PROCEDURE FOR MAKING PROJECT

Building project in the proper manner is really an art, something which must

be prectised and learned through trial and error, it is not all that difficult. The

main thing is to remember to take each step slowly and carefully according to

the instructions giving making since that everything at it should be before

proceeding further.

TOOLS:

The electronics workbench is an actual place of work with comfortably & conveniently & should be supplied with compliment of those tools must often use in project building. Probably the most important device is a soldering tool. Other tool which should be at the electronic work bench includes a pair of needle nose pliers, diagonal wire cutter, a small knife, an assortment of screw driver, nut driver, few nuts & bolts, electrical tape, plucker etc. Diagonal wire cutter will be used to cut away any excess lead length from copper side of P.C.B. 7 to cut section of the board after the circuit is complete. The needle nose pliers are most often using to bend wire leads & wrap them in order to form a strong mechanical connection.

MOUNTING & SOLDERING:

Soldering is process of joining together two metallic parts. It is actually a process of function in which an alloy, the solder, with a comparatively low melting point penetrates the surface of the metal being joined & makes a firm joint between them on cooling & solidifying.

THE SOLDERING KIT

1.SOLDERING IRON:

As soldering is a process of joining together two metallic parts, the instrument, which is used, for doing this job is known as soldering Iron. Thus it is meant for melting the solder and to setup the metal parts being joined. Soldering Iron is rated according to their wattage, which varies from 10- 200 watts.

2.SOLDER:

The raw material used for soldering is solder. It is composition of lead & tin. The good quality solder (a type of flexible naked wire) is 60% Tin +40% Lead which will melt between 180 degree to 200 degree C temperature.

3.FLUXES OR SOLDERING PASTE:

When the points to solder are heated, an oxide film forms. This must be removed at once so that solder may get to the surface of the metal parts. This is done by applying chemical substance called Flux, which boils under the heat of the iron remove the oxide formation and enable the metal to receive the solder.

4.BLADES OR KNIFE:

To clean the surface & leads of components to be soldered is done by this common instrument.

5.SAND PAPER:

The oxide formation may attack at the tip of your soldering iron & create the problem. To prevent this, clean the tip with the help of sand paper time to time or you may use blade for doing this job. Apart from all these tools, the working bench for soldering also includes desoldering pump, wink wire (used for desoldering purpose), file etc.

HOW TO SOLDER?

Mount components at their appropriate place; bend the leads slightly outwards to prevent them from falling out when the board is turned over for soldering. No cut the leads so that you may solder them easily. Apply a small amount of flux at these components leads with the help of a screwdriver. Now fix the bit or iron with a small amount of solder and flow freely at the point and the P.C.B copper track at the same time. A good solder joint will appear smooth & shiny. If all appear well, you may continue to the next solder connections.

TIPS FOR GOOD SOLDERING

Use right type of soldering iron. A small efficient soldering iron (about 10-25 watts with 1/8 or 1/4 inch tip) is ideal for this work.

Keep the hot tip of the soldering iron on a piece of metal so that excess heat is dissipated.

Make sure that connection to the soldered is clean. Wax frayed insulation and other substances cause poor soldering connection. Clean the leads, wires, tags etc. before soldering.

Use just enough solder to cover the lead to be soldered. Excess solder can cause a short circuit.

Use sufficient heat. This is the essence of good soldering. Apply enough heat to the component lead. You are not using enough heat, if the solder barely melts and forms a round ball of rough flaky solder. A good solder joint will look smooth, shining and spread type. The difference between good & bad soldering is just a few seconds extra with a hot iron applied firmly.

PRECAUTIONS

Mount the components at the appropriate places before soldering. Follow the circuit description and components details, leads identification etc. Do not start soldering before making it confirm that all the components are mounted at the right place.

Do not use a spread solder on the board, it may cause short circuit.

Do not sit under the fan while soldering.

Position the board so that gravity tends to keep the solder where you want it.

