CONSTANT VOLTAGE CONSTANT CURRENT DC POWER SUPPLY

CV_CC DC POWERSUPPLY

CHAPTER 1

INTRODUCTION OF POWER ELECTRONICS

Definition:

A device for the conversion of available power of one set of characteristics to

another set of characteristics to meet specified requirements.stypical application

of power supplies include to convert raw input power to a controlled or stabilised

voltage for operation of electrical equipment.

Power electronics is the application of solid-state electronics for the control

and conversion of electricp ower. It also refers to a subject of research in

electrical engineering which deals with design, control, computation and

integration of nonlinear, time varying energy processing electronic systems with

fast dynamics.

The first high power electronic devices were mercury-arc valves. In modern

systems the conversion is performed with semiconductor switching devices such

as diodes, thyristors and transistors, as pioneered by R. D. Middle brook and

others beginning in the 1950s. In contrast to electronic systems concerned with

transmission and processing of signals and data, in power electronics substantial

amounts of electrical energy are processed. An AC/DC converter (rectifier) is the

most typical power electronics device found in many consumer electronic

devices, e.g. television sets, personal computers, battery chargers, etc. The power

range is typically from tens of watts to several hundred watts. In industry a

common application is the variable speed drive (VSD) that is used to control

an induction motor. The power range of VSDs starts from a few hundred watts

and end at tens of megawatts.

The power conversion systems can be classified according to the type of the

input and output power

AC to DC (rectifier)

DC to AC (inverter)

DC to DC (DC-to-DC converter)

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AC to AC (AC-to-AC converter)

History of power electronics

Power electronics started with the development of mercury arc rectifier. Invented

by Peter Cooper Hewitt in 1902, the mercury arc rectifier was used to convert

alternating current (AC) into direct current (DC). From the 1920s on, research

continued on applying thyratrons and grid-controlled mercury arc valves to power

transmission. It was not until Uno Lamm developed a valve with grading

electrodes that mercury valves were usable for high voltage direct

current transmission. In 1933 selenium rectifiers were invented.

In 1947 the bipolar point-contact transistor was invented by Walter H.

Brattain and John Bardeen under the direction of William Shockley at the Bell

Telephone Laboratory. In 1948 the invention of the bipolar junction transistor by

Shockley improved the stability and performance of transistors, and reduced the

cost of manufacture. By the 1950s, semiconductor power diodes became available

and started replacing vacuum tubes. Then in 1956 the Silicon Controlled

Rectifier (SCR) was introduced by General Electric, greatly increasing the range

of power electronics applications.

In the 1960s the switching speed of BJTs allowed for high frequency DC/DC

converters. In 1976 power MOSFET becomes commercially available. In 1982

the Insulated Gate Bipolar Transistor (IGBT) was introduced.

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1.1 Basics of Power Supplies

A regulated power supply provides electrical energy which is precisely

controlled. Power supplies can be of the type Constant-Voltage, Constant-

Current, and the Constant-Voltage/Constant-Current sources.

A Constant-Voltage (CV) supply provides a DC voltage that can be set to any

desired value over a specified range. An ideal constant -voltage supply has

zero output impedance, as illustrated in Figure 1a. On the other hand, a

constant-current (CC) supply gives a regulated current independent of the

voltage over the load (up to the maximum allowable voltage), as shown in

Figure 1b. 

FIG1.1: OUTPUT CHARACTERISTIC OF A CONSTANT-VOLTAGE (A) AND CONSTANT-CURRENT (B) SUPPLY.

A more versatile power supply is the Constant-Voltage/Constant-

Current supply which can be used to provide either a constant voltage or a

constant current. Figure 1.2 illustrates the I-V characteristic of such a supply.

The values Es and is selected by the operator from the front panel or

programmed through the GPIB interface.

FIG1.2: OUTPUT CHARACTERISTIC OF A CONSTANT-VOLTAGE/ CONSTANT-CURRENT SUPPLY.

Let’s look at the operating modes of such a CV/CC power supply.

Assume that one connects a resistive load to the power supply as shown in above

fig. The supply has been set at a voltage V=Vs and current I=is (see later on how

to do this). The current through the resistor is then given by Ohm's law: I=V/R.

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As long as the current is below the maximum value Is, the voltage over the

resistor will be constant and equal to Vs. The power supply operates thus in the

CV mode as shown in Figure 3. However, if one decreases the resistance such

that the current exceeds the maximum allowable value is, the current will be

limited to be and the power supply operates in the CC mode. The resistance

RC=Vs/is is called the critical resistance and determines whether one operates in

the CV (RL>RC) or CC (RL>RC) mode.

1.2 How to switch between Constant Voltage (CV) and

Constant Current (CC) mode on a DC power supply

This article primarily applies to DC power supplies that can switch

between constant voltage (CV) and constant current (CC) mode automatically. 

Many of B&K Precision's DC power supplies are categorized as this type of

power supply, and depending on the application it is important and useful to

know how to switch between the two modes.

Upon powering a DC power supply of this type, with nothing connected the

default mode should be in CV mode.  This means that the power supply will

have voltage control of the output power.  These power supplies also allow you

to set a current value, which is often referred to as the current limit.   This

setting can either be entered in through the supply's interface, or in some cases

users must short the positive and negative terminals to adjust the current limit

value.

The current limit value is the crossover point between CV and CC mode. 

Meaning, if a load connected to the power supply draws or needs to draw more

current than the limit, the supply will automatically go into CC module.

