Introduction

A viscometer (also called viscosimeter) is an instrument used to measure the viscosity of a fluid.

For liquids with viscosities which vary with flow conditions, an instrument called a rheometer is

used. Viscometers only measure under one flow condition.

In general, either the fluid remains stationary and an object moves through it, or the object is

stationary and the fluid moves past it. The drag caused by relative motion of the fluid and a

surface is a measure of the viscosity. The flow conditions must have a sufficiently small value of

Reynolds number for there to be laminar flow.

At 20.00 degrees Celsius the viscosity of water is 1.002 mPa·s and its kinematic viscosity (ratio

of viscosity to density) is 1.0038 mm2/s. These values are used for calibrating certain types of

viscometer.

These viscometers measure the viscosity of a fluid with a known density.

Literature Survey

U-tube viscometers

These are also known as Ostwald viscometers named after Wilhelm Ostwald or glass capillary

viscometers. Another type is the Ubbelohde viscometer. They basically consist of a glass tube in

the shape of a U held vertically in a controlled temperature bath. In one arm of the U is a vertical

section of precise narrow bore (the capillary). Above this is a bulb, there is another bulb lower

down in the other arm. In use, liquid is drawn into the upper bulb by suction, then allowed to

flow down through the capillary into the lower bulb. Two marks (one above and one below the

upper bulb) indicate a known volume. The time taken for the level of the liquid to pass between

these marks is proportional to the kinematic viscosity. Most commercial units are provided with

a conversion factor, or can be calibrated by a fluid of known properties.

The time it takes for the test liquid to flow through a capillary of a known diameter of a certain

factor between 2 marked points is measured. By multiplying the time taken by the factor of the

viscometer, the kinematic viscosity is obtained. The viscometers are usually placed in a constant

temperature water bath as temperature affects viscosity.

Such viscometers are also classified as direct flow or reverse flow. Reverse flow viscometers

have the reservoir above the markings and direct flow are those with the reservoir below the

markings. Such classifications exists so that the level can be determined even when opaque or

staining liquids are measured, otherwise the liquid will cover the markings and make it

impossible to gauge the time the level passes the mark. This also allows the viscometer to have

more than 1 set of marks to allow for an immediate timing of the time it takes to reach the 3rd

mark, therefore yielding 2 timings and allowing for subsequent calculation of Determinability to

ensure accurate results.

Falling sphere viscometers

creeping flow past a sphereStokes' law is the basis of the falling sphere viscometer, in which the

fluid is stationary in a vertical glass tube. A sphere of known size and density is allowed to

descend through the liquid. If correctly selected, it reaches terminal velocity, which can be

measured by the time it takes to pass two marks on the tube. Electronic sensing can be used for

opaque fluids. Knowing the terminal velocity, the size and density of the sphere, and the density

of the liquid, Stokes' law can be used to calculate the viscosity of the fluid. A series of steel ball

bearings of different diameter is normally used in the classic experiment to improve the accuracy

of the calculation. The school experiment uses glycerine as the fluid, and the technique is used

industrially to check the viscosity of fluids used in processes. It includes many different oils, and

polymer liquids such as solutions.

In 1851, George Gabriel Stokes derived an expression for the frictional force (also called drag

force) exerted on spherical objects with very small Reynolds numbers (e.g., very small particles)

in a continuous viscous fluid by solving the small fluid-mass limit of the generally unsolvable

Navier-Stokes equations:

where:

F is the frictional force,

r is the radius of the spherical object,

η is the fluid viscosity, and

v is the particle's velocity.

If the particles are falling in the viscous fluid by their own weight, then a terminal velocity, also

known as the settling velocity, is reached when this frictional force combined with the buoyant

force exactly balance the gravitational force. The resulting settling velocity (or terminal velocity)

is given by:

where:

Vs is the particles' settling velocity (m/s) (vertically downwards if ρp > ρf, upwards if ρp < ρf),

r is the Stokes radius of the particle (m),

g is the gravitational acceleration (m/s2),

ρp is the density of the particles (kg/m3),

ρf is the density of the fluid (kg/m3), and

μ is the (dynamic) fluid viscosity (Pa s).

Note that Stokes flow is assumed, so the Reynolds number must be small.

Vibrational viscometers

Vibrational viscometers date back to the 1950s Bendix instrument, which is of a class that

operates by measuring the damping of an oscillating electromechanical resonator immersed in a

fluid whose viscosity is to be determined. The resonator generally oscillates in torsion or

transversely (as a cantilever beam or tuning fork). The higher the viscosity, the larger the

damping imposed on the resonator. The resonator's damping may be measured by one of several

methods:

Measuring the power input necessary to keep the oscillator vibrating at a constant amplitude. The

higher the viscosity, the more power is needed to maintain the amplitude of oscillation.

Measuring the decay time of the oscillation once the excitation is switched off. The higher the

viscosity, the faster the signal decays.

Measuring the frequency of the resonator as a function of phase angle between excitation and

response waveforms. The higher the viscosity, the larger the frequency change for a given phase

change.

The vibrational instrument also suffers from a lack of a defined shear field, which makes it

unsuited to measuring the viscosity of a fluid whose flow behaviour is not known before hand.

Vibrating viscometers are rugged industrial systems used to measure viscosity in the process

condition. The active part of the sensor is a vibrating rod. The vibration amplitude varies

according to the viscosity of the fluid in which the rod is immersed. These viscosity meters are

suitable for measuring clogging fluid and high-viscosity fluids even with fibers (up to 1,000

Pa·s). Currently, many industries around the world consider these viscometers as the most

efficient system to measure viscosity of any fluid, contrasted to rotational viscometers, which

require more maintenance, inability to measure clogging fluid, and frequent calibration after

intensive use. Vibrating viscometers has no moving parts, no weak parts and the sensitive part is

very small. Actually even the very basic or acid fluid can be measured by adding a special

coating or by changing the material of the sensor to a material such as 316L, SUS316, Hastelloy,

or enamel.

Rotational viscometers

Rotational viscometers use the idea that the torque required to turn an object in a fluid, can

indicate the viscosity of that fluid. They do so by measuring the required torque for rotating a

disk or bob in a fluid at known speed.

'Cup and bob' viscometers work by defining the exact volume of sample which is to be sheared

within a test cell, the torque required to achieve a certain rotational speed is measured and

plotted. There are two classical geometries in "cup and bob" viscometers, known as either the

"Couette" or "Searle" systems - distinguished by whether the cup or bob rotates. The rotating cup

is preferred in some cases, because it reduces the onset of Taylor vortices, but is more difficult to

thermostat accurately.

'Cone and Plate' viscometers use a cone of very shallow angle in bare contact with a flat plate.

With this system the shear rate beneath the plate is constant to a modest degree of precision and

deconvolution of a flow curve; a graph of shear stress (torque) against shear rate (angular

velocity) yields the viscosity in a straightforward manner.

Stabinger viscometer

Stabinger viscometer (SVM 3000)By modifying the classic Couette rotational viscometer, an

accuracy comparable to that of kinematic viscosity determination is achieved. The internal

cylinder in the Stabinger Viscometer is hollow and specifically lighter than the sample, thus

floats freely in the sample, centered by centrifugal forces. The formerly inevitable bearing

friction is thus fully avoided. The speed and torque measurement is implemented without direct

contact, by a rotating magnetic field and an eddy current brake. This allows for a previously

unprecedented torque resolution of 50 pN·m and an exceedingly large measuring range from 0.2

to 20,000 mPa·s with a single measuring system. A built-in density measurement based on the

oscillating U-tube principle allows the determination of kinematic viscosity from the measured

dynamic viscosity employing the relation

The Stabinger Viscometer was presented for the first time by Anton Paar GmbH at the

ACHEMA in the year 2000. The measuring principle is named after its inventor Dr. Hans

Stabinger.

Stormer viscometer

The Stormer viscometer is a rotation instrument used to determine the viscosity of paints,

commonly used in paint industries. It consists of a paddle-type rotor that is spun by an internal

motor, submerged into a cylinder of viscous substance. The rotor speed can be adjusted by

changing the amount of load supplied onto the rotor. For example, in one brand of viscometers,

pushing the level upwards decreases the load and speed, downwards increases the load and

speed.

The viscosity can be found by adjusting the load until the rotation velocity is 200 rotations per

minute. By examining the load applied and comparing tables found on ASTM D 562, one can

find the viscosity in Krebs units (KU), unique only to the Stormer type viscometer.

This method is intended for paints applied by brush or roller.

Bubble viscometer

Bubble viscometers are used to quickly determine kinematic viscosity of known liquids such as

resins and varnishes. The time required for an air bubble to rise is directly proportional to the

visosity of the liquid, so the faster the bubble rises, the lower the viscosity. The Alphabetical

Comparison Method uses 4 sets of lettered reference tubes, A5 through Z10, of known viscosity

to cover a viscosity range from 0.005 to 1,000 stokes. The Direct Time Method uses a single 3-

line times tube for determining the "bubble seconds", which may then be converted to stokes.

Objective

The objective of the project is to make a rotational viscometer. Rotational viscometers use the

idea that the torque required to turn an object in a fluid, can indicate the viscosity of that fluid.

They do so by measuring the required torque for rotating a disk or bob in a fluid at known speed.

Block Diagram

Theory

A fixed voltage source will give constant torque to the motor. The motor will be coupled to

paddle that will rotate in the solution. The transducer will measure the speed of rotation in the

liquid to be measured and give output on a digital screen.

Units of measureUnits of measure

Dynamic viscosity

The usual symbol for dynamic viscosity used by mechanical engineers and fluid dynamicists is

the Greek letter mu (μ). The SI physical unit of dynamic viscosity is the pascal-second (Pa·s),

which is identical to kg·m−1·s−1. If a fluid with a viscosity of one Pa·s is placed between two

plates, and one plate is pushed sideways with a shear stress of one pascal, it moves a distance

equal to the thickness of the layer between the plates in one second.

The cgs physical unit for dynamic viscosity is the poise (P), named after Jean Louis Marie

Poiseuille. It is more commonly expressed, particularly in ASTM standards, as centipoise (cP).

Water at 20 °C has a viscosity of 1.0020 cP.

1 P = 1 g·cm−1·s−1

The relation between poise and pascal-seconds is:

10 P = 1 kg·m−1·s−1 = 1 Pa·s

1 cP = 0.001 Pa·s = 1 mPa·s

The name 'poiseuille' was proposed for this unit (after Jean Louis Marie Poiseuille who

formulated Poiseuille's law of viscous flow), but not accepted internationally. Care must be taken

in not confusing the poiseuille with the poise named after the same person.