Do not over heat the components at the board. Excess heat may damage the components or board.

The board should not vibrate while soldering otherwise you have a dry or a cold joint.

Do not put the kit under or over voltage source. Be sure about the voltage either dc or ac while operating the gadget.

Do spare the bare ends of the components leads otherwise it may short circuit with the other components. To prevent this use sleeves at the component leads or use sleeved wire for connections.

Do not use old dark colour solder. It may give dry joint. Be sure that all the joints are clean and well shiny.

Do make loose wire connections especially with cell holder, speaker, probes etc. Put knots while connections to the circuit board, otherwise it may get loose.

POWER SUPPLY

In alternating current the electron flow is alternate, i.e. the electron flow increases to maximum in one direction, decreases back to zero. It then increases in the other direction and then decreases to zero again. Direct current flows in one direction only. Rectifier converts alternating current to flow in one direction only. When the anode of the diode is positive with respect to its cathode, it is forward biased, allowing current to flow. But when its anode is negative with respect to the cathode, it is reverse biased and does not allow current to flow. This unidirectional property of the diode is useful for rectification. A single diode arranged back-to-back might allow the electrons to flow during positive half cycles only and suppress the negative half cycles. Double diodes arranged back-to-back might act as full wave rectifiers as they may allow the electron flow during both positive and negative half cycles. Four diodes can be arranged to make a full wave bridge rectifier. Different types of filter circuits are used to smooth out the pulsations in amplitude of the output voltage from a rectifier. The property of capacitor to oppose any change in the voltage applied across them by storing energy in the electric field of the capacitor and of inductors to oppose any change in the current flowing through them by storing energy in the magnetic field of coil may be utilized. To remove pulsation of the direct current obtained from the rectifier, different types of combination of capacitor, inductors and resistors may be also be used to increase to action of filtering.

NEED OF POWER SUPPLY

Perhaps all of you are aware that a ‘power supply’ is a primary requirement for the ‘Test Bench’ of a home experimenter’s mini lab. A battery eliminator can eliminate or replace the batteries of solid-state electronic equipment and the equipment thus can be operated by 230v A.C. mains instead of the batteries or dry cells. Nowadays, the use of commercial battery eliminator or power supply unit has become increasingly popular as power source for household appliances like transreceivers, record player, cassette players, digital clock etc.

THEORY

U SE OF DIODES IN RECTIFIERS:

Electric energy is available in homes and industries in India, in the form of alternating voltage. The supply has a voltage of 220V (rms) at a frequency of 50 Hz. In the USA, it is 110V at 60 Hz. For the operation of most of the devices in electronic equipment, a dc voltage is needed. For instance, a transistor radio requires a dc supply for its operation. Usually, this supply is provided by dry cells. But sometime we use a battery eliminator in place of dry cells. The battery eliminator converts the ac voltage into dc voltage and thus eliminates the need for dry cells. Nowadays, almost all-electronic equipment includes a circuit that converts ac voltage of mains supply into dc voltage. This part of the equipment is called Power Supply. In general, at the input of the power supply, there is a power transformer. It is followed by a diode circuit called Rectifier. The output of the rectifier goes to a smoothing filter, and then to a voltage regulator circuit. The rectifier circuit is the heart of a power supply.

RECTIFICATION

Rectification is a process of rendering an alternating current or voltage into a unidirectional one. The component used for rectification is called ‘Rectifier’. A rectifier permits current to flow only during the positive half cycles of the applied AC voltage by eliminating the negative half cycles or alternations of the applied AC voltage. Thus pulsating DC is obtained. To obtain smooth DC power, additional filter circuits are required.

A diode can be used as rectifier. There are various types of diodes. But, semiconductor diodes are very popularly used as rectifiers. A semiconductor diode is a solid-state device consisting of two elements is being an electron emitter or cathode, the other an electron collector or anode. Since electrons in a semiconductor diode can flow in one direction only-from emitter to collector- the diode provides the unilateral conduction necessary for rectification. Out of the semiconductor diodes, copper oxide and selenium rectifier are also commonly used.