For example, suppose the power supply is set to 5 V and current limit is set to

1 A.  When the load connected draws current less than 1 A, the power supply

will maintain voltage control in CV mode.  If the load draws current at 1 A, the

supply will automatically switch to CC mode.  In these cases, the current will

output at 1 A or at the current limit, while the voltage will decrease down to a

value that's dependent on the load.  Note that the voltage drop is always

dependent on the load.  Ohm's law (V = IR) must always be satisfied.  For

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example, if the load is a 5 ohms resistor, than the power supply should be able

to supply 5V and 1 A since 5V/1A = 5 ohms according to the formula. (Note: 

the power supply may fluctuate between CV and CC mode since this would hit

the current limit).  Now, if the resistor was 2 ohms instead, the power supply

will switch to CC mode, and the voltage will no longer be 5 V while

maintaining 1 A current.  It will instead be approximately 2 V since 1A x 2

ohms = 2 V.

In short, to switch between CV and CC mode, users must configure a current

limit at a value in which they want to run CC mode.  Then, based on the load

connected, the supply will automatically switch to CC from CV when current

limit condition is satisfied.

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CHAPTER 2

SPECIFICATIONS

1) BLANK PCB -1

2) Diodes IN 4007 - 4

3) Capacitor 1000/25 - 1

4) Transistors SL100 - 1

BC547 - 2

5) Zener Diode 4.7 V Z - 1

6) Resistor 1K - 3

7) Resistor 100 E – 1

8) Potentiometer 10 K POT – 1

9) Step down Transformer (18V)

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CHAPTER 3

SYSTEM REQUIREMENTS

3.1 TRANSFORMER

A transformer is a static electrical device that transfers energy by inductive

coupling between its winding circuits. A varying current in the primary winding

creates a varying magnetic flux in the transformer's core and thus a varying

magnetic flux through the secondary winding. This varying magnetic

flux induces a varying electromotive force (EMF), or "voltage", in the secondary

winding.

Transformers range in size from thumbnail-sized units hidden

inside microphones  to units weighing hundreds of tons used in power

grid applications. A wide range of transformer designs are used in electronic

and electric power applications. Transformers are essential for the transmission,

distribution, and utilization of electric power.

3.1.1 Basic principles

The transformer is based on two principles: first, that an electric current can

produce a magnetic field (electromagnetism) and second that a changing

magnetic field within a coil of wire induces a voltage across the ends of the

coil (electromagnetic induction). Changing the current in the primary coil

changes the magnetic flux that is developed. The changing magnetic flux

induces a voltage in the secondary coil.

An ideal transformer is shown in the figure below. Current passing through the

primary coil creates a magnetic field. The primary and secondary coils are

wrapped around a core of very high magnetic permeability, such as iron, so

that most of the magnetic flux passes through both the primary and secondary

coils. If a load is connected to the secondary winding, the load current and

voltage will be in the directions indicated, given the primary current and

voltage in the directions indicated (each will be alternating current in practice).

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3.1.2 Induction law

FIG 3.1: TRANSFORMER

An ideal voltage step-down transformer. The secondary current arises from

the action of the secondary EMF on the (not shown) load impedance.

The voltage induced across the secondary coil may be calculated

from Faraday's law of induction, which states that:

Where Vs is the instantaneous voltage, Ns are the number of turns in the secondary

coil and Φ is the magnetic flux through one turn of the coil. If the turns of the coil

are oriented perpendicularly to the magnetic field lines, the flux is the product of

the magnetic flux density B and the area A through which it cuts. The area is

constant, being equal to the cross-sectional area of the transformer core, whereas the

magnetic field varies with time according to the excitation of the primary. Since the

same magnetic flux passes through both the primary and secondary coils in an ideal

transformer, the instantaneous voltage across the primary winding equals

Taking the ratio of the two equations for Vs and VP gives the basic equation for

stepping up or stepping down the voltage

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Nap/Ns are known as the turn’s ratio, and are the primary functional

characteristic of any transformer. In the case of step-up transformers, this may

sometimes be stated as the reciprocal, Ns/Nap. Turns ratio is commonly

expressed as an irreducible or ratio: for example, a transformer with primary

and secondary windings of, respectively, 100 and 150 turns is said to have a

turns ratio of 2:3 rather than 0.667 or 100:150.

3.1.3 Types of transformer

There are two types of transformers: Step up transformer

Step down transformer

A transformer in which Ns>Nap is called a step up transformer. A step up

transformer is a transformer which converts low alternate voltage to high

alternate voltage.

A transformer in which Np>Ns is called a step down transformer. A step

down transformer is a transformer which converts high alternate voltage to

low alternate voltage.

Step down transformer

Step down transformer is one whose secondary voltage is less than its primary

voltage. It is designed to reduce the voltage from the primary winding to the

secondary winding. This kind of transformer “steps down” the voltage applied

to it.

As a step-down unit, the transformer converts high-voltage, low-current power

into low-voltage, high-current power. The larger-gauge wire used in the

secondary winding is necessary due to the increase in current. The primary

winding, which doesn’t have to conduct as much current, may be made of

smaller-gauge wire.

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Step-Down Transformer Considerations

FIG3.2: STEP DOWN TRANSFORMER

It is possible to operate either of these transformer types backwards

(powering the secondary winding with an AC source and letting the primary

winding power a load) to perform the opposite function: a step-up can function

as a step-down and visa-versa. One convention used in the electric power

industry is the use of “H” designations for the higher-voltage winding (the

primary winding in a step-down unit; the secondary winding in a step-up) and

“X” designations for the lower-voltage winding.

One of the most important considerations to increase transformer efficiency

and reduce heat is choosing the metal type of the windings. Copper windings

are much more efficient than aluminum and many other winding metal

choices, but it also costs more. Transformers with copper windings cost more

to purchase initially, but save on electrical cost over time as the efficiency

more than makes up for the initial cost.

Step-down transformers are commonly used to convert the 220 volt electricity

found in most parts of the world to the 110 volts required by North American

equipment.

3.1.4 How to Wire a Step down Transformer

1 .Observe and identify the schematic and rating of the step down transformer to be

installed. Remove the terminal connection box cover placed at the lower side of the

transformer. Only the high amperage types will have this enclosure, while lower powered

transformers will have an exposed screw terminal.

