Repote File

7/30/2019 Repote File

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ROOM LIGHT CONTROLLER

After getting 220 v ac supply transformer step down it 12v ac, using bridge

Rectifier we convert ac to dc using four diode (1N4007, and 1000uf electrolytic

Capacitor ) this 12 dc supply goes to relay coil, 7805 regulator ic convert it 12v to

5v because our micro controller AT89S52, LCD 16 X 2 and sensor circuit works

5v, after getting 5v supply mc start display the welcome message to lcd, and wait

for input, if some come from in out room, in sensor take a input, if input is

greater then 0 say 1,2 then mc give signal to transistor, it on the relay, relay on

the house light, and lcd display the total number of person, if start increment, if

some goes out from our room out sensor get input and provide to mc, mc

decrement it, if total = 0 then wait for 10 second, if no one is come in this period

mc off the relay with help of transistor, it off the relay, relay off the room light.


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Basic Electronics

When a beginner to electronics first looks at a circuit board full of

components he/she is often overwhelmed by the diversity of do-dads. In

these next few sections we will help you to identify some of the simple

components and their schematical symbol. Then you should be able to call

them resistors and transistors instead of Whatchamacallits.

Electronic component are classed into either being Passive devices

or Active devices.

A Passive Device is one that contributes no power gain (amplification)

to a circuit or system. It has not control action and does not require any

input other than a signal to perform its function. In other words, A


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components with no brains! Examples are Resistors, Capactitors and

Inductors

Active Devices are components that are capable of controlling voltages

or currents and can create a switching action in the circuit. In other

words, Devices with smarts! Examples are Diodes, Transistors and

Integrated circuits. Most active components are semiconductors.


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Resistors:

This is the most common component in electronics. It is used mainly to

control current and voltage within the circuit. You can identify a simple

resistor by its simple cigar shape with a wire lead coming out of each end. It

uses a system of color coded bands to identify the value of the component

(measured in Ohms) *A surface mount resistor is in fact mere millimeters in

size but performs the same function as its bigger brother, the simple

esistor. A potentiometer is a variable resistor. It lets you vary the resistance

with a dial or sliding control in order to alter current or voltage on the fly.

This is opposed to the fixed simple resistors.

Resistor values - the resistor colour code

Resistance is measured in ohms, the symbol for ohm is an omega .

1 is quite small so resistor values are often given in k and M .

1 k = 1000 1 M = 1000000 .

Resistor values are normally shown using coloured bands.

Each colour represents a number as shown in the table.

Most resistors have 4 bands:

The first band gives the first digit.

The second band gives the second digit. The third band indicates the number of zeros.

The fourth band is used to shows the tolerance (precision) of the resistor,

this may be ignored for almost all circuits but further details aregiven below.
http://www.kpsec.freeuk.com/components/resist.htm#tolerancehttp://www.kpsec.freeuk.com/components/resist.htm#tolerance


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This resistor has red (2), violet (7), yellow (4 zeros) and gold bands.

So its value is 270000 = 270 k .

On circuit diagrams the is usually omitted and the value is written

270K.

Small value resistors (less than 10 ohm)

The standard colour code cannot show values of less than 10 . To show

these small values two special colours are used for the third

band:gold which means 0.1 and silverwhich means 0.01. The first

and second bands represent the digits as normal.

For example:

red, violet, gold bands represent 27 0.1 = 2.7

blue, green, silverbands represent 56 0.01 = 0.56

Tolerance of resistors (fourth band of colour code)

The tolerance of a resistor is shown by the fourth band of the colour

code. Tolerance is the precision of the resistor and it is given as a

percentage. For example a 390 resistor with a tolerance of 10% will

have a value within 10% of 390 , between 390 - 39 = 351 and 390 +

39 = 429 (39 is 10% of 390).

A special colour code is used for the fourth band tolerance:


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silver10%, gold 5%, red 2%, brown 1%.

If no fourth band is shown the tolerance is 20%.

Tolerance may be ignored for almost all circuits because precise resistor

values are rarely required.

