TOPIC 41 – ELECTRONICS

  1. Introduction
    1. Definition – Study of free electrons in motion.
    2. Uses – pocket calculators, digital watches, musical instruments, radios, TVs, computers, robots, etc
  2. Classes of materials and their differences:-
    1. Conductors – Have free electrons on the outer shell – copper, silver, aluminium, etc
    2. Insulators – Electrons tightly bound to the nucleus – rubber, plastics, ceramics, etc
    3. Semi-conductors – conducting properties between conductor & insulators – silicon, Germanium,
  3. Energy Bands or Levels:-
    1. Conduction band – electrons are free to move under an influence of an electric current.
    2. Valance band – Electrons are not free to move
    3. Forbidden band – represents the energy level where electron cannot occupy
    4. Materials and the three energy levels
  4. Intrinsic and Extrinsic Semi-conductors
    1. Intrinsic semiconductors:
  5. Definition: Pure semi-conductors – with properties of a pure substance –
    1. Examples – Group 4 elements – silicon & Germanium
    2. Extrinsic semi-conductors
  6. Definition – With added impurities –
    1. Doped to obtain a desired electrical property. G3 or G5
  7. The Doping Process – Extrinsic Semi-conductors
    • Doping is a process of introducing a very small quantity of impurity to a pure semiconductor to obtain a desired electrical property.
  8. N-Type – formed by doping G4 with G5 – Pentavalent atom – Phosphorous, Antimony, Arsenic
  1. Majority charge carriers are electrons and minority charge carriers are hole.
  2. ii) Illustrate with sketches
    1. P-Type – formed by doping G4 with G3 – Trivalent atom – Boron, aluminium, Indium etc-
  3. Majority charge carriers are holes and minority charge carriers are electrons.
  4. ii) Illustrate with sketches
  5. Junction Diodes – The P-N Junction Diode
    1. Definition of a diode – device which allows current to flow in one direction only. It is a one way valve
    2. Formation of p-n junction diode – device in which the p-side is connected to Anode and n-side to Cathode
    3. Depletion layer – region between p-side and n-side having very high resistance – conducts poorly.
    4. Difference between a thermionic diode and p-n junction diode – and their circuit symbols
  6. Biasing -Two biasing systems of P-N Diodes:-
    1. Forward bias – low resistance – current flows – conducts well – draw circuit diagram
    2. Reverse bias – high resistance – current through the diode is virtually zero – draw circuit diagram
  7. Characteristics curves for p-n junction diodes
    1. Current I against Voltage for Silicon
    2. Current I against Voltage for Germanium
    3. Reason why silicon is preferred to Germanium.
  8. Applications of P-N Junction diode
    1. Protection – from reverse power supply
    2. Rectification – changing Alternation current to Direct current – AC to DC
  9. Rectification and smoothing.
    1. Definition of rectification & use of diodes
    2. Definition of smoothing & use of capacitors
    3. Types of rectification
  10. Half-wave rectification – use of one diode
    1. Full-wave rectification:
      1. Use of Two Diodes – centre-tap-transformer & Use of Four diodes – bridge rectifier
  1. Project work – Simple radio Receiver
  1. Introduction
  1. Definition – Study of motion of free electrons in electrical circuits.
  2. Uses – pocket calculators; digital watches; heart pacemakers; musical instruments; radios, TVs, computers for industry, commerce and scientific research; traffic lights; microwave ovens; video cassette recorders; Personal computers (PCs); electronic games; multimedia applications; computer aided design (CAD); electronic limbs; “keyhole” surgery; data processing; electronic cash dispensers; digital telephone links; fax; e-mail; World Wide Web; Robots, etc
    1. Classes of materials and their differences:-
  3. Conductors:
  4. Have free electrons on the outer shell
    1. Electrons not tightly bound to the nucleus of the atom
  • The materials have very low electrical resistance
  1. Good conductors of electricity.
  2. Examples – silver, copper, aluminium, etc
  3. Resistance increases with rise in temperature, caused by collision between moving free electrons and the vibrating atoms.
  1. Insulators
  2. Electrons are not free.
  3. Electrons are tightly bound to the nucleus of the atoms.
  • The materials have very high electrical resistance
  1. They do not conduct electricity.
  2. Examples – Rubber, plastics, ceramics,
  3. Insulators have negative temperature coefficient of resistance.
  4. Semi-conductors
  5. Materials with conducting properties between conductors and insulators
  6. Their electrical conductivities are higher than those of insulators but less than those of conductors.
  • Examples – silicon, Germanium, Indium, Gallium Arsenide, Cadmium Sulphite, etc
  1. Semiconductors have negative temperature coefficient of resistance; i.e. their electrical resistance decrease with increase in temperature.
  2. The extent to which a semiconductor conducts electricity is considerably affected by the presence of impurities.