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2. Know termination identification follows for all step down transformers: H1, H2, H3

and H4 signify the high voltage side or power feed end of the transformer. This holds

true regardless of the size of the transformer. Interconnection of the transformer will

vary depending on the manufacturer and voltage used for feeding the transformer.

1. Terminate the feed power wires first by cutting the wires to length. If you are

using large wire lugs be sure to take into consideration the length of the lug and

the amount of wire that can be inserted into the female crimp area.

2. Strip back the outer insulating of the wires with the pocketknife or wire strippers.

Insert the eye ring or wire lug over the bare copper wire and crimp the connection

device, using the appropriate-size crimper, permanently to the wire.

3. Terminate the high side, high voltage of the step down transformer. If the high

side terminals are bolts, be sure to follow any torque requirements that are listed

by the manufacturer.

4. Terminate the low side, low voltage of the transformer. Note these terminals will

be identified by X1, X2, X3 and X4. Again follow the manufacturer’s individual

schematics for that particular type of transformer. Note that on small control

transformers there will only be an X1 and X2. X1 is the power or “hot” side and

X2 is generally the grounding and neutral portion of the low voltage.

5. Terminate the small control transformer for X1 and X2. X1 will go directly to the

control circuit after passing through a small fuse that is rated for the circuit. X2

will be terminated not only to the neutral side of the control circuit, but the

grounding safety as well. In other words, the X2 side of the small control

transformer must be tied to the grounding system of the electrical circuit.

6. Replace all covers on the transformer and any enclosures that protect you from

electricity. Apply the high voltage to the transformer by switching on the feeder

power circuit. Turn on the low side safety circuit control.

7. Use a volt meter to test for proper voltage on the step down side of the

transformer. It should be the same that is listed on the specs tag provided by the

manufacturer.

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How to Check a Step down Transformer

1. Remove all wires from the transformer terminals using the screwdriver. Identify

the wires if they are not already identified. Use a clear tape and pen. Write the

terminal that the wires are attached to and place the identified tape on the wire’s

end.

2. Turn the volt ohmmeter to the “Ohms” position and place the red lead into the

connector identified as “Ohms.” Touch the black lead to the metal frame of the

transformer.

3. Touch the red lead to the transformer’s terminals in the following order: H1, H2,

X1 and then X2. The meter should read infinite ohms or wide open. Infinite ohms

on a digital meter will be identified as a blank screen or a wide open will have the

word “Open” displayed. If the meter registers any form of resistance, there is an

internal problem with the windings. The copper coils may be shorted to the metal

frame of the transformer. The transformer will have to be replaced.

4. Check the continuity of each separate coil using the ohmmeter. Touch the black

lead to H1 and the red lead to H2. The meter should give a resistance reading.

Generally, it should read in the range of 3 to 100 ohms, depending on the style

and type of transformer. Perform the same test to the X1 and X2 terminals. You

should receive the same results. If the meter reads infinite ohms or a wide open

when checking between the terminals of the same coil, the wires are broken.

Replace the transformer.

5. Use the ohmmeter to conduct the transformers isolation circuit. Touch the red

lead to H1 and the black lead to X1. The meter should read infinite ohms or a

wide-open circuit. Perform the same test, but to H2 and X2 respectively. If any

resistance at all is read on the meter other than a wide-open circuit, the isolation

of the transformer has been compromised and must be replaced.

3.1.5 STEP DOWN TRANSFORMER (18V)

This step down transformer is used to convert the 230 ac to 18v dc and its output

is further passed to the remaining circuitry

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3.1.6 Advantages and Applications of Step down Transformer

Fast turnaround of custom transformers - often within days

Electro-static screen can be placed between primary & secondary windings

Multiple secondary windings

Tapped input and/or output windings

Choice of mounting brackets

Encased in powder coated steel (IP21) enclosure

Terminal blocks or flying leads

Tough Varnish Coating - either clear or black.

Available in a wide power range - from 2VA to 500KVA

Insulation classes up to class H - 180°C

Australian Standard AS61558 - 1997 compliant

Transformers fetch more demand among the customer for reasonable

prices. Transformers are more saleable products among the people and they

are used to produce energy or current from the circuits. Transformers come in

different types and them all suits for wide applications. Among the different

kinds of custom transformers, step down transformer is the most saleable and

required kind of transformer. Step down transformer are the newly designed

and produced specially transformer for affordable prices.

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3.2 DIODE

FIG.3.3 BASIC DIODE

A diode is a two-terminal electronic component with an asymmetric transfer

characteristic, with low (ideally zero) resistance to current flow in one direction,

and high (ideally infinite) resistance in the other. A semiconductor diode, the

most common type today, is a crystalline piece of semiconductor material with

a p-n junction connected to two electrical terminals. A vacuum tube diode is

a vacuum tube with two electrodes, a plate (anode) and heated cathode.

The most common function of a diode is to allow an electric current to pass in

one direction (called the diode's forward direction), while blocking current in

the opposite direction (the reverse direction). Thus, the diode can be viewed as

an electronic version of a check valve. This unidirectional behavior is called

rectification, and is used to convert alternating current to direct current,

including extraction of modulation from radio signals in radio receivers—these

diodes are forms of rectifiers.

However, diodes can have more complicated behavior than this simple on–off

action. Semiconductor diodes begin conducting electricity only if a certain

threshold voltage or cut-in voltage is present in the forward direction (a state in

which the diode is said to be forward-biased). The voltage drop across a

forward-biased diode varies only a little with the current, and is a function of

temperature; this effect can be used as a temperature sensor or voltage

reference.

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Semiconductor diodes' nonlinear current–voltage characteristic can be tailored

by varying the semiconductor materials and doping, introducing impurities into

the materials. These are exploited in special-purpose diodes that perform many

different functions. For example, diodes are used to regulate voltage (Zener

diodes), to protect circuits from high voltage surges (avalanche diodes), to

electronically tune radio and TV receivers (varactor diodes), to generate

radio oscillations (tunnel diodes, Gunn diodes, IMPATT diodes), and to

produce light (light emitting diodes). Tunnel diodes exhibit negative resistance,

which makes them useful in some types of circuits.