Kinematic viscosity

In many situations, we are concerned with the ratio of the viscous force to the inertial force, the

latter characterised by the fluid density ρ. This ratio is characterised by the kinematic viscosity

(Greek letter nu, ν), defined as follows:

ν=µ/ρ,

where μ is the dynamic viscosity (Pa·s) and ρ is the density (kg/m3), and ν is the kinematic

viscosity (m2/s).

The cgs physical unit for kinematic viscosity is the stokes (St), named after George Gabriel

Stokes. It is sometimes expressed in terms of centistokes (cSt or ctsk). In U.S. usage, stoke is

sometimes used as the singular form.

1 stokes = 100 centistokes = 1 cm2·s−1 = 0.0001 m2·s−1.

1 centistokes = 1 mm2·s-1 = 10-6m2·s−1

The kinematic viscosity is sometimes referred to as diffusivity of momentum, because it is

comparable to and has the same unit (m2s−1) as diffusivity of heat and diffusivity of mass. It is

therefore used in dimensionless numbers which compare the ratio of the diffusivities.

Description of various modules

MOTOR DRIVER CIRCUIT

The principle of conversion of electrical energy into mechanical energy by electromagnetic

means was demonstrated by the British scientist Michael Faraday in 1821 and consisted of a

free-hanging wire dipping into a pool of mercury. A permanent magnet was placed in the middle

of the pool of mercury. When a current was passed through the wire, the wire rotated around the

magnet, showing that the current gave rise to a circular magnetic field around the wire. This

motor is often demonstrated in school physics classes, but brine (salt water) is sometimes used in

place of the toxic mercury. This is the simplest form of a class of electric motors called

homopolar motors. A later refinement is the Barlow's Wheel. These were demonstration devices,

unsuited to practical applications due to limited power.

The first commutator-type direct-current electric motor capable of a practical application was

invented by the British scientist William Sturgeon. He was self-educated in the natural sciences

and the science of electricity, and he spent much time experimenting with electricity and

lecturing on the topic. In 1825, he delivered a lecture to his class at the Royal Military College in

which he demonstrated a 7-ounce electromagnet capable of carrying 9 pounds (4 kilograms) of

iron when a current from a single cell was sent through the electromagnet coils. In 1832,

Sturgeon invented an electric motor which had a commutator, the critical part of a modern DC

motor. His other achievements include the improvement of the electrochemical battery,

contributions to the theory of thermo electricity, and the discovery that the atmosphere in serene

weather is positively charged with respect to the earth.

A commutator-type direct-current electric motor built with the intention of commercial use was

invented by the American Thomas Davenport and patented in 1837. Although several motors

were built and operated equipment such as a printing press, due to the high cost of primary

battery power, the motors were unsuccessful commercially and Davenport went bankrupt.

Although several inventors followed Sturgeon in the development of DC motors, in the days

before electric power distribution these motors had to depend on expensive primary battery

power. This meant that these motors had no practical commercial market.

The modern DC motor was invented by accident in 1873, when Zénobe Gramme connected a

spinning dynamo to a second similar unit, driving it as a motor. The Gramme machine was the

first industrially useful electric motor; earlier inventions were used as toys or laboratory

curiosities.

Electric motors of various sizes.

DC motor rotation

A simple DC electric motor. When the

coil is powered, a magnetic field is

generated around the armature. The left

side of the armature is pushed away

from the left magnet and drawn toward

the right, causing rotation.

The armature

continues to rotate.

When the armature becomes

horizontally aligned, the

commutator reverses the direction

of current through the coil,

reversing the magnetic field. The

process then repeats.

If the shaft of a DC motor is turned by an external force, the motor will act like a generator and

produce an Electromotive force (EMF). During normal operation, the spinning of the motor

produces a voltage, known as the counter-EMF (CEMF) or back EMF, because it opposes the

applied voltage on the motor. This is the same EMF that is produced when the motor is used as a

generator (for example when an electrical load (resistance) is placed across the terminals of the

motor and the motor shaft is driven with an external torque). Therefore, the voltage drop across a

motor consists of the voltage drop, due to this CEMF, and the parasitic voltage drop resulting

from the internal resistance of the armature's windings. The current through a motor is given by

the following equation:

I = (Vapplied − Vcemf) / Rarmature

The mechanical power produced by the motor is given by:

P = I * (Vapplied − Vcemf)

Mechanism of the DC motors:

When current passes through the coil wound around a soft iron core the side of the positive pole

is acted upon by an upwards force, while the other side is acted upon by a downward force.

According to Fleming's left hand rule, the forces cause a turning effect on the coil making it

rotate; to make the motor rotate in a constant direction "direct current" commutators make the

current reverse in direction every half a cycle thus causing the motor to rotate in the same

direction.The problem facing the motor is when the plane of the coil is parallel to the magnetic

field;i.e. the turning effect is ZERO-when coil is at 90 degree from its original position-yet, the

coil continues to rotate by inertia.

Since the CEMF is proportional to motor speed, when an electric motor is first started or is

completely stalled, there is zero CEMF. Therefore the current through the armature is much

higher. This high current will produce a strong magnetic field which will start the motor

spinning. As the motor spins, the CEMF increases until it is equal to the applied voltage, minus

the parasitic voltage drop. At this point, there will be a smaller current flowing through the

motor. Basically, the following three equations can be used to find the speed, current, and back

EMF of a motor under a load:

Load = Vcemf * I

Vapplied = I * Rarmature + Vcemf

Vcemf = speed * Fluxarmature

Generally, the rotational speed of a DC motor is proportional to the voltage applied to it, and the

torque is proportional to the current. Speed control can be achieved by variable battery tappings,

variable supply voltage, resistors or electronic controls. The direction of a wound field DC motor

can be changed by reversing either the field or armature connections but not both. This is

commonly done with a special set of contactors (direction contactors).

The effective voltage can be varied by inserting a series resistor or by an electronically

controlled switching device made of thyristors, transistors, or, formerly, mercury arc rectifiers. In

a circuit known as a chopper, the average voltage applied to the motor is varied by switching the

supply voltage very rapidly. As the "on" to "off" ratio is varied to alter the average applied

voltage, the speed of the motor varies. The percentage "on" time multiplied by the supply voltage

gives the average voltage applied to the motor. Therefore, with a 100 V supply and a 25% "on"

time, the average voltage at the motor will be 25 V. During the "off" time, the armature's

inductance causes the current to continue flowing through a diode called a "flywheel diode", in

parallel with the motor. At this point in the cycle, the supply current will be zero, and therefore

the average motor current will always be higher than the supply current unless the percentage

"on" time is 100%. At 100% "on" time, the supply and motor current are equal. The rapid

switching wastes less energy than series resistors. This method is also called pulse width

modulation, or PWM, and is often controlled by a microprocessor. An output filter is sometimes

installed to smooth the average voltage applied to the motor and reduce motor noise.

Since the series-wound DC motor develops its highest torque at low speed, it is often used in

traction applications such as electric locomotives, and trams. Another application is starter

motors for petrol and small diesel engines. Series motors must never be used in applications

where the drive can fail (such as belt drives). As the motor accelerates, the armature (and hence

field) current reduces. The reduction in field causes the motor to speed up (see 'weak field' in the

last section) until it destroys itself. This can also be a problem with railway motors in the event

of a loss of adhesion since, unless quickly brought under control, the motors can reach speeds far

higher than they would do under normal circumstances. This can not only cause problems for the

motors themselves and the gears, but due to the differential speed between the rails and the

wheels it can also cause serious damage to the rails and wheel treads as they heat and cool

rapidly. Field weakening is used in some electronic controls to increase the top speed of an

electric vehicle. The simplest form uses a contactor and field weakening resistor, the electronic

control monitors the motor current and switches the field weakening resistor into circuit when

the motor current reduces below a preset value (this will be when the motor is at its full design

speed). Once the resistor is in circuit, the motor will increase speed above its normal speed at its

rated voltage. When motor current increases, the control will disconnect the resistor and low

speed torque is made available.

One interesting method of speed control of a DC motor is the Ward-Leonard control. It is a

method of controlling a DC motor (usually a shunt or compound wound) and was developed as a

method of providing a speed-controlled motor from an AC supply, though it is not without its

advantages in DC schemes. The AC supply is used to drive an AC motor, usually an induction

motor that drives a DC generator or dynamo. The DC output from the armature is directly

connected to the armature of the DC motor (sometimes but not always of identical construction).

The shunt field windings of both DC machines are independently excited through variable

resistors. Extremely good speed control from standstill to full speed, and consistent torque, can

be obtained by varying the generator and/or motor field current. This method of control was the

de facto method from its development until it was superseded by solid state thyristor systems. It

found service in almost any environment where good speed control was required, from passenger

lifts through to large mine pit head winding gear and even industrial process machinery and

electric cranes. Its principal disadvantage was that three machines were required to implement a

scheme (five in very large installations, as the DC machines were often duplicated and controlled

by a tandem variable resistor). In many applications, the motor-generator set was often left

permanently running, to avoid the delays that would otherwise be caused by starting it up as

required. Although electronic (thyristor) controllers have replaced most small to medium Ward

Leonard systems, some very large ones (thousands of horsepower) remain in service. The field

currents are much lower than the armature currents, allowing a moderate sized thryistor unit to

control a much larger motor than it could control directly. For example, in one installation, a 300

amp thyristor unit controls the field of the generator. The generator output current is in excess of

15,000 amps, which would be prohibitively expensive (and inefficient) to control directly with

thyristors.

Power SupplyPower Supply

SignificanceSignificance

It is the most important component of the projectIt is the most important component of the project

Performance of all other modules depends on performance of power supplyPerformance of all other modules depends on performance of power supply

Ripple or spike if present in power supply will effect all other cards like Analog stage,Ripple or spike if present in power supply will effect all other cards like Analog stage,

Microcontroller etc.Microcontroller etc.

Problems like noise in analog and hanging in Microcontroller are then encounteredProblems like noise in analog and hanging in Microcontroller are then encountered

If regulation is not proper then under-voltage or over-voltage may occur. Under-voltageIf regulation is not proper then under-voltage or over-voltage may occur. Under-voltage

can cause errors while over-voltage can damage circuitrycan cause errors while over-voltage can damage circuitry

RequirementRequirement

Project has op-amps, microcontroller, digital ICs, motors , relays etc which do not workProject has op-amps, microcontroller, digital ICs, motors , relays etc which do not work

on 220V AC.on 220V AC.

Op-Amplifiers operate on +, - 12 V DC.Op-Amplifiers operate on +, - 12 V DC.

Microcontrollers, digital ICs operate on +5V DC.Microcontrollers, digital ICs operate on +5V DC.

Power Supply is required generate +5V, +12V or +5V, +12V, -12V DC from 220V AC.Power Supply is required generate +5V, +12V or +5V, +12V, -12V DC from 220V AC.

Constituent processesConstituent processes

To Step down the available 220V AC.To Step down the available 220V AC.