 

 

  1. Energy Bands or Levels:-
  • Conduction band – here electrons are free to move under the influence of an electric current.
  • Valance band – here electrons are not free to move
  • Forbidden band – represents the energy level that cannot be occupied by electrons. The width of the band determines the conductivity of the material.

Materials and the three energy levels:-

  1. For Conductors
  2. Conduction band:- have free electrons
  3. Valence band – few electrons – unfilled because some electrons are in the conduction band
  • Forbidden band – No forbidden band –conduction and valence band overlap.
  1. For Insulators
  2. Conduction band – has no electrons – Empty
  3. Valance band – Filled with electrons – full of electrons
  • Forbidden band – has very wide gap – high resistance – 3 to 5 eV
  1. For Semiconductors
  1. Conduction band: – Has no electrons at 0K Empty at very low temperatures

– Partially filled at room temperature.

  1. Valance band – – Filled with electrons at 0K i.e. filled at very low temperatures

– Unfilled at room temperature – few electrons at room temperature.

  • Forbidden band – Has a narrow gap – 1 eV

Diagrams to illustrate energy levels for materials

 

NOTE: For Semiconductors –

  • At room temperature – it has holes in the valance band and electrons in the conduction band
  • At 0K – it behaves like an insulator
  • HOLES – Holes are created when an electron moves from valance band to conduction band.
  • Holes are very important for conduction of electric current in semiconductors.

 

  1. Intrinsic and Extrinsic Semi-conductors
  1. Intrinsic semiconductors:
  1. They are pure semi-conductors, with electrical properties of a pure substance.
  2. Has equal number of holes and electrons
  • Conductivity is very low, insulator at low temperature.
  1. Usually not used in the pure state.
  2. Examples – Group 4 elements – silicon & Germanium
  3. Diagrams
  • An atom of silicon has four valence electrons and in the lattice each one is shared with a nearby atom to form four covalent bonds.
  • A strong crystal lattice results in which it is difficult for electrons to escape from their atoms.
  • Pure silicon is therefore a very good insulator, being perfect at near absolute zero (- 273 0C).

 

  1. b) Extrinsic semi-conductors
  2. i) They are semiconductors with added impurities
  3. ii) They are doped to obtain a desired electrical property; doped with group 3 or 5 elements.
    • All semiconductors in practical use have added impurities

 

  1. The Doping Process – Extrinsic Semi-conductors
  • Doping is a process of introducing a very small quantity of impurity to a pure semiconductor to obtain a desired electrical property.

 

  • There are two types of extrinsic semiconductors:-
  1. N-Type Semiconductor
  2. Formed by doping G4 with G5 – Pentavalent atom – Phosphorous, Antimony, Arsenic
  3. Group 4 elements – Tetravalent – silicon, germanium, etc
  • Formation of an N-Type Semiconductor:-
    • Formed by adding a Pentavalent atom (Phosphorus) to a group 4 semiconductor (silicon) and an extra electron is left unpaired and is available for conduction.
    • Majority charge carriers are electrons and minority charge carriers are positive hole.
    • Conduction of electricity is now possible because of extra electrons.
    • Phosphorous is called a DONOR. Silicon has now more electrons.