FULL WAVE RECTIFIERIt is possible to rectify both alternations of the input voltage by using two diodes in the circuit arrangement. Assume 6.3 V rms (18 V p-p) is applied to the circuit. Assume further that two equal-valued series-connected resistors R are placed in parallel with the ac source. The 18 V p-p appears across the two resistors connected between points AC and CB, and point C is the electrical midpoint between A and B. Hence 9 V p-p appears across each resistor. At any moment during a cycle of vin, if point A is positive relative to C, point B

is negative relative to C. When A is negative to C, point B is positive relative to C. The effective voltage in proper time phase which each diode "sees" is in Fig. The voltage applied to the anode of each diode is equal but opposite in polarity at any given instant.

When A is positive relative to C, the anode of D1 is positive with respect to

its cathode. Hence D1 will conduct but D2 will not. During the second

alternation, B is positive relative to C. The anode of D2 is therefore positive

with respect to its cathode, and D2 conducts while D1 is cut off.

There is conduction then by either D1 or D2 during the entire input-voltage

cycle.

Since the two diodes have a common-cathode load resistor RL, the output

voltage across RL will result from the alternate conduction of D1 and D2.

The output waveform vout across RL, therefore has no gaps as in the case of

the half-wave rectifier.

The output of a full-wave rectifier is also pulsating direct current. In the diagram, the two equal resistors R across the input voltage are necessary to provide a voltage midpoint C for circuit connection and zero reference. Note that the load resistor RL is connected from the cathodes to this center

reference point C.

An interesting fact about the output waveform vout is that its peak amplitude

is not 9 V as in the case of the half-wave rectifier using the same power

source, but is less than 4½ V. The reason, of course, is that the peak positive voltage of A relative to C is 4½ V, not 9 V, and part of the 4½ V is lost across R.

Though the full wave rectifier fills in the conduction gaps, it delivers less than half the peak output voltage that results from half-wave rectification.

BRIDGE RECTIFIERA more widely used full-wave rectifier circuit is the bridge rectifier. It requires four diodes instead of two, but avoids the need for a centre-tapped transformer. During the positive half-cycle of the secondary voltage, diodes D2 and D4 are conducting and diodes D1 and D3 are non-conducting. Therefore, current flows through the secondary winding, diode D2, load resistor RL and diode D4. During negative half-cycles of the secondary voltage, diodes D1 and D3 conduct, and the diodes D2 and D4 do not conduct. The current therefore flows through the secondary winding, diode D1, load resistor RL and diode D3. In both cases, the current passes through the load resistor in the same direction. Therefore, a fluctuating, unidirectional voltage is developed across the load.

Filtration

The rectifier circuits we have discussed above deliver an output voltage that always has the same polarity: but however, this output is not suitable as DC power supply for solid-state circuits. This is due to the pulsation or ripples of the output voltage. This should be removed out before the output voltage can be supplied to any circuit. This smoothing is done by incorporating filter networks. The filter network consists of inductors and capacitors. The inductors or choke coils are generally connected in series with the rectifier output and the load. The inductors oppose any change in the magnitude of a current flowing through them by storing up energy in a magnetic field. An inductor offers very low resistance for DC whereas; it offers very high resistance to AC. Thus, a series connected choke coil in a rectifier circuit helps to reduce the pulsations or ripples to a great extent in the output voltage. The fitter capacitors are usually connected in parallel with the rectifier output and the load. As, AC can pass through a capacitor but DC

cannot, the ripples are thus limited and the output becomes smoothed. When the voltage across its plates tends to rise, it stores up energy back into voltage and current. Thus, the fluctuations in the output voltage are reduced considerable. Filter network circuits may be of two types in general:

CHOKE INPUT FILTER

If a choke coil or an inductor is used as the ‘first- components’ in the filter network, the filter is called ‘choke input filter’. The D.C. along with AC pulsation from the rectifier circuit at first passes through the choke (L). It opposes the AC pulsations but allows the DC to pass through it freely. Thus AC pulsations are largely reduced. The further ripples are by passed through the parallel capacitor C. But, however, a little nipple remains unaffected, which are considered negligible. This little ripple may be reduced by incorporating a series a choke input filters.