To rectify, this available AC.To rectify, this available AC.

To filter the rectified AC.To filter the rectified AC.

Lastly to regulate the available voltage signal.Lastly to regulate the available voltage signal.

These processes are not exchangeable These processes are not exchangeable

We cannot first rectify then step down because a transformer operates only on AC. TheWe cannot first rectify then step down because a transformer operates only on AC. The

voltage induced in the secondary is proportional to L(dI/dt). Thus voltage will be inducedvoltage induced in the secondary is proportional to L(dI/dt). Thus voltage will be induced

only if current in the primary is alternating.only if current in the primary is alternating.

A filter is needed to remove the ripple from the rectified AC which still not a constant DCA filter is needed to remove the ripple from the rectified AC which still not a constant DC

Transformer StageTransformer Stage

The transformer is designed so as to step down the 220 V AC to a 12V AC.The transformer is designed so as to step down the 220 V AC to a 12V AC.

The turns ratio depends on the voltage ratio. The turns ratio depends on the voltage ratio.

Hence a 12-0-12 Transformer (which has a fixed turns ratio) is used.Hence a 12-0-12 Transformer (which has a fixed turns ratio) is used.

Secondly a center tapped transformer is used if our requirement is +12 V as well as -12VSecondly a center tapped transformer is used if our requirement is +12 V as well as -12V

DC.DC.

RectificationRectification

Types of rectificationTypes of rectification

Half-wave rectificationHalf-wave rectification

Full-wave rectificationFull-wave rectification

Bridge rectificationBridge rectification

Half-Wave RectificationHalf-Wave Rectification

The rectifier conducts current only during the positive half cycles of input a.c. voltageThe rectifier conducts current only during the positive half cycles of input a.c. voltage

The a.c. supply delivers power only half the time thus efficiency is lowThe a.c. supply delivers power only half the time thus efficiency is low

Rectifier efficiency = 0.406 / (1+RDIODE / RLOAD)Rectifier efficiency = 0.406 / (1+RDIODE / RLOAD)

Maximum Rectifier efficiency = 40.6% (If RDIODE is negligible as compared toMaximum Rectifier efficiency = 40.6% (If RDIODE is negligible as compared to

RLOAD )RLOAD )

Full-Wave RectificationFull-Wave Rectification

The current flows through the load in the same direction for both half-cycles of input a.c.The current flows through the load in the same direction for both half-cycles of input a.c.

voltage using two diodes working alternately.voltage using two diodes working alternately.

Two TypesTwo Types

Centre- Tapped Full-wave RectifierCentre- Tapped Full-wave Rectifier

Full- Wave Bridge RectifierFull- Wave Bridge Rectifier

Rectifier efficiency = 0.812 / (1+RDIODE / RLOAD)Rectifier efficiency = 0.812 / (1+RDIODE / RLOAD)

Maximum Rectifier efficiency = 81.2 % (If RDIODE is negligible as compared toMaximum Rectifier efficiency = 81.2 % (If RDIODE is negligible as compared to

RLOAD )RLOAD )

Choice of rectification: Centre-tapped Bridge rectification is preferred due toChoice of rectification: Centre-tapped Bridge rectification is preferred due to

The requirement of both +ve and –ve o/p voltages.The requirement of both +ve and –ve o/p voltages.

Higher efficiencyHigher efficiency

Filter StageFilter Stage

A simple capacitive low pass filter has to be designed so as to by-pass the AC rippleA simple capacitive low pass filter has to be designed so as to by-pass the AC ripple

component in the rectified AC.component in the rectified AC.

The capacitors can have values of 1,10,100,1000The capacitors can have values of 1,10,100,1000FF

RequirementRequirement

The time constant (RC) of the filter should be so high that once if it charges during theThe time constant (RC) of the filter should be so high that once if it charges during the

rising part of the signal, it should discharge very slowly as the signal falls. rising part of the signal, it should discharge very slowly as the signal falls.

By then the next rectified half cycle recharges the capacitor. By then the next rectified half cycle recharges the capacitor.

Hence the highest easily available capacitor (1000Hence the highest easily available capacitor (1000F) is used in the filter stage.F) is used in the filter stage.

Regulation StageRegulation Stage

Purpose of regulationPurpose of regulation

The transformer is designed so as to step down the 220 V AC to a 12V AC. The turnsThe transformer is designed so as to step down the 220 V AC to a 12V AC. The turns

ratio depends on the voltage ratio. Hence a 12-0-12 Transformer (which has been used)ratio depends on the voltage ratio. Hence a 12-0-12 Transformer (which has been used)

will have a fixed turns ratio. So it will generate 12 V AC only if the input is exactly 220Vwill have a fixed turns ratio. So it will generate 12 V AC only if the input is exactly 220V

AC.AC.

Is supply ever at exactly 220V AC (in India) ? NO!!Is supply ever at exactly 220V AC (in India) ? NO!!

Thus with variation in input supply the output will also vary from 12V to may be 15V ACThus with variation in input supply the output will also vary from 12V to may be 15V AC

or 9V AC.or 9V AC.

The rectified and filtered DC will also not be exactly 12V DC.The rectified and filtered DC will also not be exactly 12V DC.

The role of regulation now comes into play.The role of regulation now comes into play.

Principle of RegulationPrinciple of Regulation

No matter what the input DC is, the output of a regulator IC is fixed according to itsNo matter what the input DC is, the output of a regulator IC is fixed according to its

numbernumber

IC no. 7812 would give an output of +12V DCIC no. 7812 would give an output of +12V DC

IC no. 7805 would give an output of +5 V DCIC no. 7805 would give an output of +5 V DC

IC no. 7912 would give an output of -12V DCIC no. 7912 would give an output of -12V DC

This IC is based on zener breakdown diodes , which after breakdown keep the voltageThis IC is based on zener breakdown diodes , which after breakdown keep the voltage

across them constant.across them constant.

TransformerTransformer

A transformer is a device that transfers electrical energy from one circuit to another by magnetic

coupling without requiring relative motion between its parts. It usually comprises two or more

coupled windings, and, in most cases, a core to concentrate magnetic flux.

An alternating voltage applied to one winding creates a time-varying magnetic flux in the core,

which induces a voltage in the other windings. Varying the relative number of turns between

primary and secondary windings determines the ratio of the input and output voltages, thus

transforming the voltage by stepping it up or down between circuits.

The transformer principle was demonstrated in 1831 by Faraday, though practical designs did not

appear until the 1880s. Within less than a decade, the transformer was instrumental during the

"War of Currents" in seeing alternating current systems triumph over their direct current

counterparts, a position in which they have remained dominant. The transformer has since

shaped the electricity supply industry, permitting the economic transmission of power over long

distances. All but a fraction of the world's electrical power has passed through a series of

transformers by the time it reaches the consumer.

Amongst the simplest of electrical machines, the transformer is also one of the most efficient,

with large units attaining performances in excess of 99.75%. Transformers come in a range of

sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge

giga VA-rated units used to interconnect portions of national power grids. All operate with the

same basic principles and with many similarities in their parts, though a variety of transformer

designs exist to perform specialized roles throughout home and industry.

Coupling by mutual induction

The principles of the transformer are illustrated by consideration of a hypothetical ideal

transformer consisting of two windings of zero resistance around a core of negligible reluctance.

[4] A voltage applied to the primary winding causes a current, which develops a magnetomotive

force (MMF) in the core. The current required to create the MMF is termed the magnetising

current; in the ideal transformer it is considered to be negligible. The MMF drives flux around

the magnetic circuit of the core.[4]

An electromotive force (EMF) is induced across each winding, an effect known as mutual

inductance.[5] The windings in the ideal transformer have no resistance and so the EMFs are

equal in magnitude to the measured terminal voltages. In accordance with Faraday's law of

induction, they are proportional to the rate of change of flux:

     and     

where:

and are the induced EMFs across primary and secondary windings,

and are the numbers of turns in the primary and secondary windings,

and are the time derivatives of the flux linking the primary and secondary windings.

In the ideal transformer, all flux produced by the primary winding also links the secondary, [6] and

so , from which the well-known transformer equation follows:

The ratio of primary to secondary voltage is therefore the same as the ratio of the number of

turns;[4] alternatively, that the volts-per-turn is the same in both windings.

Under load

The ideal transformer as a circuit element

If a load impedance is connected to the secondary winding, a current will flow in the secondary

circuit so created. The current develops an MMF over the secondary winding in opposition to

that of the primary winding, so acting to cancel the flux in the core. [6] The now decreased flux

reduces the primary EMF, causing current in the primary circuit to increase to exactly offset the

effect of the secondary MMF, and returning the flux to its former value.[7] The core flux thus

remains the same regardless of the secondary current, provided the primary voltage is sustained.

[6] In this way, the electrical energy fed into the primary circuit is delivered to the secondary

circuit.

The primary and secondary MMFs differ only to the extent of the negligible magnetising current

and may be equated, and so: , from which the transformer current relationship

emerges:

From consideration of the voltage and current relationships, it may be readily shown that

impedance in one circuit is transformed by the square of the turns ratio,[6] a secondary impedance

thus appearing to the primary circuit to have a value of .

In general, operation of a transformer at its designed voltage but at a higher frequency than

intended will lead to reduced magnetising current. At a frequency lower than the design value,

with the rated voltage applied, the magnetising current may increase to an excessive level.

Operation of a transformer at other than its design frequency may require assessment of voltages,

losses, and cooling to establish if safe operation is practical. For example, transformers may need

to be equipped with "volts per hertz" over-excitation relays to protect the transformer from

overvoltage at higher than rated frequency.

Energy losses

An ideal transformer would have no energy losses, and would therefore be 100% efficient.

Despite the transformer being amongst the most efficient of electrical machines, with

experimental models using superconducting windings achieving efficiencies of 99.85%, [9] energy

is dissipated in the windings, core, and surrounding structures. Larger transformers are generally

more efficient, and those rated for electricity distribution usually perform better than 95%. [10] A

small transformer such as a plug-in "power brick" used for low-power consumer electronics may

be less than 85% efficient.

Transformer losses are attributable to several causes and may be differentiated between those

originating in the windings, sometimes termed copper loss, and those arising from the magnetic

circuit, sometimes termed iron loss. The losses vary with load current, and may furthermore be

expressed as "no-load" or "full-load" loss, or at an intermediate loading. Winding resistance

dominates load losses, whereas hysteresis and eddy currents losses contribute to over 99% of the

no-load loss.

Losses in the transformer arise from:

Winding resistance

Current flowing through the windings causes resistive heating of the conductors. At higher

frequencies, skin effect and proximity effect create additional winding resistance and losses.

Eddy currents

Ferromagnetic materials are also good conductors, and a solid core made from such a material

also constitutes a single short-circuited turn throughout its entire length. Induced eddy currents

therefore circulate within the core in a plane normal to the flux, and are responsible for resistive

heating of the core material.