Resistor values - the resistor colour code

The ResistorColour Code

Colour Number

Black 0

Brown 1

Red 2

Orange 3

Yellow 4

Green 5

Blue 6

Violet 7

Grey 8White 9


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Condensors/Capacitors:

Capacitors, or "caps", vary in size and shape - from a small surface mount model

up to a huge electric motor cap the size of a paint can. It storages electrical

energy in the form of electrostatic charge. The size of a capacitor generally

determines how much charge it can store. A small surface mount or ceramic cap

will only hold a minuscule charge. A cylindrical electrolytic cap will store a much

larger charge. Some of the large electrolytic caps can store enough charge to kill

a person. Another type, called Tantalum Capacitors, store a larger charge in a

smaller package.This is a measure of a capacitor's ability to store charge. A large

capacitance means that more charge can be stored. Capacitance is

measured in farads, symbol F. However 1F is very large, so prefixes are

used to show the smaller values.

Three prefixes (multipliers) are used, (micro), n (nano) and p (pico):

means 10-6 (millionth), so 1000000F = 1F

n means 10-9 (thousand-millionth), so 1000nF = 1F

p means 10-12 (million-millionth), so 1000pF = 1nF

Capacitor values can be very difficult to find because there are many

types of capacitor with different labelling systems!

There are many types of capacitor but they can be split into two

groups, polarised and unpolarised. Each group has its own circuit symbol.

Polarised capacitors (large values, 1F +)
http://www.kpsec.freeuk.com/components/capac.htm#polarisedhttp://www.kpsec.freeuk.com/components/capac.htm#unpolarisedhttp://www.kpsec.freeuk.com/components/capac.htm#polarisedhttp://www.kpsec.freeuk.com/components/capac.htm#unpolarised


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Examples: Circuit symbol:

Electrolytic Capacitors

Electrolytic capacitors are polarised and they must be connected the

correct way round, at least one of their leads will be marked + or -.

They are not damaged by heat when soldering.

There are two designs of electrolytic capacitors; axial where the leads

are attached to each end (220F in picture) and radial where both leads

are at the same end (10F in picture). Radial capacitors tend to be a

little smaller and they stand upright on the circuit board.

It is easy to find the value of electrolytic capacitors because they are

clearly printed with their capacitance and voltage rating. The voltage

rating can be quite low (6V for example) and it should always be

checked when selecting an electrolytic capacitor. It the project parts list

does not specify a voltage, choose a capacitor with a rating which is

greater than the project's power supply voltage. 25V is a sensible

minimum for most battery circuits.

Tantalum Bead Capacitors

Tantalum bead capacitors are polarised and have low voltage ratings

like electrolytic capacitors. They are expensive but very small, so they

are used where a large capacitance is needed in a small size.

Modern tantalum bead capacitors are printed with their capacitance and


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voltage in full. However older ones use a colour-code system which has

two stripes (for the two digits) and a spot of colour for the number of

zeros to give the value in F. The standard colour code is used, but for

the spot, grey is used to mean 0.01 and white means 0.1 so that

values of less than 10F can be shown. A third colour stripe near the

leads shows the voltage (yellow 6.3V, black 10V, green 16V, blue 20V,

grey 25V, white 30V, pink 35V).

For example: blue, grey, black spot means 68F

For example: blue, grey, white spot means 6.8F

For example: blue, grey, grey spot means 0.68F

Unpolarised capacitors (small values, up to 1F)

Examples: Circuit symbol:

Small value capacitors are unpolarised and may be connected either

way round. They are not damaged by heat when soldering, except for

one unusual type (polystyrene). They have high voltage ratings of at

least 50V, usually 250V or so. It can be difficult to find the values of

these small capacitors because there are many types of them and

several different labelling systems!
http://www.kpsec.freeuk.com/components/capac.htm#colourshttp://www.kpsec.freeuk.com/components/capac.htm#colours


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Many small value capacitors have their value printed but without a

multiplier, so you need to use experience to work out what the multiplier

should be!

For example 0.1 means 0.1F = 100nF.

Sometimes the multiplier is used in place of the decimal point:

For example: 4n7 means 4.7nF.

Capacitor Number Code

A number code is often used on small capacitors where printing is

difficult: the 1st number is the 1st digit,

the 2nd number is the 2nd digit,

the 3rd number is the number of zeros to give the capacitance in pF.