CAPACITOR INPUT FILTER

If a capacitor is placed before the inductors of a choke-input filter network, the filter is called capacitor input filter. The D.C. along with AC ripples from the rectifier circuit starts charging the capacitor C. to about peak value. The AC ripples are then diminished slightly. Now the capacitor C, discharges through the inductor or choke coil, which opposes the AC ripples, except the DC. The second capacitor C by passes the further AC ripples. A small ripple is still present in the output of DC, which may be reduced by adding additional filter network in series.

CIRCUIT DIAGRAM

MAKING PRINTED CIRCUIT BOARD (P.C.B.)

INTRODUCTION--

Making a Printed Circuit Board is the first step towards building electronic equipment by any electronic industry. A number of methods are available for making P.C.B., the simplest method is of drawing pattern on a copper clad board with acid resistant (etchants) ink or paint or simple nail polish on a copper clad board and do the etching process for dissolving the rest of copper pattern in acid liquid.

MATERIAL REQUIRED

The apparatus needs for making a P.C.B. is :- Copper Clad Sheet Nail Polish or Paint Ferric Chloride Powder (Fecl) Plastic Tray

PROCEDURE

The first and foremost in the process is to clean all dirt from copper sheet with say spirit or trichloro ethylene to remove traces grease or oil etc. and then wash the board under running tap water. Dry the surface with forced warm air or just leave the board to dry naturally for some time.

Making of the P.C.B. drawing involves some preliminary consideration such as thickness of lines/ holes according to the components. Now draw the sketch of P.C.B. design (tracks, rows, square) as per circuit diagram with the help of nail polish or enamel paint or any other acid resistant liquid. Dry the point surface in open air, when it is completely dried, the marked holes in P.C.B. may be drilled using 1Mm drill bits. In case there is any shorting of lines due to spilling of paint, these may be removed by scraping with a blade or a knife, after the paint has dried.

After drying, 22-30 grams of ferric chloride in 75 ml of water may be heated to about 60 degree and poured over the P.C.B. , placed with its copper side upwards in a plastic tray of about 15*20 cm. Stirring the solution helps speedy etching. The dissolution of unwanted copper would take about 45 minutes. If etching takes longer, the solution may be heated again and the process repeated. The paint on the pattern can be removed P.C.B. may then be washed and dried. Put a coat of varnish to retain the shine. Your P.C.B. is ready.

REACTIONFecl3 + Cu ----- CuCl3 + Fe

Fecl3 + 3H2O --------- Fe (OH)3 + 3HCL

PRECAUTION

Add Ferric Chloride (Fecl3) carefully, without any splashing. Fecl3 is

irritating to the skin and will stain the clothes. Place the board in solution with copper side up. Try not to breathe the vapours. Stir the solution by giving see-saw

motion to the dish and solution in it. Occasionally warm if the solution over a heater-not to boiling. After

some time the unshaded parts change their colour continue to etch. Gradually the base material will become visible. Etch for two minutes more to get a neat pattern.

Don't throw away the remaining Fecl3 solution. It can be used again

for next Printed Circuit Board P.C.B.

USES

Printed Circuit Board are used for housing components to make a circuit for compactness, simplicity of servicing and case of interconnection. Thus we can define the P.C.B. as : Prinked Circuit Boards is actually a sheet of bakelite (an insulating material) on the one side of which copper patterns are made with holes and from another side, leads of electronic components are inserted in the proper holes and soldered to the copper points on the back. Thus leads of electronic components terminals are joined to make electronic circuit.