Hysteresis losses

Each time the magnetic field is reversed, a small amount of energy is lost to hysteresis within the

magnetic core, the amount being dependant on the particular core material.

Magnetostriction

Magnetic flux in the core causes it to physically expand and contract slightly with the alternating

magnetic field, an effect known as magnetostriction. This produces the familiar buzzing sound,

and in turn causes losses due to frictional heating in susceptible cores.

Mechanical losses

In addition to magnetostriction, the alternating magnetic field causes fluctuating electromagnetic

forces between the primary and secondary windings. These incite vibrations within nearby

metalwork, adding to the buzzing noise, and consuming a small amount of power.

Stray losses

Not all the magnetic field produced by the primary is intercepted by the secondary. A portion of

the leakage flux may induce eddy currents within nearby conductive objects, such as the

transformer's support structure, and be converted to heat.

Cooling system

Large power transformers may be equipped with cooling fans, oil pumps or water-cooled heat

exchangers designed to remove heat. The power used to operate the cooling system is typically

considered part of the losses of the transformer.

Cores

Laminated core transformer showing edge of laminations at top of unit.

Steel cores

Transformers for use at power or audio frequencies typically have cores made of high

permeability silicon steel. By concentrating the magnetic flux, more of it usefully links both

primary and secondary windings, and the magnetising current is greatly reduced. Early

transformer developers soon realised that cores constructed from solid iron resulted in

prohibitive eddy-current losses, and their designs mitigated this effect with cores consisting of

bundles of insulated iron wires. Later designs constructed the core by stacking layers of thin steel

laminations, a principle which has continued to the present. Each lamination is insulated from its

neighbours by a coat of non-conducting paint.

The effect of laminations is to confine eddy currents to highly elliptical paths that enclose little

flux, and so reduce their magnitude. Thinner laminations result in lower eddy currents, and hence

reduce losses,[11] but are more laborious and expensive to construct. Thin laminations are

generally used on high frequency transformers, with some types of very thin steel laminations

able to operate up to 10 kHz.

E-I core construction, windings omitted

One common design of laminated core is made from interleaved stacks of E-shaped steel sheets

capped with I-shaped pieces, leading to its name of "E-I transformer". The cut-core or C-core

type is made by winding a steel strip around a rectangular form and then bonding the layers

together. It is then cut in two, forming two C shapes, and the core assembled by binding the two

C halves together with a steel strap. They have the advantage that the flux is always oriented

parallel to the metal grains, reducing reluctance.

A steel core's remanence means that it retains a static magnetic field when power is removed.

When power is then reapplied, the residual field will cause a high inrush current until the effect

of the remanent magnetism is reduced, usually after a few cycles of the applied alternating

current. Overcurrent protection devices such as fuses must be selected to allow this harmless

inrush to pass. On transformers connected to long overhead power transmission lines, induced

currents due to geomagnetic disturbances during solar storms can cause saturation of the core,

and false operation of transformer protection devices.

Distribution transformers can achieve low off-load losses by using cores made with low loss high

permeability silicon steel and amorphous (non-crystalline) steel, so-called "metal glasses". The

high initial cost of the core material is offset over the life of the transformer by its lower losses at

light load.

The conducting material used for the winding depends upon the application. Small power and

signal transformers are wound with solid copper wire, insulated usually with enamel, and

sometimes additional insulation. Larger power transformers may be wound with wire, copper, or

aluminium rectangular conductors. Strip conductors are used for very heavy currents. High

frequency transformers operating in the tens to hundreds of kilohertz will have windings made of

Litz wire to minimize the skin effect losses in the conductors. Large power transformers use

multiple-stranded conductors as well, since even at low power frequencies non-uniform

distribution of current would otherwise exist in high-current windings. Each strand is insulated

from the other, and the strands are arranged so that at certain points in the winding, or throughout

the whole winding, each portion occupies different relative positions in the complete conductor.

This "transposition" equalizes the current flowing in each strand of the conductor, and reduces

eddy current losses in the winding itself.

Windings on both the primary and secondary of power transformers may have external

connections (called taps) to intermediate points on the winding to allow adjustment of the

voltage ratio. Taps may be connected to an automatic, on-load tap changer type of switchgear for

voltage regulation of distribution circuits. Audio-frequency transformers, used for the

distribution of audio to public address loudspeakers, have taps to allow adjustment of impedance

to each speaker. A center-tapped transformer is often used in the output stage of an audio power

amplifier in a push-pull type circuit. Modulation transformers in AM transmitters are very

similar. Tapped transformers are also used as components of amplifiers, oscillators, and for

feedback linearization of amplifier circuits.

Winding insulation

The turns of the windings must be insulated from each other to ensure that the current travels

through the entire winding. The potential difference between adjacent turns is usually small, so

that enamel insulation may suffice for small power transformers. Supplemental sheet or tape

insulation is usually employed between winding layers in larger transformers.

The transformer may also be immersed in transformer oil that provides further insulation.

Although the oil is primarily used to cool the transformer, it also helps to reduce the formation of

corona discharge within high voltage transformers. By cooling the windings, the insulation will

not break down as easily due to heat. To ensure that the insulating capability of the transformer

oil does not deteriorate, the transformer casing is completely sealed against moisture ingress.

Thus the oil serves as both a cooling medium to remove heat from the core and coil, and as part

of the insulation system.

Rectifier

AC, half-wave and full wave rectified signals

A rectifier is an electrical device that converts alternating current to direct current, a process

known as rectification. Rectifiers are used as components of power supplies and as detectors of

radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc

valves, and other technologies.

When just one diode is used to rectify AC (by blocking the negative or positive portion of the

waveform) the difference between the term diode and the term rectifier is merely one of usage,

i.e., the term rectifier describes a diode that is being used to convert AC to DC. Almost all

rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting

AC to DC than is possible with just a single diode. Before the development of solid state

rectifiers, vacuum tube diodes and copper oxide or selenium rectifier stacks were used.

Early radio receivers called crystal sets, used a "cat's whisker" of fine wire pressing on a crystal

of galena (lead sulfide) to serve as a point contact rectifier or "crystal detector". In gas heating

systems flame rectification can be used to detect a flame. Two metal electrodes in the outer layer

of the flame provide a current path and rectification of an applied alternating voltage, but only

while the flame is present.

Half-wave rectification

A half wave rectifier is a special case of a clipper. In half wave rectification, either the positive

or negative half of the AC wave is passed easily while the other half is blocked, depending on

the polarity of the rectifier. Because only one half of the input waveform reaches the output, it is

very inefficient if used for power transfer. Half wave rectification can be achieved with a single

diode in a one phase supply.

Full-wave rectification

Full-wave rectification converts both polarities of the input waveform to DC, and is more

efficient. However, in a circuit with a non-center tapped transformer, four rectifiers are required

instead of the one needed for half-wave rectification. This is due to each output polarity requiring

two rectifiers each, for example, one for when AC terminal 'X' is positive and one for when AC

terminal 'Y' is positive. The other DC output requires exactly the same, resulting in four

individual junctions (See semiconductors/diode). Four rectifiers arranged this way are called a

bridge rectifier:

A full wave rectifier converts the whole of the input waveform to one of constant polarity

(positive or negative) at its output by reversing the negative (or positive) portions of the

alternating current waveform. The positive (negative) portions thus combine with the reversed

negative (positive) portions to produce an entirely positive (negative) voltage/current waveform.

For single phase AC, if the transformer is center-tapped, then two diodes back-to-back (i.e.

anodes-to-anode or cathode-to-cathode) form a full wave rectifier.

Rectifier output smoothing

While half- and full-wave rectification suffices to deliver a form of DC output, neither produces

constant voltage DC. In order to produce steady DC from a rectified AC supply, a smoothing

circuit, sometimes called a filter, is required. In its simplest form this can be what is known as a

reservoir capacitor, Filter capacitor or smoothing capacitor, placed at the DC output of the

rectifier. There will still remain an amount of AC ripple voltage where the voltage is not

completely smoothed.

Sizing of the capacitor represents a tradeoff. For a given load, a larger capacitor will reduce

ripple but will cost more and will create higher peak currents in the transformer secondary and in

the supply feeding it. In extreme cases where many rectifiers are loaded onto a power

distribution circuit, it may prove difficult for the power distribution authority to maintain a

correctly shaped sinusoidal voltage curve.

For a given tolerable ripple the required capacitor size is proportional to the load current and

inversely proportional to the supply frequency and the number of output peaks of the rectifier per

input cycle. The load current and the supply frequency are generally outside the control of the

designer of the rectifier system but the number of peaks per input cycle can be effected by the

choice of rectifier design.

A half wave rectifier will only give one peak per cycle and for this and other reasons is only used

in very small power supplies. A full wave rectifier achieves two peaks per cycle and this is the

best that can be done with single phase input. For three phase inputs a three phase bridge will

give six peaks per cycle and even higher numbers of peaks can be achieved by using transformer

networks placed before the rectifier to convert to a higher phase order.

To further reduce this ripple, a capacitor-input filter can be used. This complements the reservoir

capacitor with a choke and a second filter capacitor, so that a steadier DC output can be obtained

across the terminals of the filter capacitor. The choke presents a high impedance to the ripple

current.

If the DC load is very demanding of a smooth supply voltage, a voltage regulator will be used

either instead of or in addition to the capacitor-input filter, both to remove the last of the ripple

and to deal with variations in supply and load characteristics.

Regulator

7805 is an integrated three-terminal positive fixed linear voltage regulator. It supports an input

voltage of 10 volts to 35 volts and output voltage of 5 volts. It has a current rating of 1 amp

although lower current models are available. Its output voltage is fixed at 5.0V. The 7805 also

has a built-in current limiter as a safety feature. 7805 is manufactured by many companies,

including National Semiconductors and Fairchild Semiconductors. The 7805 will automatically

reduce output current if it gets too hot.

It belongs to a family of three-terminal positive fixed regulators with similar specifications and

differing fixed voltages from 8 to 15 volts. These are usually packaged in TO220 chip carriers,

but smaller surface-mount and larger TO3 packages are also available. The last two digits

represent the voltage; for instance, the 7812 is a 12-volt regulator. The 78xx series of regulators

is designed to work in complement with the 79xx series of negative voltage regulators in systems

that provide both positive and negative regulated voltages, since the 78xx series can't regulate

negative voltages in such a system.

The 7805 is one of the most common and well-known of the 78xx series regulators, as its small

component count and medium-power regulated 5V make it useful for powering TTL devices.

7812 is an integrated three-terminal positive fixed linear voltage regulator. It supports an input

voltage of 18 volts to 35 volts and an output voltage of 12 volts. It has a current rating of 1 amp

although lower current models are available as of 2006. Its output voltage is fixed at 12 volts.