Ignore any letters - they just indicate tolerance and voltage rating.

For example: 102 means 1000pF = 1nF (not 102pF!)

For example: 472J means 4700pF = 4.7nF (J means 5% tolerance).

Diodes:

Diodes are basically a one-way valve for electrical current. They let it flow in one

direction (from positive to negative) and not in the other direction. This is used to

perform rectification or conversion of AC current to DC by clipping off the negative

portion of a AC waveform. The diode terminals are cathode and anode and the

arrow inside the diode symbol points towards the cathode, indicating current flow

in that direction when the diode is forward biased and conducting current. Most

diodes are similar in appearance to a resistor and will have a painted line on one


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end showing the direction or flow (white side is negative). If the negative side is

on the negative end of the circuit, current will flow. If the negative is on the ositive

side of the circuit no current will flow.

Light Emitting Diodes (LEDs)

Example: Circuit symbol:

Function

LEDs emit light when an electric current passes through them.

Connecting and soldering

LEDs must be connected the correct way round, the diagram may be

labelled a or + for anode and k or - for cathode (yes, it really is k, not c,

for cathode!). The cathode is the short lead and there may be a slight flat

on the body of round LEDs. If you can see inside the LED the cathode is

the larger electrode (but this is not an official identification method).

LEDs can be damaged by heat when soldering, but the risk is small

unless you are very slow. No special precautions are needed for

soldering most LEDs.

Testing an LED

Never connect an LED directly to a battery or power supply!

It will be destroyed almost instantly because too much current will pass


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through and burn it out.

LEDs must have a resistor in series to limit the current to a safe value,

for quick testing purposes a 1k resistor is suitable for most LEDs if your

supply voltage is 12V or less. Remember to connect the LED the

correct way round!

Colours of LEDs

LEDs are available in red, orange, amber, yellow, green, blue and white. Blue andwhite LEDs are much more expensive than the other colours.

The colour of an LED is determined by the semiconductor material, not by thecolouring of the 'package' (the plastic body). LEDs of all colours are available inuncoloured packages which may be diffused (milky) or clear (often described as

'water clear'). The coloured packages are also available as diffused (the standardtype) or transparent.

Switch :


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This is a mechanical part which when pressed makes the current to flow through

it. If the switch is released the current stops flowing through it. This helps to

control a circuit.

Transistors:

The transistor performs two basic functions. 1) It acts as a switch turning current

on and off. 2) It acts as a amplifier. This makes an output signal that is a

magnified version of the input signal. Transistors come in several sizes depending

on their application. It can be a big power transistor such as is used inpower applifiers in your stereo, down to a surface mount (SMT) and even down

to .5 microns wide (I.E.: Mucho Small!) such as in a microprocessor or Integrated


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Circuit.

NPN Transistor: Bipolar junction perform the function of amplificationswhere

a small varying voltage or current applied to the base (the lead on the left

side of the symbol) is proportionately replicated by a much larger voltage or

current between the collector and emitter leads. Bipolar junction refers to

sandwich construction of the semiconductor, where a wedge of "P" material is

placed between two wedges of "N" material. In this NPN construction a small

base current controls the larger current flowing from collector to emitter (the lead

with the arrow).

PNP Transistor:


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Similar to NPN transistors, PNP's have a wedge of "N" material

between two wedges of "P" material. In this design, a base current regulates the

larger current flowing from emitter to collector, as indicated by the direction of the

arrow on the emitter lead. In CED players, PNP transistors are used less

frequently that the NPN type for amplification functions.

Batteries:

Symbol of batteries shows +ve terminal by a longer line than the ve terminal.

For low power circuit dry batteries are used.

Speakers:


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These convert electrical signals to accoustic viberations. It comprises apermanent

magnet and a moving coil (through which electrical signal is passed). This movingcoil is

fixed to the diaphram which vibrates to produce sound

ICs (Integrated Circuits):Integrated Circuits, or ICs, are complex circuits inside one simple package. Silicon

and metals are used to simulate resistors, capacitors, transistors, etc. It is a space

saving miracle. These components come in a wide variety of packages and sizes.