In the boards copper cladding is done by pasting thin copper foil on the boards during curing. The copper on the board is about 2 mm thick and weights an ounce per square foot.

The process of making a Printed Circuit for any application has the following steps (opted professionally):

Preparing the layout of the track. Transferring this layout photographically M the copper. Removing the copper in places which are not needed, by the process of

etching (chemical process) . Drilling holes for components mounting.

PRINTED CIRCUIT BOARD

Printed circuit boards are used for housing components to make a circuit, for comactness, simplicity of servicing and ease of interconnection. Single sided, double sided and double sided with plated-through-hold (PYH) types of p.c boards are common today.

Boards are of two types of material (1) phenolic paper based material (2) Glass epoxy material. Both materials are available as laminate sheets with copper cladding.

Printed circuit boards have a copper cladding on one or both sides. In both boards, pasting thin copper foil on the board during curing does this. Boards are prepared in sizes of 1 to 5 metre wide and upto 2 metres long. The thickness of the boards is 1.42 to 1.8mm. The copper on the boards is about 0.2 thick and weighs and ounce per square foot.

TRANSFORMER

PRINCIPLE OF THE TRANSFORMER:-

Two coils are wound over a Core such that they are magnetically coupled. The two coils are known as the primary and secondary windings.

In a Transformer, an iron core is used. The coupling between the coils is source of making a path for the magnetic flux to link both the coils. A core as in fig.2 is used and the coils are wound on the limbs of the core. Because of high permeability of iron, the flux path for the flux is only in the iron and hence the flux links both windings. Hence there is very little ‘leakage flux’. This term leakage flux denotes the part of the flux, which does not link both the coils, i.e., when coupling is not perfect. In the high frequency transformers, ferrite core is used. The transformers may be step-up, step-down, frequency matching, sound output, amplifier driver etc. The basic principles of all the transformers are same.

MINIATURE TRANSFORMER

CONVENTIONAL POWER TRANSFORMER

TRANSISTOR

The name is transistor derived from ‘transfer resistors’ indicating a solid state Semiconductor device. In addition to conductor and insulators, there is a third class of material that exhibits proportion of both. Under some conditions, it acts as an insulator, and under other conditions it’s a conductor. This phenomenon is called Semi-conducting and allows a variable control over electron flow. So, the transistor is semi conductor device used in electronics for amplitude. Transistor has three terminals, one is the collector, one is the base and other is the emitter, (each lead must be connected in the circuit correctly and only then the transistor will function). Electrons are emitted via one terminal and collected on another terminal, while the third terminal acts as a control element. Each transistor has a number marked on its body. Every number has its own specifications.

There are mainly two types of transistor (i) NPN & (ii) PNP

NPN Transistors:

When a positive voltage is applied to the base, the transistor begins to conduct by allowing current to flow through the collector to emitter circuit. The relatively small current flowing through the base circuit causes a much greater current to pass through the emitter / collector circuit. The phenomenon is called current gain and it is measure in beta.

PNP Transistor:

It also does exactly same thing as above except that it has a negative voltage on its collector and a positive voltage on its emitter.

Transistor is a combination of semi-conductor elements allowing a controlled current flow. Germanium and Silicon is the two semi-conductor elements used for making it. There are two types of transistors such as POINT CONTACT and JUNCTION TRANSISTORS. Point contact construction is defective so is now out of use. Junction triode transistors are in many respects analogous to triode electron tube.

A junction transistor can function as an amplifier or oscillator as can a triode tube, but has the additional advantage of long life, small size, ruggedness and absence of cathode heating power.

Junction transistors are of two types which can be obtained while manufacturing.

The two types are: -

1)PNP TYPE: This is formed by joining a layer of P type of germanium to an N-P Junction

2)NPN TYPE: This is formed by joining a layer of N type germanium to a P-N Junction.