The 7812 also has a built-in current limiter as a safety feature. 7812 is manufactured by many

companies, including National Semiconductors and Fairchild Semiconductors. The 7812 will

automatically reduce output current if it gets too hot.

It belongs to a family of three-terminal positive fixed regulators with similar specifications and

differing fixed voltages from 8 to 15 volts. These are usually packaged in TO220 chip carriers,

but smaller surface-mount or larger metal can packages are also available. The last two digits

represent the voltage; for instance, the 7812 is a 12-volt regulator. The 78xx series of regulators

is designed to work in complement with the 79xx series of negative voltage regulators in systems

that provide both positive and negative regulated voltages, since the 78xx series can't regulate

negative voltages in such a system

Filtering of Ripple

A capacitor is an electrical device that can store energy in the electric field between a pair of

closely-spaced conductors (called 'plates'). When voltage is applied to the capacitor, electric

charges of equal magnitude, but opposite polarity, build up on each plate.

Capacitors are used in electrical circuits as energy-storage devices. They can also be used to

differentiate between high-frequency and low-frequency signals and this makes them useful in

electronic filters.

Capacitors are occasionally referred to as condensers. This is now considered an antiquated term.

The capacitor's capacitance (C) is a measure of the amount of charge (Q) stored on each plate for

a given potential difference or voltage (V) which appears between the plates:

In SI units, a capacitor has a capacitance of one farad when one coulomb of charge is stored due

to one volt applied potential difference across the plates. Since the farad is a very large unit,

values of capacitors are usually expressed in microfarads (µF), nanofarads (nF), or picofarads

(pF).

The capacitance is proportional to the surface area of the conducting plate and inversely

proportional to the distance between the plates. It is also proportional to the permittivity of the

dielectric (that is, non-conducting) substance that separates the plates.

DC sources

Electrons cannot easily pass directly across the dielectric from one plate of the capacitor to the

other as the dielectric is carefully chosen so that it is a good insulator. When there is a current

through a capacitor, electrons accumulate on one plate and electrons are removed from the other

plate. This process is commonly called 'charging' the capacitor -- even though the capacitor is at

all times electrically neutral. In fact, the current through the capacitor results in the separation of

electric charge, rather than the accumulation of electric charge. This separation of charge causes

an electric field to develop between the plates of the capacitor giving rise to voltage across the

plates. This voltage V is directly proportional to the amount of charge separated Q. Since the

current I through the capacitor is the rate at which charge Q is forced through the capacitor

(dQ/dt), this can be expressed mathematically as:

AC sources

The current through a capacitor due to an AC source reverses direction periodically. That is, the

alternating current alternately charges the plates: first in one direction and then the other. With

the exception of the instant that the current changes direction, the capacitor current is non-zero at

all times during a cycle. For this reason, it is commonly said that capacitors "pass" AC.

However, at no time do electrons actually cross between the plates, unless the dielectric breaks

down. Such a situation would involve physical damage to the capacitor and likely to the circuit

involved as well.

Since the voltage across a capacitor is proportional to the integral of the current, as shown above,

with sine waves in AC or signal circuits this results in a phase difference of 90 degrees, the

current leading the voltage phase angle. It can be shown that the AC voltage across the capacitor

is in quadrature with the alternating current through the capacitor. That is, the voltage and

current are 'out-of-phase' by a quarter cycle. The amplitude of the voltage depends on the

amplitude of the current divided by the product of the frequency of the current with the

capacitance, C.

Capacitor types

Metal film: Made from high quality polymer film and metal foil (usually polycarbonate,

polystyrene, polypropylene, polyester (Mylar), and for high quality capacitors polysulfone), with

a layer of metal deposited on surface. They have good quality and stability, and are suitable for

timer circuits. Suitable for high frequencies.

Mica: Similar to metal film. Often high voltage. Suitable for high frequencies. Expensive.

Excellent tolerance.

Paper: Used for relatively high voltages. Now obsolete.

Glass: Used for high voltages. Expensive. Stable temperature coefficient in a wide range of

temperatures.

Ceramic: Chips of altering layers of metal and ceramic. Depending on their dielectric, whether

Class 1 or Class 2, their degree of temperature/capacity dependence varies. They often have

(especially the class 2) high dissipation factor, high frequency coefficient of dissipation, their

capacity depends on applied voltage, and their capacity changes with aging. However they find

massive use in common low-precision coupling and filtering applications. Suitable for high

frequencies.

Aluminum electrolytic: Polarized. Constructionally similar to metal film, but the electrodes are

made of etched aluminium to acquire much larger surfaces. The dielectric is soaked with liquid

electrolyte. They can achieve high capacities but suffer from poor tolerances, high instability,

gradual loss of capacity especially when subjected to heat, and high leakage. Tend to lose

capacity in low temperatures. Bad frequency characteristics make them unsuited for high-

frequency applications. Special types with low equivalent series resistance are available.

Tantalum electrolytic: Similar to the aluminum electrolytic capacitor but with better frequency

and temperature characteristics. High dielectric absorption. High leakage. Has much better

performance in low temperatures.

Supercapacitors: Made from carbon aerogel, carbon nanotubes, or highly porous electrode

materials. Extremely high capacity. Can be used in some applications instead of rechargeable

batteries.

Energy storage

A capacitor can store electric energy when disconnected from its charging circuit, so it can be

used like a temporary battery. Capacitors are commonly used in electronic devices to maintain

power supply while batteries are being changed. (This prevents loss of information in volatile

memory.)

Capacitors are used in power supplies where they smooth the output of a full or half wave

rectifier. They can also be used in charge pump circuits as the energy storage element in the

generation of higher voltages than the input voltage.

Capacitors are connected in parallel with the power circuits of most electronic devices and larger

systems (such as factories) to shunt away and conceal current fluctuations from the primary

power source to provide a "clean" power supply for signal or control circuits. Audio equipment,

for example, uses several capacitors in this way, to shunt away power line hum before it gets into

the signal circuitry. The capacitors act as a local reserve for the DC power source, and bypass

AC currents from the power supply. This is used in car audio applications, when a stiffening

capacitor compensates for the inductance and resistance of the leads to the lead-acid car battery.

Tuned circuits

Capacitors and inductors are applied together in tuned circuits to select information in particular

frequency bands. For example, radio receivers rely on variable capacitors to tune the station

frequency. Speakers use passive analog crossovers, and analog equalizers use capacitors to select

different audio bands.

In a tuned circuit such as a radio receiver, the frequency selected is a function of the inductance

(L) and the capacitance (C) in series, and is given by:

This is the frequency at which resonance occurs in an LC circuit.

High-voltage

Above and beyond usual hazards associated with working with high voltage, high energy

circuits, there are a number of dangers that are specific to high voltage capacitors. High voltage

capacitors may catastrophically fail when subjected to voltages or currents beyond their rating, or

as they reach their normal end of life. Dielectric or metal interconnection failures may create

arcing within oil-filled units that vaporizes dielectric fluid, resulting in case bulging, rupture, or

even an explosion that disperses flammable oil, starts fires, and damages nearby equipment.

Rigid cased cylindrical glass or plastic cases are more prone to explosive rupture than

rectangular cases due to an inability to easily expand under pressure. Capacitors used in RF or

sustained high current applications can overheat, especially in the center of the capacitor rolls.

The trapped heat may cause rapid interior heating and destruction, even though the outer case

remains relatively cool. Capacitors used within high energy capacitor banks can violently

explode when a fault in one capacitor causes sudden dumping of energy stored in the rest of the

bank into the failing unit. And, high voltage vacuum capacitors can generate soft X-rays even

during normal operation. Proper containment, fusing, and preventative maintenance can help to

minimize these hazards.

High voltage capacitors can benefit from a pre-charge to limit in-rush currents at power-up of

HVDC circuits. This will extend the life of the component and may mitigate high voltage

hazards.

Background

Various types of capacitors. From left: multilayer ceramic, ceramic disc, multilayer polyester

film, tubular ceramic, polystyrene (twice: axial and radial), electrolytic. Major scale divisions are

cm.

Various Capacitors

In October 1745, Ewald Georg von Kleist of Pomerania invented the first recorded capacitor: a

glass jar coated inside and out with metal. The inner coating was connected to a rod that passed

through the lid and ended in a metal sphere. By having this thin layer of glass insulation (a

dielectric) between two large, closely spaced plates, von Kleist found the energy density could be

increased dramatically compared with the situation with no insulator.

In January 1746, before Kleist's discovery became widely known, a Dutch physicist Pieter van

Musschenbroek independently invented a very similar capacitor. It was named the Leyden jar,

after the University of Leyden where van Musschenbroek worked. Daniel Gralath was the first to

combine several jars in parallel into a "battery" to increase the total possible stored charge.

The earliest unit of capacitance was the 'jar', equivalent to about 1 nF.

Early capacitors were also known as condensers, a term that is still occasionally used today. It

was coined by Volta in 1782 (derived from the Italian condensatore), with reference to the

device's ability to store a higher density of electric charge than a normal isolated conductor. Most

non-English languages still use a word derived from "condensatore", like the French

"condensateur", the German, Norwegian, Swedish or Polish "Kondensator", the Portuguese

"condensador" or the Czech "kondenzátor".

7 - SEGMENT DISPLAY

A seven-segment display (abbreviation: "7-seg(ment) display"), less commonly known as a

seven-segment indicator, is a form of display device that is an alternative to the more complex

dot-matrix displays. Seven-segment displays are commonly used in electronics as a method of

displaying decimal numeric feedback on the internal operations of devices.

Concept and visual structure

A typical 7-segment LED display component

A seven segment display, as its name indicates, is composed of seven elements. Individually on

or off, they can be combined to produce simplified representations of the Hindu-Arabic

numerals. Each of the numbers 0, 6, 7 and 9 may be represented by two or more different glyphs

on seven-segment displays.

The seven segments are arranged as a rectangle of two vertical segments on each side with one

horizontal segment on the top and bottom. Additionally, the seventh segment bisects the

rectangle horizontally. There are also fourteen-segment displays and sixteen-segment displays

(for full alphanumerics); however, these have mostly been replaced by dot-matrix displays.

Often the seven segments are arranged in an oblique, or italic, arrangement, which aids

readability.

The segments of a 7-segment display are referred to by the letters A to G, as follows:

where the optional DP decimal point (an "eighth segment") is used for the display of non-integer

numbers.

Implementations

A mechanical seven segment display for displaying petrol prices.

Most separate 7-segment displays use an array of light-emitting diodes (LEDs), though other

types exist using alternative technologies such as cold cathode gas discharge, vacuum

fluorescent, incandescent filament, liquid crystal display (LCD), etc. For gas price totems and

other large signs, electromagnetically flipped light-reflecting segments (sometimes called

"vanes") are still commonly used. An alternative to the 7-segment display in the 1950s through

the 1970s was the vacuum tube-like nixie tube.