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You can tell them by their "monolithic shape" that has a ton of "pins" coming out

of them. Their applications are as varied as their packages. It can be a simple

timer, to a complex logic circuit, or even a microcontroller (microprocessor with a

few added functions) with erasable memory built inside.

SOLDERING INSTRUCTIONS

1.1 Cleaning for soldering:1. Ensure that parts to be soldered and the PCB are clean and free from dirt or

grease.

2. Use isopropyl alcohol with the help of non-static bristol brush for

cleaning.

3. Use lint-free muslin cloth for wiping or alternatively use mild soap

solution followed by thorough rinsing with water and drying.

1.2 Tips for good Soldering:

1. Use 15 to 25 watt soldering iron for general work involving small

joints and for CMOS ICs, FETS and ASICS use temprature controlled

soldering station ensuring that the tip temperature is maintained

within 330-350 deg. centigrade.

2. For bigger joints use elevated temperature as per job.

3. Before using a new tip, ensure that it is tinned and before applying

the tip to the job, wipe it using a wet sponge.

4. Use 60 : 40 (tin : lead) resin core (18-20 SWG) solder.

5. Ensure that while applying the tip to the job, the tip of the soldering

iron is held at an angle such that the tip grazes the surface to


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be heated and ensure that it does not transfer heat to other joints/

components in its vicinity at the same time heating all parts of

joint equally.

6. Heat the joint for just the.right amount of time, during which a

very short length of solder flows over the joint and then smoothly

withdraw the tip.

7. Do not carry molten solder to the joint.

8. Do not heat the electronic parts for more than 2-4 seconds since

most of them are sensitive to heat.

9. Apply one to three mm solder which is neither too less nor too

much and adequate for a normal joint.

10. Do not move the components until the molten solder, at the joint

has cooled._

1.3 Tips for de-soldering:

1. Remove and re-make if a solder joint is bad or dry.

2. Use a de-soldering pump which is first cocked and then the joint

is heated in the same way as during soldering, and when the

solder melts, push the release button to disengage the pump.

3. Repeat the above operation 2-3 times until the soldered component

can be comfortably removed using tweezers or long nose

pliers.

4. Deposit additional solder before using the de-soldering pump for

sucking it in case of difficulty in sucking the solder if it is too


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sparse as this will hasten the de-soldering operation.

5. Alternatively, use the wet de-soldering wick using soldering flux

which is nothing but a fine copper braid used as a shield in coaxial

cables etc. and then press a short length of the wick using

the tip of the hot iron against the joint to be desoldered so that

the iron melts the solder which is drawn into the braid.

6. Do not allow the solder to cool while the braid is still adhering to

the joint.

7. Solder the component again after cleaning by repeating the steps

under sub para A and B above.

8. Allow it to cool and check for continuity.

1.4 Precautions:

1. Mount the components at the appropriate places before soldering.

Follow the circuit discription and components details, leads

identification etc. Do not start soldering before making it confirm

that all the components are mounted at the right place.

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

circuit.

3. Do not sit under the fan while soldering

4. Position the board so that gravity tends to keep the solder where

you want it.

5. Do not over heat the components at the board. Excess heat may

damage the components or board.


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6. The board should not vibrate while soldering otherwise you have

a dry or a cold joint.

7. Do not put the kit under or over voltage source. Be sure about the

voltage either dc or ac while operating the gadget.

8. 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.

9. Do not use old dark colour solder. It may give dry joint. Be sure

that all the joints are clean and well shiny.

1.5 Illustrations showing correct/wrong insertion of components

and their soldering:

Corrected assembling and soldering process can provide the product

in the best performance.


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HOW TO MAKE GOOD

HOMEMADE PCBs?

Do not use sodium hydroxide for developing photoresist laminates. It is completely a

dreadful stuff for developing PCBs. Apart from its causticity, it is very sensitive to

both temperature and concentration, and made-up solution doesnt last long


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POWER SUPPLY:


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Power supply is a reference to a source of electrical power. A device orsystem that supplieselectrical or other types of energy to an output load or group of loads is calleda power supply unitor PSU. The term is most commonly applied to electrical energy supplies, lessoften to mechanical

ones, and rarely to others.Here in our application we need a 5v DC power supply for all electronicsinvolved in the project.