Diodes were the first semiconductor electronic devices. The discovery

of crystals' rectifying abilities was made by German physicist Ferdinand

Braun in 1874. The first semiconductor diodes, called cat's whisker diodes,

developed around 1906, were made of mineral crystals such as galena. Today

most diodes are made of silicon, but other semiconductors such

as germanium are sometimes used.

3.2.1 DIODE 1N 4007

The 1N4007 diode is a 1000V diode and has a 1A rating. Voltage drop is 0.7V

3.2.2 Junction diodes

Most diodes today are silicon junction diodes. A junction is formed between the p and n regions which is also called a depletion region

a) p–n junction diode:

A p–n junction diode is made of a crystal of semiconductor. Impurities are

added to it to create a region on one side that contains negative charge

carriers (electrons), called n-type semiconductor, and a region on the other side

that contains positive charge carriers (holes), called p-type semiconductor.

When two materials i.e. n-type and p-type are attached together, a momentary

flow of electrons occur from n to p side resulting in a third region where no

charge carriers are present. It is called Depletion region due to the absence of

charge carriers (electrons and holes in this case). The diode's terminals are

attached to each of these regions. The boundary between these two regions,

called a p–n junction, is where the action of the diode takes place. The crystal

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allows electrons to flow from the N-type side (called the cathode) to the P-type

side (called the anode), but not in the opposite direction.

b) Scotty diode

Another type of junction diode, the Schottky diode, is formed from

a metal–semiconductor junction rather than a p–n junction, which reduces

capacitance and increases switching speed.

c) Current–voltage characteristic

A semiconductor diode’s behavior in a circuit is given by its current–

voltage characteristic, or I–V graph (see graph below). The shape of the curve is

determined by the transport of charge carriers through the so-called depletion

layer or depletion region that exists at the p–n junction between differing

semiconductors. When a p–n junction is first created, conduction-band (mobile)

electrons from the N-doped region diffuse into the P-doped region where there is

a large population of holes (vacant places for electrons) with which the electrons

"recombine". When a mobile electron recombines with a hole, both hole and

electron vanish, leaving behind an immobile positively charged donor (dopant)

on the N side and negatively charged acceptor (dopant) on the P side. The region

around the p–n junction becomes depleted of charge carriers and thus behaves as

an insulator.

However, the width of the depletion region (called the depletion width)

cannot grow without limit. For each electron–hole pair that recombines, a

positively charged dopant ion is left behind in the N-doped region, and a

negatively charged dopant ion is left behind in the P-doped region. As

recombination proceeds more ions are created, an increasing electric field

develops through the depletion zone that acts to slow and then finally stop

recombination. At this point, there is a "built-in" potential across the depletion

zone.

If an external voltage is placed across the diode with the same polarity as

the built-in potential, the depletion zone continues to act as an insulator,

preventing any significant electric current flow (unless electron/hole pairs are

actively being created in the junction by, for instance, light. see photodiode).

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This is the reverse bias phenomenon. However, if the polarity of the external

voltage opposes the built-in potential, recombination can once again proceed,

resulting in substantial electric current through the p–n junction (i.e. substantial

numbers of electrons and holes recombine at the junction). For silicon diodes,

the built-in potential is approximately 0.7 V (0.3 V for Germanium and 0.2 V

for Schottky). Thus, if an external current is passed through the diode, about

0.7 V will be developed across the diode such that the P-doped region is

positive with respect to the N-doped region and the diode is said to be "turned

on" as it has a forward bias.

A diode’s I–V characteristic can be approximated by four regions of

operation.

FIG3.4: I–V CHARACTERISTICS OF A P–N JUNCTION DIODE

(Not to scale—the current in the reverse region is magnified compared to

the forward region, resulting in the apparent slope discontinuity at the origin;

the actual I–V curve is smooth across the origin)

At very large reverse bias, beyond the peak inverse voltage or PIV, a process

called reverse breakdown occurs that causes a large increase in current (i.e., a

large number of electrons and holes are created at, and move away from the p–n

junction) that usually damages the device permanently. The avalanche diode is

deliberately designed for use in the avalanche region. In the Zener diode, the

concept of PIV is not applicable. A Zener diode contains a heavily doped p–n

junction allowing electrons to tunnel from the valence band of the p-type

material to the conduction band of the n-type material, such that the reverse

voltage is "clamped" to a known value (called the Zener voltage), and avalanche

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does not occur. Both devices, however, do have a limit to the maximum current

and power in the clamped reverse-voltage region. The device does not attain its

full blocking capability until the reverse current ceases.

The second region, at reverse biases more positive than the PIV, has only a

very small reverse saturation current. In the reverse bias region for a normal P–

N rectifier diode, the current through the device is very low (in the µA range).

However, this is temperature dependent, and at sufficiently high temperatures,

a substantial amount of reverse current can be observed (mA or more).

The third region is forward but small bias, where only a small forward

current is conducted.

As the potential difference is increased above an arbitrarily defined "cut-in

voltage" or "on-voltage" or "diode forward voltage drop (Vd)", the diode

current becomes appreciable (the level of current considered "appreciable" and

the value of cut-in voltage depends on the application), and the diode presents

a very low resistance. The current–voltage curve is exponential. In a normal

silicon diode at rated currents, the arbitrary cut-in voltage is defined as 0.6 to

0.7 volts. The value is different for other diode types—Schottky diodes can be

rated as low as 0.2 V, Germanium diodes 0.25 to 0.3 V, and red or blue light-

emitting diodes (LEDs) can have values of 1.4 V and 4.0 V respectively. At

higher currents the forward voltage drop of the diode increases. A drop of 1 V

to 1.5 V is typical at full rated current for power diodes.