In a simple LED package, each LED is typically connected with one terminal to its own pin on

the outside of the package and the other LED terminal connected in common with all other LEDs

in the device and brought out to a shared pin. This shared pin will then make up all of the

cathodes (negative terminals) OR all of the anodes (positive terminals) of the LEDs in the

device; and so will be either a "Common Cathode" or "Common Anode" device depending how

it is constructed. Hence a 7 segment plus DP package will only require nine pins to be present

and connected.

Integrated displays also exist, with single or multiple digits. Some of these integrated displays

incorporate their own internal decoder, though most do not – each individual LED is brought out

to a connecting pin as described.

Seven segment displays can be found in patents as early as 1908 (in U.S. Patent 974,943 , F W

Wood invented an 8-segment display, which displayed the number 4 using a diagonal bar), but

did not achieve widespread use until the advent of LEDs in the 1970s. They are sometimes even

used in unsophisticated displays like cardboard "For sale" signs, where the user either applies

color to pre-printed segments, or (spray)paints color through a seven-segment digit template, to

compose figures such as product prices or telephone numbers.

For many applications, dot-matrix LCDs have largely superseded LED displays, though even in

LCDs 7-segment displays are very common. Unlike LEDs, the shapes of elements in an LCD

panel are arbitrary since they are formed on the display by a kind of printing process. In contrast,

the shapes of LED segments tend to be simple rectangles, reflecting the fact that they have to be

physically moulded to shape, which makes it difficult to form more complex shapes than the

segments of 7-segment displays. However, the high common recognition factor of 7-segment

displays, and the comparatively high visual contrast obtained by such displays relative to dot-

matrix digits, makes seven-segment multiple-digit LCD screens very common on basic

calculators.

Alphabetic display

In addition to the ten numerals, seven segment displays can be used to show letters of the latin,

cyrillic and greek alphabets including punctuation, but only few representations are unambiguous

and intuitive at the same time: uppercase A, B, C, E, F, G, H, I, J, L, N, O, P, S, U, Y, Z, and

lowercase a, b, c, d, e, g, h, i, n, ñ, o, q, r, t, u. Detailed tables of alternative seven-segment

symbols for letters and punctuation are given in the section Character representations, below.

The representation of digits and/or letters with seven-segment displays is not standardized by any

relevant entity (e.g. ISO, IEEE or IEC). Two basic conventions can be seen for digits: one lights

the additional segment in six (a), seven (f) and nine (d), the other, more anglophone one does

not. Military, mission critical, and safety-of-life applications prefer the latter. The idea is to use a

display font such that a single burned out or missing segment in a digit will not display as a

different valid digit.

Ad hoc and corporate solutions dominate the field of alphabetic seven-segment display, which is

usually not considered essential and only used for basic notifications.

Using a restricted range of letters that look like (upside-down) digits, seven-segment displays are

commonly used by school children to form words and phrases using a technique known as

"calculator spelling".

DigitsDigits and and punctuationpunctuation

0

1

2

3

4

5

6

7

8

9

 

 

  Figure:  The 7-segment Display 

 

 

ANALOG TO DIGITAL CONVERTER

Although manufacturers use common terms to describe analog-to-digital converters (ADCs), the

way ADC makers specify the performance of ADCs in data sheets can be confusing, especially

for a newcomers. But to select the correct ADC for an application, it's essential to understand the

specifications. This guide will help engineers to better understand the specifications commonly

posted in manufacturers' data sheets that describe the performance of successive approximation

register (SAR) ADCs.

ADCs convert an analog signal input to a digital output code. ADC measurements deviate from

the ideal due to variations in the manufacturing process common to all integrated circuits (ICs)

and through various sources of inaccuracy in the analog-to-digital conversion process. The ADC

performance specifications will quantify the errors that are caused by the ADC itself.

ADC performance specifications are generally categorized in two ways: DC accuracy and

dynamic performance. Most applications use ADCs to measure a relatively static, DC-like signal

(for example, a temperature sensor or strain-gauge voltage) or a dynamic signal (such as

processing of a voice signal or tone detection). The application determines which specifications

the designer will consider the most important.

Although not a specification, one of the fundamental errors in ADC measurement is a result of

the data-conversion process itself: quantization error. This error cannot be avoided in ADC

measurements. DC accuracy, and resulting absolute error are determined by four specs—offset,

full-scale/gain error, INL, and DNL. Quantization error is an artifact of representing an analog

signal with a digital number (in other words, an artifact of analog-to-digital conversion).

Maximum quantization error is determined by the resolution of the measurement (resolution of

the ADC, or measurement if signal is over sampled). Further, quantization error will appear as

noise, referred to as quantization noise in the dynamic analysis. For example, quantization error

will appear as the noise floor in an FFT plot of a measured signal input to an ADC, which I'll

discuss later in the dynamic performance section).

The ADC0808, ADC0809 data acquisition component is a monolithic CMOS device with an 8-

bit analog-to-digital converter, 8-channel multiplexer and microprocessor compatible control

logic. The 8-bit A/D converter uses successive approximation as the conversion technique. The

converter features a high impedance chopper stabilized comparator, a 256R voltage divider with

analog switch tree and a successive approximation register. The 8-channel multiplexer can

directly access any of 8-single-ended analog signals.

The device eliminates the need for external zero and full-scale adjustments. Easy interfacing to

microprocessors is provided by the latched and decoded multiplexer address inputs and latched

TTL TRI-STATE outputs.

The design of the ADC0808, ADC0809 has been optimized by incorporating the most desirable

aspects of several A/D conversion techniques. The ADC0808, ADC0809 offers high speed, high

accuracy, minimal temperature dependence, excellent long-term accuracy and repeatability, and

consumes minimal power.

The analog signal is input from pins 1-5 and 26-28. Pin no. 6 and 7 are connected together so

that start of conversion gets triggered by end of conversion. Output enable is tied to Vcc and

thresholds are set at 0 and 5V. The clock is provided by using a 555 timer IC.

BASICS OF MICROCONTROLLER 89C51

Background

Modern semiconductor technology has evolved to such an extent that many analog and digital

circuits are available in the form of integrated circuits (IC). Manufacturers of these packages

provide application notes, which can be effectively used to implement circuit functions.

Knowledge of the characteristics and operation of devices within the IC packages is essential,

however, to understand the limitations of these ICs when they are interfaced as building blocks

in circuit designs.

The first IC was developed in the year 1958 simultaneously by Kilby at Texas Instruments USA

and Noyce and Moore at Fairchild Semiconductor, USA, marking the beginning of a new phase

in microelectronics revolution. This invention was followed by development of first commercial

IC operational amplifier, the A709, by Fairchild Semiconductor in the year 1968, the 4004

microprocessor by Intel in the year 1971, the 8-bit microprocessor by Intel in the year 1972 and

the gigabit memory chip by Intel in the year 1995, respectively.

Microprocessor is one of the most interesting technological developments among the

semiconductor devices in recent times. It has had a tremendous impact on industrial control and

instrumentation due to its high reliability and flexibility at the design and implementation stages.

A microprocessor(P) is an IC having a large number of logical circuits inbuilt on a single chip.

A microprocessor has tremendous monitoring and control capabilities. Initially its application

was limited to manufacturing electronic controllers and computers only. But gradually, it has

also entered other fields such as domestic appliances, musical instruments, medical electronics

etc. A P basically serves as the central processing unit of a computer. A P when interfaced

with an input/output device alongwith a memory gives rise to a microcomputer.

A device similar to a microprocessor is the microcontroller(C). In the year 1976 Intel

introduced the 8748, the first device in the MCS-48 family of microcontrollers.

Distinguishing feature of the microcontroller chip is the inclusion, on a single chip, of all the

resources, which permit the IC to be used as a dedicated controller in a system or an instrument.

In a typical industrial controller using a standard P such as the Intel 8085, the following

peripheral chips are essential to form a complete system.

RAM

ROM

Timer

Parallel input/output

UART

Apparently a small system no longer remains small, and the microcontroller that has all or most

of these built-in facilities within the chip itself has significant advantages, when used for

building stand-alone front ends and controllers.

The applications of microcontrollers are in washing machines, alarm clocks, thermostat, VCRs,

stereo equipment, musical instruments etc. The microcontroller offers the following advantages:

Significant reduction in controller hardware and cost.

Increased reliability

Universal hardware

Sophisticated control possible

The standard controllers widely employed for many applications are the 8051 series

microcontrollers. The one used in this application is the 8951.

Comparison between Microprocessors and Microcontrollers

Microprocessors differ from microcontrollers in their hardware architecture, applications and

instruction set features.

(a) Hardware Architecture

Whereas a microprocessor is a single chip CPU, the microcontroller contains in a single chip, a

CPU and much of the remaining circuitry of a complete microcomputer system. An important

feature of the microcontrollers is the built-in interrupt system. As control oriented devices,

microcontrollers are often called upon to respond to external stimuli (interrupts) in real time.

(b) Applications

Microprocessors are designed for and most commonly used as the CPU in microcomputer

systems. Microcontrollers, however, are found in small, minimum component designs

performing control-oriented activities. A C can aid in reducing the overall component count.

All that is required is a C, a small number of support components and a control program in

ROM. Microcontrollers are suited for control of I/O devices in designs requiring a minimum

component count, whereas microprocessors are suited to processing information in computer

systems.

(c) Instruction Set Features

Microprocessor instructions are processing intensive implying they have powerful addressing

modes with instructions catering to operations on large volumes of data. Their instructions

operate on nibbles, bytes, words or even double words. Addressing modes provide access to

large arrays of data, using address pointers and offsets. Microcontrollers, on the other hand, have

instruction set catering to the control of inputs and outputs. Microcontrollers have instructions to

set and clear individual bits and perform other bit-oriented operations such as logically ANDing,

ORing, or EXORing bits, jumping etc.

Introduction to 89C51

The AT89C51 is a low-power, high-performance CMOS 8-bit microcomputer with 4K bytes of

Flash Programmable and Erasable Read Only Memory (PEROM). The device is manufactured

using Atmel's high-density nonvolatile memory technology and is compatible with the industry

standard MCS-51(TM) instruction set and pinout. The on-chip Flash allows the program memory

to be reprogrammed in-system or by a conventional nonvolatile memory programmer. By

combining a versatile 8-bit CPU with Flash on a monolithic chip, the Atmel AT89C51 is a

powerful microcomputer, which provides a highly flexible and cost effective solution to many

embedded control applications.