This requires step down transformer, rectifier, voltage regulator, and filtercircuit for generation of5v DC power. Here a brief description of all the components is given asfollows:

TRANSFORMER:A transformer is a device that transfers electrical energy from one circuit toanother through inductivelycoupled conductors the transformer's coils or "windings". Except for air-

core transformers, theconductors are commonly wound around a single iron-rich core, or aroundseparate but magneticallycoupledcores. A varying current in the first or "primary" winding creates a varyingmagnetic field in thecore (or cores) of the transformer. This varying magnetic field induces avarying electromotive force(EMF) or "voltage" in the "secondary" winding. This effect is called mutualinduction.

TRANSFORMER:A transformer is a device that transfers electrical energy from one circuit toanother through inductively coupled conductors the transformer's coils or"windings". Except for air-core transformers, the conductors are commonly

wound around a single iron-rich core, or around separate butmagneticallycoupled cores. A varying current in the first or "primary" windingcreates a varying magnetic field in the core (or cores) of the transformer. Thisvarying magnetic field induces a varying electromotive force(EMF) or "voltage" in the "secondary" winding. This effect is called mutualinduction.


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If a load is connected to the secondary circuit, electric charge will flowin the secondary winding of the transformer and transfer energy fromthe primary circuit to the load connected in the secondary circuit. Thesecondary induced voltage VS, of an ideal transformer, is scaled fromthe primary VP by a

factor equal to the ratio of the number of turns of wire in theirrespective windings:

By appropriate selection of the numbers of turns, a transformer thusallows an alternating voltageto be stepped up by making NS morethan NP or stepped down, by making It

BASIC PARTS OF A TRANSFORMERIn its most basic form a transformer consists of: A primary coil orwinding.A secondary coil or winding. A core that supports the coils or windings.

Refer to the transformer circuit in figure as you read the followingexplanation: The primary winding is connected to a 60-hertz ac voltagesource. The magnetic field (flux) builds up (expands) and collapses(contracts) about the primary winding. The expanding and contractingmagnetic field around the primary winding cuts the secondary windingand induces an alternating voltage into the winding. This voltagecauses alternating current to flow through theload. The voltage may be stepped up or down depending on the designof the primary and secondary windings.


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THE

COMPONENTS OF A TRANSFORMER17

Two coils of wire (called windings) are wound on some type of corematerial. In some cases thecoils of wire are wound on a cylindrical or rectangular cardboard form.In effect, the corematerial is air and the transformer is called an AIR-CORETRANSFORMER. Transformers usedat low frequencies, such as 60 hertz and 400 hertz, require a core oflow-reluctance magnetic

material, usually iron. This type of transformer is called an IRON-CORETRANSFORMER.Most power transformers are of the iron-core type. The principle partsof a transformer and theirfunctions are:The CORE, which provides a path for the magnetic lines of flux.The PRIMARY WINDING, which receives energy from the ac source.The SECONDARY WINDING, which receives energy from the primary windingand delivers it tothe load.The ENCLOSURE, which protects the above components from dirt, moisture,and mechanical

damage.

BRIDGE RECTIFIERA bridge rectifier makes use of four diodes in a bridge arrangement to achievefull-wave rectification. This is a widely used configuration, both with individualdiodes wired as shown and with single component bridges where the diodebridge is wired internally.


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BASIC OPERATIONAccording to the conventional model of current flow originallyestablished by Benjamin Franklin and still followed by most engineerstoday, current is assumedto flow through electrical conductors fromthe positive to the negative pole. In actuality, free electrons in a

conductor nearly always flow from the negative to the positive pole.In the vast majority of applications, however, the actual direction ofcurrent flow is irrelevant. Therefore, in the discussion below theconventional model is retained. In the diagrams below, when the inputconnected to the left corner of the diamond is positive, and the inputconnected to the right corner is negative, current flows from theupper supply terminal to the right along the red (positive) path to theoutput, and returns to the lower supplyterminal via the blue (negative) path.