Both types are shown in figure, with their symbols for representation. The centre section is called the base, one of the outside sections-the emitter and the other outside section-the collector. The direction of the arrowhead gives the direction of the conventional current with the forward bias on the emitter. The conventional flow is opposite in direction to the electron flow.

OPERATION OF PNP TRANSISTOR:-

A PNP transistor is made by sand witching two PN germanium or silicon diodes, placed back to back. The centre of N-type portion is extremely thin in comparison to P region. The P region of the left is connected to the positive terminal and N-region to the negative terminal i.e. PN is biased in the forward direction while P region of right is biased negatively i.e. in the reverse direction as shown in Fig. The P region in the forward biased circuit is called the emitter and P region on the right, biased negatively is called collector. The centre is called base.

P N P

N P N

The majority carriers (holes) of P region (known as emitter) move to N region as they are repelled by the positive terminal of battery while the electrons of N region are attracted by the positive terminal. The holes overcome the barrier and cross the emitter junction into N region. As the width of base region is extremely thin, two to five percent of holes recombine with the free electrons of N-region which result in a small base current while the remaining holes (95% to 98%) reach the collector junction. The collector is biased negatively and the negative collector voltage aids in sweeping the hole into collector region.

As the P region at the right is biased negatively, a very small current should flow but the following facts are observed:-

A substantial current flows through it when the emitter junction is biased in a forward direction.

The current flowing across the collector is slightly less than that of the emitter, and

The collector current is a function of emitter current i.e. with the decrease or increase in the emitter current a corresponding change in the collector current is observed.

The facts can be explained as follows:-

As already discussed that 2 to 5% of the holes are lost in recombination with the electron n base region, which result in a small base current and hence the collector current is slightly less than the emitter current.

The collector current increases as the holes reaching the collector junction are attracted by negative potential applied to the collector.

When the emitter current increases, most holes are injected into the base region, which is attracted by the negative potential of the collector and hence results in increasing the collector current. In this way emitter is analogous to the control of plate current by small grid voltage in a vacuum triode.

Hence we can say that when the emitter is forward biased and collector is negatively biased, a substantial current flows in both the circuits. Since a small emitter voltage of about 0.1 to 0.5 volts permits the flow of an appreciable emitter current the input power is very small. The collector voltage can be as high as 45 volts.

RESISTANCE

Resistance is the opposition of a material to the current. It is measured in Ohms . All conductors represent a certain amount of resistance, since no conductor is 100% efficient. To control the electron flow (current) in a predictable manner, we use resistors. Electronic circuits use calibrated lumped resistance to control the flow of current. Broadly speaking, resistor can be divided into two groups viz. fixed & adjustable (variable) resistors. In fixed resistors, the value is fixed & cannot be varied. In variable resistors, the resistance value can be varied by an adjuster knob. It can be divided into (a) Carbon composition (b) Wire wound (c) Special type. The most common type of resistors used in our projects is carbon type. The resistance value is normally indicated by colour bands. Each resistance has four colours, one of the band on either side will be gold or silver, this is called fourth band and indicates the tolerance, others three band will give the value of resistance (see table). For example if a resistor has the following marking on it say red,

violet, gold. Comparing these coloured rings with the colour code, its value is 27000 ohms or 27 kilo ohms and its tolerance is ±5%. Resistor comes in various sizes (Power rating). The bigger, the size, the more power rating of 1/4 watts. The four colour rings on its body tells us the value of resistor value as given below.

COLOURS CODE

Black---------------------------------------------0Brown--------------------------------------------1Red-----------------------------------------------2Orange-------------------------------------------3Yellow-------------------------------------------4Green--------------------------------------------5Blue----------------------------------------------6Violet--------------------------------------------7Grey----------------------------------------------8White--------------------------------------------9

The first rings give the first digit. The second ring gives the second digit. The third ring indicates the number of zeroes to be placed after the digits. The fourth ring gives tolerance (gold ±5%, silver ± 10%, No colour ± 20%).