Features

Hardware features

Compatible with MCS-51(TM) Products

4K Bytes of In-System Reprogrammable Flash Memory

Endurance: 1,000 Write/Erase Cycles

Fully Static Operation: 0 Hz to 24 MHz

Three-Level Program Memory Lock

128 x 8-Bit Internal RAM

32 Programmable I/O Lines

Two 16-Bit Timer/Counters

Six Interrupt Sources

Programmable Serial Channel

Low Power Idle and Power Down Modes

Software features

Bit manipulation

Single instruction multiplication

Separate program and data memory

Four banks of eight temporary registers

Direct, indirect, register and relative addressing

The AT89C51 provides the following standard features: 4K bytes of Flash, 128 bytes of RAM,

32 I/O lines, two 16-bit timer/counters, a five vector two-level interrupt architecture, a full

duplex serial port, on-chip oscillator and clock circuitry. In addition, the AT89C51 is designed

with static logic for operation down to zero frequency and supports two software selectable

power saving modes. The Idle Mode stops the CPU while allowing the RAM, timer/counters,

serial port and interrupt system to continue functioning. The Power Down Mode saves the RAM

contents but freezes the oscillator, disabling all other chip functions until the next hardware reset.

Pin Description

VCC

Supply voltage.

GND

Ground.

Port 0

Port 0 is an 8-bit open drain bidirectional I/O port. As an output port each pin can sink eight TTL

inputs. When 1s are written to port 0 pins, the pins can be used as high-impedance inputs. Port 0

may also be configured to be the multiplexed low-order address/data bus during accesses to

external program and data memory. In this mode P0 has internal pullups. Port 0 also receives the

code bytes during Flash programming, and outputs the code bytes during program verification.

External pull-ups are required during program verification.

Port 1

Port 1 is an 8-bit bidirectional I/O port with internal pullups. The Port 1 output buffers can

sink/source four TTL inputs. When 1s are written to Port 1 pins they are pulled high by the

internal pullups and can be used as inputs. As inputs, Port 1 pins that are externally being pulled

low will source current (IIL) because of the internal pull-ups. Port 1 also receives the low-order

address bytes during Flash programming and verification.

Port 2

Port 2 is an 8-bit bi-directional I/O port with internal pull-ups. The Port 2 output buffers can

sink/source four TTL inputs. When 1s are written to Port 2 pins they are pulled high by the

internal pull-ups and can be used as inputs. As inputs, Port 2 pins that are externally being pulled

low will source current (IIL) because of the internal pull-ups. Port 2 emits the high-order address

byte during fetches from external program memory and during accesses to external data memory

that use 16-bit addresses (MOVX @ DPTR). In this application it uses strong internal pullups

when emitting 1s. During accesses to external data memory that use 8-bit addresses (MOVX @

RI), Port 2 emits the contents of the P2 Special Function Register. Port 2 also receives the high-

order address bits and some control signals during Flash programming and verification.

Port 3

Port 3 is an 8-bit bidirectional I/O port with internal pullups. The Port 3 output buffers can

sink/source four TTL inputs. When 1s are written to Port 3 pins they are pulled high by the

internal pullups and can be used as inputs. As inputs, Port 3 pins that are externally being pulled

low will source current (IIL) because of the pull-ups. Port 3 also serves the functions of various

special features of the AT89C51 as listed below.

Port Pin Alternate Functions

P3.0 RXD (serial input port)

P3.1 TXD (serial output port)

P3.2 INT0 (external interrupt 0)

P3.3 INT1 (external interrupt 1)

P3.4 T0 (timer 0 external input)

P3.5 T1 (timer 1 external input)

P3.6 WR (external data memory write strobe)

P3.7 RD (external data memory read strobe)

Port 3 also receives some control signals for Flash programming and verification.

RST

This pin is for Reset input. A high on this pin for two machine cycles while the oscillator is

running resets the device. The simple RC circuit used will supply system voltage VCC to the reset

pin until the capacitor begins to charge. At a threshold of approximately 2.5V, the reset input

reaches a low level and the system begins to run. Internal reset circuitry has hysteresis

necessitated by the slow fall time of RC circuit. The addition of a reset button enables the user to

reset the system without having to turn the power off/on.

ALE/PROG

Address Latch Enable output pulse for latching the low byte of the address during accesses to

external memory. This pin is also the program pulse input (PROG) during Flash programming.

In normal operation ALE is emitted at a constant rate of 1/6 the oscillator frequency, and may be

used for external timing or clocking purposes. Note, however, that one ALE pulse is skipped

during each access to external Data Memory. If desired, ALE operation can be disabled by

setting bit 0 of SFR location 8EH. With the bit set, ALE is active only during a MOVX or

MOVC instruction. Otherwise, the pin is weakly pulled high. Setting the ALE-disable bit has no

effect if the microcontroller is in external execution mode.

PSEN

Program Store Enable is the read strobe to external program memory. When the AT89C51 is

executing code from external program memory, PSEN is activated twice each machine cycle,

except that two PSEN activations are skipped during each access to external data memory.

EA/VPP

External Access Enable. EA must be strapped to GND in order to enable the device to fetch code

from external program memory locations starting at 0000H up to FFFFH. Note, however, that if

lock bit 1 is programmed, EA will be internally latched on reset. EA should be strapped to VCC

for internal program executions. This pin also receives the 12-volt programming enable voltage

(VPP) during Flash programming, for parts that require 12-volt VPP.

XTAL1

Input to the inverting oscillator amplifier and input to the internal clock operating circuit.

XTAL2

Output from the inverting oscillator amplifier.

Oscillator Characteristics

XTAL1 and XTAL2 are the input and output, respectively, of an inverting amplifier, which can

be configured for use as an on-chip oscillator. Either a quartz crystal or ceramic resonator may

be used. To drive the device from an external clock source, XTAL2 should be left unconnected

while XTAL1 is driven. There are no requirements on the duty cycle of the external clock signal,

since the input to the internal clocking circuitry is through a divide-by-two flip-flop, but

minimum and maximum voltage high and low time specifications must be observed.

Idle Mode

In idle mode, the CPU puts itself to sleep while all the on-chip peripherals remain active. The

mode is invoked by software. The content of the on-chip RAM and all the special function

registers remain unchanged during this mode. The idle mode can be terminated by any enabled

interrupt or by a hardware reset. It should be noted that when idle is terminated by a hard ware

reset, the device normally resumes program execution, from where it left off, up to two machine

cycles before the internal reset algorithm takes control. On-chip hardware inhibits access to

internal RAM in this event, but access to the port pins is not inhibited. To eliminate the

possibility of an unexpected write to a port pin when Idle is terminated by reset, the instruction

following the one that invokes Idle should not be one that writes to a port pin or to external

memory.

Power Down Mode

In the power down mode the oscillator is stopped, and the instruction that invokes power down

mode is the last instruction executed. The on-chip RAM and special function registers retain

their values until the power down mode is terminated. The only exit from power down is a

hardware reset. Reset redefines the SFRs but does not change the on-chip RAM. The reset

should not be activated before Vcc is restored to its normal operating level and must be held

active long enough to allow the oscillator to restart and stabilize.

Microcontroller Hardware

The 89C51 Oscillator and Clock

The heart of the microcontroller is the circuit that generates the clock that generates the clock

pulses by which all internal operations are synchronized. Pins XTAL1 and XTAL2 are provided

for connecting a resonant network to form an oscillator. A quartz crystal and capacitors are

employed. The crystal frequency is the basic internal clock frequency of the microcontroller.

There are various designs which can run at specified maximum or minimum frequencies.

Minimum frequencies imply that some internal memories are dynamic and must always operate

above a minimum frequency or data will be lost. The 89C51 has limits 3-24 MHz. In the present

application 89C51 is running at 6MHz.

The clock frequency (f) establishes the smallest interval of time within the microcontroller,

called the pulse, p, time. The smallest interval of time to accomplish any simple instruction, or

part of a complex instruction, however, is the machine cycle. The machine cycle is itself made of

6 states. A state is the basic time interval for discrete operations of the microcontroller such as

fetching an opcode byte, decoding an opcode, executing an opcode, or writing a data byte. Two

oscillator pulses define each state.i.e. one machine cycle is made up of 12 oscillator pulses.

Program instructions may require one, two or four machine cycles to be executed, depending

upon the type of the instruction. Instructions are fetched and executed by the microcontroller

automatically, beginning with the instruction located at ROM memory address 0000h at the time

the microcontroller is first reset.

To calculate the time of execution of any particular instruction, find the number of machine

cycles, C and then multiply by 12 and divide the product by crystal frequency:

Tinst = C 12d (Eq. 3.1)

crystal frequency

Program Counter and Data Pointer

The program counter (PC) and data pointer (DPTR) are two 16-bit registers. Each is used to hold

the address of a byte in memory.

Program instruction bytes are fetched from locations in memory that are addressed by PC.

Program ROM is on the chip at addresses 0000h - 0FFFh. The PC is automatically incremented

after every instruction byte is fetched and is also altered by certain instructions. The PC is the

only register that does not have an internal address.

The DPTR register is made up of two 8-bit registers, named DPH and DPL, which are used to

furnish memory addresses for internal and external data access. The DPTR is under the control

of program instructions and can be specified by its 16-bit name, DPTR, or by each individual

byte name, DPH and DPL. DPTR does not have a single internal address; DPH and DPL are

assigned an address each.

A and B CPU Registers

The 89C51 contains 34 general-purpose or working registers. Two of these, registers A and B

hold results of many instructions, particularly math and logical operations, of the central

processing unit. The other 32 are arranged as a part of internal RAM in four banks of eight

registers each and comprise the mathematical core.

The A (accumulator) register is the most versatile of the two CPU registers and is used for many

operations, including addition, subtraction, integer multiplication and division, and Boolean bit

manipulations. The A register is also used for all data transfers between the microcontroller and

any external memory. The B register is used with the A register for multiplication and division

operations and has no other function than as a location where data may be stored.

Flags and the Program Status Word (PSW)

Flags are 1-bit registers provided to store the results of certain program instructions. Other

instructions can test the condition of the flags and make decisions based on flag states. In order

that the flags may be conveniently addressed, they are grouped inside the program status word

(PSW) and the power control (PCON) registers. The program status word is shown in Table 3.1.

The PSW contains the math flag, user program flag F0, and the register select bit that identify

which of the four general purpose register banks is currently in use by the program. The

remaining two user flags, GF0 and GF1, are stored in PCON.

Internal Memory

A functioning computer must have memory for program code bytes, commonly in ROM, and

RAM memory for variable data that can be altered as the program runs. The microcontroller has

internal RAM and ROM memory for these functions.