When the input connected to the left corner is negative, and the inputconnected to the right corner is positive, current flows from thelower supply terminal to the right along the red path to the output,and returns to the upper supply terminal via the blue path.

In each case, the upper right output remains positive and lower rightoutput negative. Since this is true whether the input is AC or DC, this


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circuit not only produces a DC output from an AC input, it can alsoprovide what is sometimes called "reverse polarity protection". That is,it permits normal functioning of DC-powered equipment when batterieshave been installed backwards, or when the leads (wires) from a DCpower source have been reversed, and protects

the equipment from potential damage caused by reverse polarity. Priorto availability of integrated electronics, such a bridge rectifier wasalways constructed from discrete components. Since about 1950, asingle four-terminal component containing the four diodes connected inthe bridge configuration became a standard commercial componentand is now available with various voltage and current ratings.

OUTPUT SMOOTHINGFor many applications, especially with single phase AC where the full-wave bridge serves to convert an AC input into a DC output, theaddition of a capacitor may be desired because the bridge alone

supplies an output of fixed polarity but continuously varying or"pulsating" magnitude (see diagram above).

The function of this capacitor, known as a reservoir capacitor (orsmoothing capacitor) is to lessen the variation in (or 'smooth') therectified AC output voltage waveform from the bridge. One explanation

of 'smoothing' is that the capacitor provides a low impedance path tothe AC component of the output, reducing the AC voltage across, andAC current through, the resistive load. In less technical terms, any dropin the output voltage and current of the bridge tends to becanceled by loss of charge in the capacitor. This charge flows out asadditional current through the load. Thus the change of load currentand voltage is reduced relative to what would occur without thecapacitor. Increases of voltage correspondingly store excess charge in


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the capacitor, thus moderating the change in output voltage / current.The simplified circuit shown has a well-deserved reputation for beingdangerous, because, in some applications, the capacitor can retain alethal charge after the AC power source is removed.If supplying a dangerous voltage, a practical circuit should include a

reliable way to safely discharge the capacitor. If the normal load cannotbe guaranteed to perform this function, perhaps because it can bedisconnected, the circuit should include a bleeder resistor connected asclose as practical across the capacitor. This resistor should consume acurrent large enough to discharge the capacitor in a reasonable time,but small enough to minimize unnecessary powerwaste. Because a bleeder sets a minimum current drain, the regulationof the circuit, defined as percentage voltage change from minimum tomaximum load, is improved. However in many cases the improvementis of insignificant magnitude.The capacitor and the load resistance have a typical time constant =

RC where C and R are the capacitance and load resistance respectively.As long as the load resistor is large enough so that this time constant ismuch longer than the time of one ripple cycle, the above configurationwill produce a smoothed DC voltage across the load. In some designs, aseries resistor at the load side of the capacitor is added. The smoothingcan then be improved by adding additional stages of capacitorresistorpairs, often done only for subsupplies to critical high-gain circuits thattend to be sensitive to supply voltage noise. The idealized waveformsshown above are seen for both voltage and current when the load onthe bridge is resistive. When the load includes a smoothing capacitor,both the voltage and the current waveforms will be greatly changed.

While the voltage is smoothed, as described above, current will flowthrough the bridge only during the time when the input voltage isgreater than the capacitor voltage. For example, if the load draws anaverage current of n Amps, and the diodes conduct for 10% of the time,the average diode current during conduction must be 10nAmps. This non-sinusoidal current leads to harmonic distortion and apoor power factor in the AC supply. In a practical circuit, when acapacitor is directly connected to the output of a bridge, the bridgediodes must be sized to withstand the current surge that occurs whenthe power is turned on at the peak of the AC voltage and the capacitoris fully discharged. Sometimes a small series

resistor is included before the capacitor to limit this current, though inmost applications the power supply transformer's resistance is alreadysufficient.Output can also be smoothed using a choke and second capacitor. Thechoke tends to keep the current (rather than the voltage) moreconstant. Due to the relatively high cost of an effective chokecompared to a resistor and capacitor this is not employed in modernequipment. Some early console radios created the speaker's constant


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field with the current from the high voltage ("B +") power supply, whichwas then routed to the consuming circuits, (permanentmagnets were then too weak for good performance) to create thespeaker's constant magnetic field. The speaker field coil thusperformed 2 jobs in one: it acted as a choke, filtering the power supply,

and it produced the magnetic field to operate the speaker.