3.3 BRIDGE RECTIFIER

A diode bridge is an arrangement of four (or more) diodes in a bridge

circuit configuration that provides the same polarity of output for either polarity

of input. When used in its most common application, for conversion of

an alternating current (AC) input into a direct current (DC) output, it is known

as abridge rectifier. A bridge rectifier provides full-wave rectification from a

two-wire AC input, resulting in lower cost and weight as compared to a rectifier

with a 3-wire input from a transformer with a center-tapped secondary winding.

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The essential feature of a diode bridge is that the polarity of the output is the

same regardless of the polarity at the input. The diode bridge circuit is also

known as the Graetz circuit after its inventor, physicist Leo Graetz, and the

single-phase version, with four diodes, may also be referred to as an H bridge.

Basic operation

According to the conventional model of current flow (originally established

by Benjamin Franklin and still followed by most engineers today[2]), current is

assumed to flow through electrical conductors from the positive to

the negative pole. In actuality, free electrons in a conductor nearly always flow

from the negative to the positive pole. In the vast majority of applications,

however, the actual direction of current flow is irrelevant. Therefore, in the

discussion below the conventional model is retained.

In the diagrams below, when the input connected to the left corner of the

diamond is positive, and the input connected to the right corner is negative,

current flows from the upper supply terminal to the right along

the red (positive) path to the output, and returns to the lower supply terminal

via the blue (negative) path.

FIG.3.5: FIRST HALF RECTIFICATION

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

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FIG 3.6: SECOND HALF RECTIFICATION

In each case, the upper right output remains positive and lower right output

negative. Since this is true whether the input is AC or DC, this circuit not only

produces a DC output from an AC input, it can also provide what is sometimes

called "reverse polarity protection". That is, it permits normal functioning of

DC-powered equipment when batteries have been installed backwards, or

when the leads (wires) from a DC power source have been reversed, and

protects the equipment from potential damage caused by reverse polarity.

FIG.3.7: RECTIFIED OUTPUT

Prior to the availability of integrated circuits, a bridge rectifier was

constructed from "discrete components", i.e., separate diodes. Since about

1950, a single four-terminal component containing the four diodes connected

in a bridge configuration became a standard commercial component and is now

available with various voltage and current ratings.

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The purpose of a bridge rectifier in electronic circuit is to convert the ac

voltage (wave) into Dc voltage. Usually it is located near the ac supply area. It

can come in the 4 separate diodes arrangement or in a single package as seen

from the photo. The specification that you need to know for a bridge rectifier is

the peak reverse voltage (PRV) and the ampere. How do we know the spec of

a bridge rectifier? By looking at part number printed on the body.  The part

number usually starts with KBLXXX and you have to search a data book to

look for the specification. Always replace the bridge rectifier with the same or

a higher specification.

If you have the time, you can always join four separate diodes to form

a bridge rectifier. Use the general purpose part number such as the 1N4007 to

make a bridge and sometime this rectifier can even work better than the

original one.

Whenever a fuse opens in electronic equipment, you must always check

the bridge rectifier. Chances are high that the bridge may go shorted too. If you

experience enough, you may check the bridge rectifier while it is still on board.

If you are a beginner, i suggest that you remove the bridge and test it with you

multimeter. In Monitor repair, usuallly a replacement of a fuse and a bridge

rectifier will restore to normal operation.

3.3.1 Advantages

With the availabilities of low-cost, highly reliable and small-sized

silicon diodes bridge rectifier is becoming more and more popular in

comparison to center-tap and half-wave rectifier. It has many advantages over

a center-tap and half-wave rectifier, as given below.

1.The rectification efficiency of full-wave rectifier is double of that of a rectifier.

2.The ripple voltage is low and of higher frequency in rectifier so simple filter.

3.Higher output voltage, higher Transformer Utilization Factor (TUF).

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In a full-wave rectifier, there is no problem due to dc saturation of the core because the

dc current in the two halves of the two halves of the transformer secondary flow in

opposite directions.

No centre tap is required in the transformer secondary so in case of a bridge rectifier

the transformer required is simpler. If stepping up or stepping down of voltage is not

required, transformer can be eliminated even.

The PIV is one half that of centre-tap rectifier. Hence bridge rectifier is highly suited

for high voltage applications.

Transformer utilization factor, in case of a bridge rectifier, is higher than that of a

centre-tap rectifier.

For a given power output, power transformer of smaller size can be used in case of the

bridge rectifier because current in both (primary and secondary) windings of the

supply transformer flow for the entire ac cycle.

3.3.2 Disadvantages

The main disadvantage of a bridge rectifier is that it needs four diodes, two of

which conduct in alternate half-cycles. Because of this the total voltage drop in

diodes becomes double of that in case of centre-tap rectifier, losses are increased

and rectification efficiency is somewhat reduced. This poses a problem when

low voltages are required. Another disadvantage of bridge rectifier is that the

load resistor RL and the supply source have no common point which may be

earthed.

3.4 CAPACITOR

A capacitor (originally known as condenser) is a passive two-

terminal electrical component used to store energy in an electric field. The

forms of practical capacitors vary widely, but all contain at least

two electrical conductors separated by a dielectric (insulator); for example,

one common construction consists of metal foils separated by a thin layer of

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insulating film. Capacitors are widely used as parts of electrical circuits in

many common electrical devices.

When there is a potential difference (voltage) across the conductors, a

static electric field develops across the dielectric, causing positive charge to

collect on one plate and negative charge on the other plate. Energy is stored in

the electrostatic field. An ideal capacitor is characterized by a single constant

value, capacitance, measured in farads. This is the ratio of the electric

charge on each conductor to the potential difference between them.

The capacitance is greatest when there is a narrow separation between large

areas of conductor; hence capacitor conductors are often called plates, referring

to an early means of construction. In practice, the dielectric between the plates

passes a small amount of leakage current and also has an electric field strength

limit, resulting in a breakdown voltage, while the conductors

and leads introduce an undesired inductance and resistance.

