding the flow of such a current,
whereas 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

45. RECTIFICATION OF ALTERNATING
CURRENT
 Rectification is the conversion of AC into DC
 This can be accomplished by either the metal rectifier or the diode
valve
 There are two types of rectification:
1. Half wave Rectification
- if one valve or metal rectifier is included in the circuit, current can
pass in one direction only and the flow is blocked during alternate half
cycles of alternating current
- in figure, the continuous lines represents the current flow, and the
dotted lines which are the reverse waves represent no flow of current

46.  The resulting current that is obtained is unidirectional,
pulsating and interrupted, and the process by which it is
obtained is called half wave rectification

47. 2. Full wave rectification
- It is the process by which a unidirectional, pulsating but
uninterrupted current flow is produced
- The circuits that produce the full wave rectification are such
that, the direction of the current is reversed during alternate
half cycles of AC, as shown in fig

48. SMOOTHING CIRCUIT
 Though the current that is obtained from the rectifying
circuit is unidirectional, it still varies considerably in
intensity
 In order to eliminate these variations and render the
current suitable for application to patients, a circuit is
necessary called the smoothing circuit
 The circuit consists of one or two condensers wired in
parallel to the output circuit and a choke coil in series
with the circuit

50.  When the EMF of the rectified current rises, current flows in
the external circuit and at the same time the condensers are
charged
 When the EMF falls, the intensity of the current in the output
circuit falls but the condensers discharge round this circuit
and augment the current flow so that the intensity does not
fall to zero
 Thus the variations in the intensity of the current are
reduced
 The condensers have a large capacity, so that they offer lite
impedance to the charging current and hold a considerable
quantity of electricity to discharge round the circuit.

51.  As the current varies in intensity, a self induced EMF
is set up the choke coil.
 When the intensity of the current is rising, these self
induced EMF opposes the applied EMF and retards
the rising current
 When the intensity of the current is falling, the self
induced EMF is in the same direction as the applied
EMF and prolongs the current flow
 These effects further reduce the variations of the
intensity of the current
 Though the current that is obtained from the
smoothing circuit varies slightly in intensity, it is still
suitable for constant DC treatments.

52. THANK YOU…..!!!