REGULATOR IC (78XX)It is a three pin IC used as a voltage regulator. It converts unregulated DCcurrent into regulated DC current

Normally we get fixed output by connecting the voltage regulator at theoutput of the filtered DC (see in above diagram). It can also be used incircuits to get a low DC voltage from a high DC voltage (for example weuse 7805 to get 5V from 12V). There are two types of voltageregulators 1. fixed voltage regulators (78xx, 79xx) 2. variable voltageregulators (LM317) In fixed voltage regulators there is anotherclassification 1. +ve voltage regulators 2. -ve voltage regulatorsPOSITIVE VOLTAGE REGULATORS This include 78xx voltage regulators.Themost commonly used ones are 7805 and 7812. 7805 gives fixed 5V DCvoltage if input voltage is in (7.5V, 20V).


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The CAPACITOR FILTERThe simple capacitor filter is the most basic type of power supply filter.The application of the simple capacitor filter is very limited. It is

sometimes used on extremely high-voltage, low current power suppliesfor cathode ray and similar electron tubes, which require very little loadcurrent from the supply. The capacitor filter is also used where thepower-supply ripplefrequency is not critical; this frequency can be relatively high. Thecapacitor (C1) shown in figure 4-15 is a simple filter connected acrossthe output of the rectifier in parallel with the load.

Full-wave rectifier with a capacitor filter. When this filter is used, the RCcharge time of the filter capacitor (C1) must be short and the RCdischarge time must be long to eliminate ripple action. In other words,

the capacitor must chargeup fast, preferably with no discharge at all. Better filtering also resultswhen the input frequency is high; therefore, the full-wave rectifieroutput is easier to filter than that of the half-wave rectifier because ofits higher frequency.For you to have a better understanding of the effect that filtering hason Eavg, a comparison of a rectifier circuit with a filter and one without afilter is illustrated in views A and B of figure 4-16. The outputwaveforms in figure 4-16 represent the unfiltered and filtered outputsof the halfwave rectifier circuit. Current pulses flow through the loadresistance (RL) each time a diode conducts. The dashed line indicates

the average value of output voltage. For the half-wave rectifier, Eavg isless than half (or approximately 0.318) of the peak output voltage. Thisvalue isstill much less than that of the applied voltage. With no capacitorconnected across the output of the rectifier circuit, the waveform inview A has a large pulsating component (ripple) compared with theaverage or dc component. When a capacitor is connected across the


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output (view B), the average value of output voltage (Eavg) is increaseddue to the filtering action of capacitor C1.

The value of the capacitor is fairly large (several microfarads), thus itpresents a relatively low reactance to the pulsating current and itstores a substantial charge. The rate of charge for the capacitor islimited only by the resistance of the conducting diode, which isrelatively low. Therefore, the RC charge time of the circuit is relativelyshort. As a result, when the pulsating voltage is first applied to the


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circuit, the capacitor charges rapidly and almost reaches the peakvalue of the rectified voltage within the first few cycles. The capacitorattempts to

charge to the peak value of the rectified voltage anytime a diode isconducting, and tends to retain its charge when the rectifier output fallsto zero. (The capacitor cannot discharge immediately.) The capacitorslowly discharges through the load resistance (RL) during the time therectifier is non-conducting.The rate of discharge of the capacitor is determined by the value ofcapacitance and the value of the load resistance. If the capacitance andload-resistance values are large, the RC discharge time for the circuit is

relatively long.A comparison of the waveforms shown in figure 4-16 (view A and viewB) illustrates that the addition of C1 to the circuit results in an increasein the average of the output voltage (Eavg) and a reduction in theamplitude of the ripple component (Er) which is normally present acrossthe load resistance.Now, let's consider a complete cycle of operation using a half-waverectifier, a capacitive filter (C1), and a load resistor (RL). As shown inview A of figure 4-17, the capacitive filter (C1) is assumed to be largeenough to ensure a small reactance to the pulsating rectified current.The resistance of RL is assumed to be much greater than the reactance