Capacitors are widely used in electronic circuits for blocking direct

current while allowing alternating current to pass, in filter networks, for

smoothing the output of power supplies, in the resonant circuits that tune

radios to particular frequencies, in electric power transmission systems for

stabilizing voltage and power flow, and for many other purposes.

Capacitors are components that are used to store an electrical charge and are

used in timer circuits a capacitor may be used with a resistor to produce a

timer. Sometimes capacitors are used to smooth a current in a circuit as they

can prevent false triggering of other components such as relays. When power is

supplied to a circuit that includes a capacitor - the capacitor charges up. When

power is turned off the capacitor discharges its electrical charge slowly.

Acts as a filter.

Used to block the ripples of the output from the bridge circuit.

A capacitor is composed of two conductors separated by an insulating

material called a DIELECTRIC. The dielectric can be paper, plastic film,

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ceramic, air or a vacuum. The plates can be aluminum discs, aluminum foil or a

thin film of metal applied to opposite sides of a solid dielectric. The

CONDUCTOR - DIELECTRIC - CONDUCTOR sandwich can be rolled into a

cylinder or left flat.

FIG.3.8: CONDUCTING PLATES

3.4.1 CAPACITOR(1000/25V)

FIG 3.9: CAPACITOR(1000/25V)

Description:

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High quality, low-profile radial electrolytic. Can be used in typical power supply

filtering applications. Capacitor is rated 1000uF at 25V. Caps are rated at 85c or

better.

3.4.2 Applications:

FIG 3.10: MYLAR FILM OIL FILLED CAPACITOR

This Mylar-film, oil-filled capacitor has very low inductance and low

resistance, to provide the high-power (70 megawatt) and high speed (1.2

microseconds) discharge needed to operate a dye laser.

3.4.3 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.)

Conventional capacitors provide less than 360 joules per kilogram

of energy density, whereas a conventional alkaline battery has a density of 590

kJ/kg.

In car audio systems, large capacitors store energy for the amplifier to use

on demand. Also for a flash tube a capacitor is used to hold the high voltage.

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3.4.4 Pulsed power and weapons

Groups of large, specially constructed, low-inductance high-voltage

capacitors (capacitor banks) are used to supply huge pulses of current for

many pulsed power applications. These include electromagnetic forming, Marx

generators, pulsed lasers (especially TEA lasers), pulse forming

networks, radar, fusion research, and particle accelerators.

Large capacitor banks (reservoir) are used as energy sources for

the exploding-bridge wire detonators or slapper detonators in nuclear

weapons and other specialty weapons. Experimental work is under way using

banks of capacitors as power sources for electromagnetic armour and

electromagnetic railgunsand coilguns.

3.4.5 Power conditioning

Reservoir 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

FIG 3.11: 1O mF CAPACITOR IN AN AMPLIFIER POWER

SUPPLY

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

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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.

3.4.6 Power factor correction

FIG 3.12: CAPACITORS IN POWER TRANSMISSION SYSTEM

A high-voltage capacitor bank used for power factor correction on a power

transmission system.

In electric power distribution, capacitors are used for power factor

correction. Such capacitors often come as three capacitors connected as a three

phase load. Usually, the values of these capacitors are given not in farads but

rather as a reactive power in volt-amperes reactive (VAr). The purpose is to

counteract inductive loading from devices like electric

motors and transmission lines to make the load appear to be mostly resistive.

Individual motor or lamp loads may have capacitors for power factor

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correction, or larger sets of capacitors (usually with automatic switching

devices) may be installed at a load center within a building or in a large

utility substation.

3.4.7 Suppression and coupling

Signal coupling

FIG 3.13: POLYESTER FILM CAPACITOR

Polyester film capacitors are frequently used as coupling capacitors.

Because capacitors pass AC but block DC signals (when charged up to the

applied dc voltage), they are often used to separate the AC and DC

components of a signal. This method is known as AC coupling or "capacitive

coupling". Here, a large value of capacitance, whose value need not be

accurately controlled, but whose reactance is small at the signal frequency, is

employed.

Decoupling

A decoupling capacitor is a capacitor used to protect one part of a circuit

from the effect of another, for instance to suppress noise or transients. Noise

caused by other circuit elements is shunted through the capacitor, reducing the

effect they have on the rest of the circuit. It is most commonly used between

the power supply and ground. An alternative name is bypass capacitor as it is

used to bypass the power supply or other high impedance component of a

circuit.

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Noise filters and snubbers

When an inductive circuit is opened, the current through the inductance

collapses quickly, creating a large voltage across the open circuit of the switch

or relay. If the inductance is large enough, the energy will generate a spark,

causing the contact points to oxidize, deteriorate, or sometimes weld together,

or destroying a solid-state switch. A snubber capacitor across the newly

opened circuit creates a path for this impulse to bypass the contact points,

thereby preserving their life; these were commonly found in contact

breaker ignition systems, for instance. Similarly, in smaller scale circuits, the

spark may not be enough to damage the switch but will

still radiate undesirable radio frequency interference (RFI), which a filter

capacitor absorbs. Snubber capacitors are usually employed with a low-value

resistor in series, to dissipate energy and minimize RFI. Such resistor-capacitor

combinations are available in a single package.

Capacitors are also used in parallel to interrupt units of a high-

voltage circuit breaker in order to equally distribute the voltage between these

units. In this case they are called grading capacitors.

In schematic diagrams, a capacitor used primarily for DC charge storage is

often drawn vertically in circuit diagrams with the lower, more negative, plate

drawn as an arc. The straight plate indicates the positive terminal of the device,

if it is polarized (see electrolytic capacitor).