Internal RAM

The 128-byte internal RAM is organized into three distinct areas:

1. Thirty-two bytes from address 00h to 1Fh that make up 32 working registers organized as four

banks of eight registers each. The four register banks are numbered 0 to 3 and are made up of

eight registers named R0 to R7. Each register can be addressed by name (when its bank is

selected) or by its RAM address. Thus R0 of bank 3 is R0 {if bank 3 is currently selected or

address 18h (whether bank 3 is selected or not. Bits RS0 and RS1 in the PSW determine which

bank of registers is currently in use at any time when the program is running. Register banks not

selected can be used as general purpose RAM. Bank 0 is selected on reset.

2. A bit addressable area of 16 bytes occupies RAM byte addresses 20h to 2Fh, forming a total of

128 addressable bits. An addressable bit may he specified by its bit address of 00h to 7Fh, or 8

bits may form any byte address from 20h to 2Fh. Thus, for example, bit address 4Fh is also bit 7

of byte address 29h. Addressable bits are useful when the program need only remember a binary

event (switch on, light off, etc.).

3. A general-purpose RAM area above the bit area, from 30h to 7Fh, addressable as bytes.

The Stack and the Stack Pointer

The stack refers to an area of internal RAM that is used in conjunction with certain op-codes to

store and retrieve data quickly. The 8-bit Stack Pointer (SP) register is used by the

microcontroller to hold an internal RAM address that is called the top of the stack. The address

held in the SP register is the location in internal RAM where the last byte of data was stored by a

stack operation.

When data is to be placed on the stack, the SP increments before storing data on the stack so that

the stack grows up as data is stored. As data is retrieved from the stack, the byte is read from the

stack, and then the SP decrements to point to the next available byte of stored data.

Special Function Registers

The microcontroller operations that do not use the internal 128-byte RAM addresses from 00h to

7Fh are done by a group of specific internal registers, each called a special function register

(SFR), which may be addressed much like internal RAM, using addresses from 80h to FFh. The

PC is not a part of the SFR and has no internal RAM address.

Internal ROM

The microcontroller is organized so that data memory and program code memory can be in two

entirely different physical memory entities. Each has the same address ranges.

The structure of the internal RAM has been discussed previously. A corresponding block of

internal program code, contained in an internal ROM, occupies code address space 0000h to

0FFFh. The PC is ordinarily used to address program code bytes from addresses 0000h to

FFFFh. Program addresses higher than 0FFFh, which exceed the internal ROM capacity, will

cause the microcontroller to automatically fetch code bytes from external program memory.

Code bytes can also be fetched exclusively from an external memory, addresses 0000h to FFFFh,

by connecting the external access pin (EA pin 31 on the DIP) to ground. The PC does not care

where the code is; the circuit designer decides whether the code is found totally in internal ROM,

totally in external ROM, or in a combination of internal and external ROM.

The block diagram of 89C51 is shown on the next page.

Block Diagram of 89C51

PORT 2 DRIVERS

PORT 3 DRIVERS

RAM ADDR. REGISTER

RAM PORT 0 LATCH

FLASHPORT 2 LATCH

B ACC

PORT 0 DRIVERS

TMP2 TMP1

ALU

PORT 1 PORT 3

STACK POINTER

PSW

INTERRUPT, SERIAL PORT, TIMER BLOCKS

TIMING AND

CONTROL

INSTRUCTION REGISTER

PORT 1 DRIVERS

PROGRAM ADDRESS REGISTER

BUFFER

PC INCREMENTER

PROGRAM COUNTER

DPTR

OSC

VCC

GND

SOLDERING

While assembling the PCB the smallest component has to be assembled first.

Hold the soldering iron in such a way that it makes 45 degree angle with the PCB and it should

be touching the component lead and the track of the PCB, Hold it for approximately 5 seconds so

that the sufficient heat is given to the lead and the track. Now bring the solder wire close to it and

solder it.

PRECAUTIONS

1. The tip of the soldering iron is very hot therefore proper care should be taken when handling

the iron.

2. The direction of the notch of the base IC should be taken care of.

3. Make sure that no two isolated paths should be short circuited during soldering.

P1.0 - P 1.7 P 3.0 - P 3.7

Soldering is the process in which two metals are joined together by means of a third metal or

alloy having a relatively low melting point. Soft soldering is characterized by the value of the

melting point of the third metal or alloy, which is below 450°C (842°F).[1] The third metal or

alloy used in the process is called solder.

Soldering is distinguished from brazing by use of a lower melting-temperature filler metal; it is

distinguished from welding since the base metal is not melted during the joining process. In a

soldering process, heat is applied to the parts to be joined, causing the solder to melt and be

drawn into the joint by capillary action and to bond to the materials to be joined by wetting

action. After the metal cools, the resulting joints are not as strong as the base metal, but have

adequate strength, electrical conductivity, and water-tightness for many uses. Soldering is an

ancient technique that has been used practically as long as humans have been making items out

of metal

Soldering filler materials are available in many different alloys for differing applications. In

electronics assembly, the eutectic alloy of 63% tin and 37% lead (or 60/40, which is almost

identical in performance to the eutectic) has been the alloy of choice. Other alloys are used for

plumbing, mechanical assembly, and other applications.

A eutectic formulation has several advantages for soldering; chief among these is the

coincidence of the liquidus and solidus temperatures, i.e. the absence of a plastic phase. This

allows for quicker wetting out as the solder heats up, and quicker setup as the solder cools. A

non-eutectic formulation must remain still as the temperature drops through the liquidus and

solidus temperatures. Any differential movement during the plastic phase may result in cracks,

giving an unreliable joint. Additionally, a eutectic formulation has the lowest possible melting

point, which minimizes heat stress on electronic components during the soldering process.

For environmental reasons, 'no-lead' solders are becoming more widely used. Unfortunately most

'no-lead' solders are not eutectic formulations, making it more difficult to create reliable joints

with them. See complete discussion below; see also RoHS.

Other common solders include low-temperature formulations (often containing bismuth), which

are often used to join previously-soldered assemblies without un-soldering earlier connections,

and high-temperature formulations (usually containing silver) which are used for high-

temperature operation or for first assembly of items which must not become unsoldered during

subsequent operations. Specialty alloys are available with properties such as higher strength,

better electrical conductivity and higher corrosion resistance.

In high-temperature metal joining processes (welding, brazing and soldering), the primary

purpose of flux is to prevent oxidation of the base and filler materials. Tin-lead solder, for

example, attaches very well to copper, but poorly to the various oxides of copper, which form

quickly at soldering temperatures. Flux is a substance which is nearly inert at room temperature,

but which becomes strongly reducing at elevated temperatures, preventing the formation of metal

oxides. Secondarily, flux acts as a wetting agent in the soldering process, reducing the surface

tension of the molten solder and causing it to better wet out the parts to be joined.

Fluxes currently available include water-soluble fluxes (no VOC's required for removal) and 'no-

clean' fluxes which are mild enough to not require removal at all. Performance of the flux needs

to be carefully evaluated -- a very mild 'no-clean' flux might be perfectly acceptable for

production equipment, but not give adequate performance for a poorly-controlled hand-soldering

operation.

Traditional rosin fluxes are available in non-activated (R), mildly activated (RMA) and activated

(RA) formulations. RA and RMA fluxes contain rosin combined with an activating agent,

typically an acid, which increases the wettability of metals to which it is applied by removing

existing oxides. The residue resulting from the use of RA flux is corrosive and must be cleaned

off the piece being soldered. RMA flux is formulated to result in a residue which is not

significantly corrosive, with cleaning being preferred but optional.

The heat source tool should be selected to provide adequate heat for the size of joint to be

completed. A soldering gun at 100 watts output may provide too much heat for printed circuit

boards, while a 23 watt iron may not provide enough heat for joining copper roof flashing or

large stained-glass lead came. Using a tool with too high a temperature can damage sensitive

components, but a tool that is too cool can cause an extended heating-up period which can also

cause extensive damage.

For attachment of electronic components to a PCB, proper selection and use of flux is the best

way to ensure that all solder pads and device terminals remain clean and oxide-free, which is

essential for good wetting and heat transfer. The soldering iron tip must be clean and pre-tinned

with solder to ensure rapid heat transfer. The devices must be mounted on the circuit board

properly. Components which dissipate large amounts of heat during operation are sometimes

elevated above the PCB a few millimeters to allow proper cooling. After inserting a through-hole

mounted component, the excess leads can be cut leaving only a length equal to the radius of the

pad. Plastic mounting clips or holders may be used with large devices to reduce mounting

stresses.

A heat sink may be used on the leads of sensitive components to reduce heat transfer to the

component (notice that the heat sink will require the use of MORE heat to complete the joint!). If

ALL copper surfaces are not properly fluxed and brought above the melting temperature of the

solder in use, the result will be an unreliable 'cold soldered' joint. To achieve a properly heated

joint, the soldering iron and the solder must be applied separately to the surfaces to be joined,

rather than the iron being applied directly to the solder. When the surfaces are adequately heated

up, the solder will melt and flow into the joint. The solder supply is then removed from the joint,

followed by the heat source. Even distribution of solder throughout the joint gives a

mechanically sound and electrically conductive joint. Since non-eutectic solder alloys have a

small plastic range, the joint must not be disturbed until the solder has cooled down through both

the liquidus and solidus temperatures. Visually, a good solder joint will appear smooth and

shiny, with the outline of the soldered wire clearly visible. Too little solder will result in a dry

and unreliable joint; too much solder (the 'solder blob' very familiar to beginners) is not

necessarily unsound, but will not allow proper inspection of the joint. Depending on the flux

used, any flux residue remaining on the joint may need to be removed, using water, alcohol or

other solvents compatible with the process. Excess solder and unconsumed flux and residues

must be wiped from the soldering iron tip between joints, but the tip of the iron must be kept

wetted with solder ("tinned") constantly when hot to prevent oxidation and vaporization of the

tip itself.

Environmental legislation in many countries, and the whole of the European Community area,

have led to a change in formulation of both solders and fluxes. Water soluble non-rosin based

fluxes have been increasingly used since the 1980's so that soldered boards can be cleaned with

water or water based cleaners. This eliminates hazardous solvents from the production

environment, and effluent.

Various problems may arise in the soldering process which lead to joints which are non-

functional either immediately or after a period of use. The most common defect when hand-

soldering results from the parts being joined not exceeding the solder's liquidus temperature,

resulting in a "cold solder" joint. An improperly selected or applied flux can cause joint failure,

or if not properly cleaned off the joint, may corrode the metals in the joint over time and cause

eventual joint failure. Without flux the joint may not be clean, or may be oxidized, resulting in an

unsound joint. Movement of metals being soldered before the solder has cooled will cause a

highly unreliable cracked joint

References

1. British Standards Institute BS ISO/TR 3666:1998 Viscosity of water

2. British Standards Institute BS 188:1977 Methods for Determination of the viscosity of

liquids

3. ASTM International (ASTM D7042)

4. http://www.pra.org.uk/viscosityoils/notes-units.htm