 

  1. Diagrams

 

  1. P-Type Semiconductor
  2. formed by doping G4 with G3 – Trivalent atom – Boron, aluminium, Indium etc-
  3. Formation of a P-Type Semiconductor:-
  • Formed by adding a trivalent atom (Boron) to a group 4 semiconductor (silicon), a fourth electron will be unpaired and a gap will be left called a positive hole.
  • Pure semiconductor is doped with an impurity of group 3 element, combination creates a positive hole which can accepts an electron.
  • The doping material creates a Positive hole, which can accept an electron – called an
  • Majority charge carriers are holes and minority charge carriers are electrons.
  • Diagrams:-

 

  1. Junction Diodes – The P-N Junction Diode
  1. Definition of a diode –
  • An electronic device with two electrodes, which allows current to flow in one direction only.
  • It is a one way valve. It is a solid device.
  1. Formation of p-n junction diode
    • It is a device in which the p-side is connected to Anode and n-side to Cathode
  • It consists of such a p-n junction with the P-side connected to the Anode and the N-side to the Cathode.
  • It is formed by doping a crystal of pure silicon so that a junction is formed between the p-type and n-type regions.
  1. Depletion layer –
  • The region between p-side and n-side having very high resistance, it conducts poorly.
  • At the junction, electrons diffuse from both sides and neutralize each other.
  • A narrow depletion layer is formed on either side of the junction free from charge carriers and of high resistance
  1. The Junction
  • The plane (boundary) between two different types of semiconductors.
  1. Diagram of unbiased P-N Junction diode.

 

  1. Difference between a thermionic diode and p-n junction diode – and their circuit symbols

 

  1. Biasing -Two biasing systems of P-N Diodes:-
  2. Forward bias – low resistance – current flows – conducts well – draw circuit diagram
  3. Reverse bias – high resistance – current through the diode is virtually zero – draw circuit diagram

 

  1. Characteristics curves for p-n junction diodes
  2. Current I against Voltage for Silicon
  3. Current I against Voltage for Germanium
  4. Reason why silicon is preferred to Germanium.

 

  1. Applications of P-N Junction diode
  2. Protection – from reverse power supply
  1. Rectification –

changing Alternation current to Direct current – AC to DC

RECTIFIERS

Overview

As we have noted when looking at the Elements of a Power Supply, the purpose of the rectifier section is to convert the incoming ac from a transformer or other ac power source to some form of pulsating dc. That is, it takes current that flows alternately in both directions as shown in the first figure to the right, and modifies it so that the output current flows only in one direction, as shown in the second and third figures below.

The circuit required to do this may be nothing more than a single diode, or it may be considerably more complex. However, all rectifier circuits may be classified into one of two categories, as follows:

 

Half-Wave Rectifiers. An easy way to convert ac to pulsating dc is to simply allow half of the ac cycle to pass, while blocking current to prevent it from flowing during the other half cycle. The figure to the right shows the resulting output. Such circuits are known as half-wave rectifiers because they only work on half of the incoming ac wave. 

Full-Wave Rectifiers. The more common approach is to manipulate the incoming ac wave so that both halves are used to cause output current to flow in the same direction. The resulting waveform is shown to the right. Because these circuits operate on the entire incoming ac wave, they are known as full-wave rectifiers. 

 

 

Rectifier circuits may also be further clasified according to their configuration, as we will see below

 

The Half-Wave Rectifier

 

 

The simplest rectifier circuit is nothing more than a diode connected in series with the ac input, as shown to the right. Since a diode passes current in only one direction, only half of the incoming ac wave will reach the rectifier output. Thus, this is a basic half-wave rectifier.

 

The orientation of the diode matters; as shown, it passes only the positive half-cycle of the ac input, so the output voltage contains a positive dc component. If the diode were to be reversed, the negative half-cycle would be passed instead, and the dc component of the output would have a negative polarity. In either case, the DC component of the output waveform is vp/π = 0.3183vp, where vp is the peak voltage output from the transformer secondary winding.