3.4.8 Motor starters

In single phase squirrel cage motors, the primary winding within the

motor housing is not capable of starting a rotational motion on the rotor, but is

capable of sustaining one. To start the motor, a secondary "start" winding has a

series non-polarized starting capacitor to introduce a lead in the sinusoidal

current. When the secondary (start) winding is placed at an angle with respect

to the primary (run) winding, a rotating electric field is created. The force of

the rotational field is not constant, but is sufficient to start the rotor spinning.

When the rotor comes close to operating speed, a centrifugal switch (or

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current-sensitive relay in series with the main winding) disconnects the

capacitor. The start capacitor is typically mounted to the side of the motor

housing. These are called capacitor-start motors that have relatively high

starting torque. Typically they can have up-to four times as much starting

torque than a split-phase motor and are used on applications such as

compressors, pressure washers and any small device requiring high starting

torques.

Capacitor-run induction motors have a permanently connected phase-

shifting capacitor in series with a second winding. The motor is much like a

two-phase induction motor.

Motor-starting capacitors are typically non-polarized electrolytic types,

while running capacitors are conventional paper or plastic film dielectric types.

3.4.9 Signal processing

The energy stored in a capacitor can be used to represent information,

either in binary form, as in DRAMs, or in analogue form, as in analog sampled

filters and CCDs. Capacitors can be used in analog as components of

integrators or more complex filters and in negative feedback loop stabilization.

Signal processing circuits also use capacitors to integrate a current signal.

3.4.10 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.

The resonant frequency f of a tuned circuit is a function of the inductance

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

Where L is in henries and C is in farads.

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3.4.11 Sensing

Most capacitors are designed to maintain a fixed physical structure.

However, various factors can change the structure of the capacitor, and the

resulting change in capacitance can be used to sense those factors.

Changing the dielectric:

The effects of varying the characteristics of the dielectric can be used for

sensing purposes. Capacitors with an exposed and porous dielectric can be

used to measure humidity in air. Capacitors are used to accurately measure the

fuel level in airplanes; as the fuel covers more of a pair of plates, the circuit

capacitance increases.

Changing the distance between the plates:

Capacitors with a flexible plate can be used to measure strain or pressure.

Industrial pressure transmitters used for process control use pressure-sensing

diaphragms, which form a capacitor plate of an oscillator circuit. Capacitors

are used as the sensor in condenser microphones, where one plate is moved by

air pressure, relative to the fixed position of the other plate. Some

accelerometers use MEMS capacitors etched on a chip to measure the

magnitude and direction of the acceleration vector. They are used to detect

changes in acceleration, in tilt sensors, or to detect free fall, as sensors

triggering airbag deployment, and in many other applications.

Some fingerprint sensors use capacitors. Additionally, a user can adjust the

pitch of a  musical instrument by moving his hand since this changes the

effective capacitance between the user's hand and the antenna.

Changing the effective area of the plates:

Capacitive touch switches are now used on many consumer electronic

products.

3.5 TRANSISTOR

The essential usefulness of a transistor comes from its ability to use a small

signal applied between one pair of its terminals to control a much larger signal at

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another pair of terminals. This property is called gain. A transistor can control

its output in proportion to the input signal; that is, it can act as an amplifier.

Alternatively, the transistor can be used to turn current on or off in a circuit as

an electrically controlled switch, where the amount of current is determined by

other circuit elements voltage at the gate can control a current between source

and drain.

The image to the right represents a typical bipolar transistor in a circuit.

Charge will flow between emitter and collector terminals depending on the

current in the base. Since internally the base and emitter connections behave

like a semiconductor diode, a voltage drop develops between base and emitter

while the base current exists. The amount of this voltage depends on the

material the transistor is made from, and is referred to as VBE.

FIG 3.15: 3.14 TRANSISTOR

Transistor as a switch

FIG 3.16: TRANSISTOR AS A SWITCH

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Transistors are commonly used as electronic switches, both for high-power

applications such as switched-mode power supplies and for low-power

applications such as logic gates.

In a grounded-emitter transistor circuit, such as the light-switch circuit

shown, as the base voltage rises, the emitter and collector currents raise

exponentially. The collector voltage drops because of the collector load

resistance (in this example, the resistance of the light bulb). If the collector

voltage were zero, the collector current would be limited only by the light bulb

resistance and the supply voltage. The transistor is then said to be saturated - it

will have a very small voltage from collector to emitter. Providing sufficient

base drive current is a key problem in the use of bipolar transistors as switches.

The transistor provides current gain, allowing a relatively large current in the

collector to be switched by a much smaller current into the base terminal. The

ratio of these currents varies depending on the type of transistor, and even for a

particular type, varies depending on the collector current. In the example light-

switch circuit shown, the resistor is chosen to provide enough base current to

ensure the transistor will be saturated.

In any switching circuit, values of input voltage would be chosen such that

the output is either completely off, [21] or completely on. The transistor is acting

as a switch, and this type of operation is common in digital circuits where only

"on" and "off" values are relevant

3.5.1Transistor as an amplifier

FIG 3.16 TRANSISTOR AS AN AMPLIFIER

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Amplifier circuit, common-emitter configuration with a voltage-divider

bias circuit.

The common-emitter amplifier is designed so that a small change in

voltage (VIN) changes the small current through the base of the transistor; the

transistor's current amplification combined with the properties of the circuit

mean that small swings in VIN produce large changes in Vought.

Various configurations of single transistor amplifier are possible, with

some providing current gain, some voltage gain, and some both.

From mobile phones to televisions, vast numbers of products include

amplifiers for sound reproduction, radio transmission, and signal processing.

The first discrete transistor audio amplifiers barely supplied a few hundred mill

watts, but power and audio fidelity gradually increased as better transistors

became available and amplifier architecture evolved.

Modern transistor audio amplifiers of up to a few hundred watts are

common and relatively inexpensive.

3.5.2 SL100

SL100 is a general purpose, medium power NPN transistor. It is mostly

used as switch in common emitter configuration. The transistor terminals

require a fixed DC voltage to operate in the desired region of its characteristic

curves. This is known as the biasing. For switching applications, SL100 is

biased in such a way that it remains fully on if there is a signal at its base. In

the absence of base signal, it gets turned off completely. 