In variable resistors, we have the dial type of resistance boxes. There is a knob with a metal pointer. This presses over brass pieces placed along a circle with some space b/w each of them.

Resistance coils of different values are connected b/w the gaps. When the knob is rotated, the pointer also moves over the brass pieces. If a gap is skipped over, its resistance is included in the circuit. If two gaps are skipped over, the resistances of both together are included in the circuit and so on.

A dial type of resistance box contains many dials depending upon the range, which it has to cover. If a resistance box has to read upto 10,000ohm, it will have three dials each having ten gaps i.e. ten resistance coils each of resistance 10ohm. The third dial will have ten resistances each of 100ohm.

The dial type of resistance boxes is better because the contact resistance in this case is small & constant.

CAPACITORS

It is an electronic component whose function is to accumulate charges and then release it.

To understand the concept of capacitance, consider a pair of metal plates which all are placed near to each other without touching. If a battery is connected to these plates the positive pole to one and the negative pole to the other, electrons from the battery will be attracted from the plate connected to the positive terminal of the battery. If the battery is then disconnected, one plate will be left with an excess of electrons, the other with a shortage, and a potential or voltage difference will exists between them. These plates will be acting as capacitors.

Capacitors are of two types: -

(1) fixed type like ceramic, polyester, electrolytic capacitors-these names refer to the material they are made of aluminium foil.

(2) Variable type like gang condenser in radio or trimmer. In fixed type capacitors, it has two leads and its value is written over its body and variable type has three leads. Unit of measurement of a capacitor is farad denoted by the symbol F. It is a very big unit of capacitance. Small unit capacitor are pico-farad denoted by pf (Ipf=1/1000,000,000,000 f) Above all, in case of electrolytic capacitors, it's two terminal are marked as (-) and (+) so check it while using capacitors in the circuit in right direction. Mistake can destroy the capacitor or entire circuit in operational.

DIODE

The simplest semiconductor device is made up of a sandwich of P-type semiconducting material, with contacts provided to connect the p-and n-type layers to an external circuit. This is a junction Diode. If the positive terminal of the battery is connected to the p-type material (cathode) and the negative terminal to the N-type material (Anode), a large current will flow. This is called forward current or forward biased.

If the connections are reversed, a very little current will flow. This is because under this condition, the p-type material will accept the electrons from the negative terminal of the battery and the N-type material will give up its free electrons to the battery, resulting in the state of electrical equilibrium since the N-type material has no more electrons. Thus there will be a small current to flow and the diode is called Reverse biased.

Thus the Diode allows direct current to pass only in one direction while blocking it in the other direction. Power diodes are used in concerting AC into DC. In this, current will flow freely during the first half cycle (forward biased) and practically not at all during the other half cycle (reverse biased). This makes the diode an effective rectifier, which convert ac into pulsating dc. Signal diodes are used in radio circuits for detection. Zener diodes are used in the circuit to control the voltage.

Some common diodes are:-

ZENER DIODE:-

A zener diode is specially designed junction diode, which can operate continuously without being damaged in the region of reverse break down voltage. One of the most important applications of zener diode is the design of constant voltage power supply. The zener diode is joined in reverse bias to d.c. through a resistance R of suitable value.

PHOTO DIODE:-

A photo diode is a junction diode made from photo- sensitive semiconductor or material. In such a diode, there is a provision to allow the light of suitable frequency to fall on the p-n junction. It is reverse biased, but the voltage applied is less than the break down voltage. As the intensity of incident light

is increased, current goes on increasing till it becomes maximum. The maximum current is called saturation current.

LIGHT EMITTING DIODE (LED):-

When a junction diode is forward biased, energy is released at the junction diode is forward biased, energy is released at the junction due to recombination of electrons and holes. In case of silicon and germanium diodes, the energy released is in infrared region. In the junction diode made of gallium arsenate or indium phosphide, the energy is released in visible region. Such a junction diode is called a light emitting diode or LED.