It is also quite possible to use two half-wave rectifiers together, as shown in the second figure to the right. This arrangement provides both positive and negative output voltages, with each output utilizing half of the incoming ac cycle

 

 

Note that in all cases, the lower transformer connection also serves as the common reference point for the output. It is typically connected to the common ground of the overall circuit. This can be very important in some applications. The transformer windings are of course electrically insulated from the iron core, and that core is normally grounded by the fact that it is bolted physically to the metal chassis (box) that supports the entire circuit. By also grounding one end of the secondary winding, we help ensure that this winding will never experience even momentary voltages that might overload the insulation and damage the transformer.

The Full-Wave Rectifier

 

While the half-wave rectifier is very simple and does work, it isn’t very efficient. It only uses half of the incoming ac cycle, and wastes all of the energy available in the other half. For greater efficiency, we would like to be able to utilize both halves of the incoming ac. One way to accomplish this is to double the size of the secondary winding and provide a connection to its center. Then we can use two separate half-wave rectifiers on alternate half-cycles, to provide full-wave rectification. The circuit is shown to the right.

 

 

Because both half-cycles are being used, the DC component of the output waveform is now 2vp/π = 0.6366vp, where vp is the peak voltage output from half the transformer secondary winding, because only half is being used at a time.

This rectifier configuration, like the half-wave rectifier, calls for one of the transformer’s secondary leads to be grounded. In this case, however, it is the center connection, generally known as the center tap on the secondary winding.

 

 

The full-wave rectifier can still be configured for a negative output voltage, rather than positive. In addition, as shown to the right, it is quite possible to use two full-wave rectifiers to get outputs of both polarities at the same time.

The full-wave rectifier passes both halves of the ac cycle to either a positive or negative output. This makes more energy available to the output, without large intervals when no energy is provided at all. Therefore, the full-wave rectifier is more efficient than the half-wave rectifier. At the same time, however, a full-wave rectifier providing only a single output polarity does require a secondary winding that is twice as big as the half-wave rectifier’s secondary, because only half of the secondary winding is providing power on any one half-cycle of the incoming ac.

Actually, it isn’t all that bad, because the use of both half-cycles means that the current drain on the transformer winding need not be as heavy. With power being provided on both half-cycles, one half-cycle doesn’t have to provide enough power to carry the load past an unused half-cycle. Nevertheless, there are some occasions when we would like to be able to use the entire transformer winding at all times, and still get full-wave rectification with a single output polarity.

 

 

The Full-Wave Bridge Rectifier

 

The four-diode rectifier circuit shown to the right serves very nicely to provide full-wave rectification of the ac output of a single transformer winding. The diamond configuration of the four diodes is the same as the resistor configuration in a Wheatstone Bridge. In fact, any set of components in this configuration is identified as some sort of bridge, and this rectifier circuit is similarly known as a bridge rectifier.

If you compare this circuit with the dual-polarity full-wave rectifier above, you’ll find that the connections to the diodes are the same. The only change is that we have removed the center tap on the secondary winding, and used the negative output as our ground reference instead. This means that the transformer secondary is never directly grounded, but one end or the other will always be close to ground, through a forward-biased diode. This is not usually a problem in modern circuits.

 

 

To understand how the bridge rectifier can pass current to a load in only one direction, consider the figure to the right. Here we have placed a simple resistor as the load, and we have numbered the four diodes so we can identify them individually.

During the positive half-cycle, shown in red, the top end of the transformer winding is positive with respect to the bottom half. Therefore, the transformer pushes electrons from its bottom end, through D3 which is forward biased, and through the load resistor in the direction shown by the red arrows. Electrons then continue through the forward-biased D2, and from there to the top of the transformer winding. This forms a complete circuit, so current can indeed flow. At the same time, D1 and D4 are reverse biased, so they do not conduct any current.

During the negative half-cycle, the top end of the transformer winding is negative. Now, D1 and D4 are forward biased, and D2 and D3 are reverse biased. Therefore, electrons move through D1, the resistor, and D4 in the direction shown by the blue arrows. As with the positive half-cycle, electrons move through the resistor from left to right.

In this manner, the diodes keep switching the transformer connections to the resistor so that current always flows in only one direction through the resistor. We can replace the resistor with any other circuit, including more power supply circuitry (such as the filter), and still see the same behavior from the bridge rectifier.

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