of C1 at the input frequency. When the circuit is energized, the diodeconducts on the positive half cycle and current flowsthrough the circuit, allowing C1 to charge. C1 will charge toapproximately the peak value of the input voltage. (The charge is lessthan the peak value because of the voltage drop across the diode (D1)).In view A of the figure, the heavy solid line on the waveform indicatesthe charge on C1. As illustrated in view B, the diode cannot conduct onthe negative half cycle because the anode of D1 is negative withrespect to the cathode. During this interval, C1 discharges throughthe load resistor (RL). The discharge of C1 produces the downwardslope as indicated by the solid line on the waveform in view B. In

contrast to the abrupt fall of the applied ac voltage from peak value tozero, the voltage across C1 (and thus across RL) during the dischargeperiod gradually decreases until the time of the next half cycle ofrectifier operation. Keep in mind that for good filtering, the filtercapacitor should charge up as fast as possible and discharge as littleas possible. Figure 4-17A. - Capacitor filter circuit (positive and negativehalf cycles). POSITIVE HALFCYCLE


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Figure 4-17B. - Capacitor filter circuit (positive and negative halfcycles). NEGATIVE HALFCYCLE

Since practical values of C1 and RL ensure a more or less gradualdecrease of the discharge voltage, a substantial charge remains on thecapacitor at the time of the next half cycle of operation. As a result, nocurrent can flow through the diode until the rising ac input voltage atthe anode of the diode exceeds the voltage on the charge remaining on

C1. The charge on C1 is the cathode potential of the diode. When thepotential on the anode exceeds the potential on the cathode (thecharge on C1), the diode again conducts, and C1 begins to charge toapproximately the peak value of the applied voltage. After thecapacitor has charged to its peak value, the diode will cut off and thecapacitor willstart to discharge. Since the fall of the ac input voltage on the anode is


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considerably more rapid than the decrease on the capacitor voltage,the cathode quickly become more positive than the anode, and the

diode ceases to conduct.Operation of the simple capacitor filter using a full-wave rectifier isbasically the same as that discussed for the half-wave rectifier.Referring to figure 4-18, you should notice that because one of thediodes is always conducting on. either alternation, the filter capacitorcharges and discharges during each half cycle. (Note that each diodeconducts only for that portion of time when the peak secondary voltageis greater than the charge across the capacitor.)Figure 4-18. - Full-wave rectifier (with capacitor filter).

Another thing to keep in mind is that the ripple component (E r) of theoutput voltage is an ac voltage and the average output voltage (Eavg) isthe dc component of the output. Since the filter capacitor offersrelatively low impedance to ac, the majority of the ac component flowsthrough the filter capacitor. The ac component is therefore bypassed(shunted) around the load resistance, and the entire dc component (orEavg) flows through the load resistance. This statement can be clarifiedby using the formula for XC in a half-wave and full-wave rectifier. First,

you must establish some values for the circuit.


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As you can see from the calculations, by doubling the frequency of therectifier, you reduce the impedance of the capacitor by one-half. Thisallows the ac component to pass through the capacitor more easily. Asa result, a full-wave rectifier output is much easier to filter than that ofa half-wave rectifier. Remember, the smaller the XC of the filtercapacitor with respect to theload resistance, the better the filtering action. Since

the largest possible capacitor will provide the best filtering.Remember, also, that the load resistance is an important consideration.If load resistance is made small, the load current increases, and theaverage value of output voltage (Eavg) decreases. The RC dischargetime constant is a direct function of the value of the load resistance;therefore, the rate of capacitor


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voltage discharge is a direct function of the current through the load.

Thegreater the load current, the more rapid the discharge of the

capacitor, and the lower the average value of output voltage. For thisreason, the simple capacitive filter is seldom used with rectifier circuitsthat must supply a relatively large load current. Using the simplecapacitive filter in conjunction with a full-wave or bridge rectifierprovides improved filtering because the increased ripple frequencydecreases the capacitive reactance of the filter capacitor.

CIRCUIT DIAGRAM OF POWER SUPPLY


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