The emitter leg of SL100 is indicated by a protruding edge in the transistor

case. The base is nearest to the emitter while collector lies at other extreme of

the casing.

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Pin Diagram: 

FIG 3.17: PIN DIAGRAM OF SL100 TRANSISTOR

3.5.3 BC547:

BC547 is an NPN bi-polar junction transistor. A transistor, stands for

transfer of resistance, is commonly used to amplify current. A small current at

its base controls a larger current at collector & emitter terminals.

BC547 is mainly used for amplification and switching purposes. It has a

maximum current gain of 800. Its equivalent transistors are BC548 and

BC549.

The transistor terminals require a fixed DC voltage to operate in the desired

region of its characteristic curves. This is known as the biasing. For

amplification applications, the transistor is biased such that it is partly on for

all input conditions. The input signal at base is amplified and taken at the

emitter. BC547 is used in common emitter configuration for amplifiers. The

voltage divider is the commonly used biasing mode.

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Pin Diagram: 

FIG 3.18: PIN DIAGRAM OF BC547

Transistor Technical Specifications:

The BC547 transistor is an NPN Epitaxial Silicon Transistor. The BC547

transistor is a general-purpose transistor in small plastic packages. It is used in

general-purpose switching and amplification BC847/BC547 series 45 V, 100

mA NPN general-purpose transistors.

The BC547 transistor is an NPN bipolar transistor, in which the letters "N"

and "P" refer to the majority charge carriers inside the different regions of the

transistor. Most bipolar transistors used today are NPN, because electron

mobility is higher than whole mobility in semiconductors, allowing greater

currents and faster operation. NPN transistors consist of a layer of P-doped

semiconductor (the "base") between two N-doped layers. A small current

entering the base in common-emitter mode is amplified in the collector output.

In other terms, an NPN transistor is "on" when its base is pulled high relative

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to the emitter. The arrow in the NPN transistor symbol is on the emitter leg and

points in the direction of the conventional current flow when the device is in

forward active mode. One mnemonic device for identifying the symbol for the

NPN transistor is "not pointing in." An NPN transistor can be considered as

two diodes with a shared anode region. In typical operation, the emitter base

junction is forward biased and the base collector junction is reverse biased. In

an NPN transistor, for example, when a positive voltage is applied to the base

emitter junction, the equilibrium between thermally generated carriers and the

repelling electric field of the depletion region becomes unbalanced, allowing

thermally excited electrons to inject into the base region. These electrons

wander (or "diffuse") through the base from the region of high concentration

near the emitter towards the region of low concentration near the collector. The

electrons in the base are called minority carriers because the base is doped p-

type which would make holes the majority carrier in the base.

3.6 ZENER DIODE

A Zener diode is a diode which allows current to flow in the forward

direction in the same manner as an ideal diode, but will also permit it to flow in

the reverse direction when the voltage is above a certain value known as

the breakdown voltage, "zener knee voltage" or "zener voltage" or "avalanche

point".

The device was named after Clarence Zener, who discovered this electrical

property. Many diodes described as "zener" diodes rely instead on avalanche

breakdown as the mechanism. Both types are used. Common applications

include providing a reference voltage for voltage regulators, or to protect other

semiconductor devices from momentary voltage pulses.

FIG 3.19: ZENER DIODE

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Operation

A conventional solid-state diode will allow significant current if it is reverse-

biased above its reverse breakdown voltage. When the reverse bias breakdown

voltage is exceeded, a conventional diode is subject to high current due

to avalanche breakdown. Unless this current is limited by circuitry, the diode

will be permanently damaged due to overheating. A zener diode exhibits almost

the same properties, except the device is specially designed so as to have a

reduced breakdown voltage, the so-called zener voltage. By contrast with the

conventional device, a reverse-biased zener diode will exhibit a controlled

breakdown and allow the current to keep the voltage across the zener diode

close to the zener breakdown voltage. For example, a diode with a zener

breakdown voltage of 3.2 V will exhibit a voltage drop of very nearly 3.2 V

across a wide range of reverse currents. The zener diode is therefore ideal for

applications such as the generation of a reference voltage (e.g. for an amplifier

stage), or as a voltage stabilizer for low-current applications.[1]

FIG 3.20: OUTPUT CHARACTERISTICS OF ZENER DIODE

Another mechanism that produces a similar effect is the avalanche effect as in

the avalanche diode.[1] The two types of diode are in fact constructed the same

way and both effects are present in diodes of this type. In silicon diodes up to

about 5.6 volts, the zener effect is the predominant effect and shows a marked

negative temperature coefficient. Above 5.6 volts, the avalanche effect becomes

predominant and exhibits a positive temperature coefficient.

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FIG 3.21: TC DEPENDING ON ZENER VOLTAGE

In a 5.6 V diode, the two effects occur together and their temperature

coefficients nearly cancel each other out, thus the 5.6 V diode is useful in

temperature-critical applications. An alternative which is used for voltage

references that need to be highly stable over long periods of time is to use a Z-

diode with a TC of +2 mV/°C (breakdown voltage 6.2-6.3 V) connected in

series with a forward-biased silicon diode (or a transistor B-E junction)

manufactured on the same chip.[3] The forward biased diode has a TC of -2

mV/°C, causing the TCs to cancel out.

Modern manufacturing techniques have produced devices with voltages

lower than 5.6 V with negligible temperature coefficients, but as higher voltage

devices are encountered, the temperature coefficient rises dramatically. A 75 V

diode has 10 times the coefficient of a 12 V diode.

Zener and avalanche diodes, regardless of breakdown voltage, are usually

marketed under the umbrella term of "zener diode".

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Zener Diode Voltage Regulator Circuit:

FIG 3.22: ZENER DIODE AS A VOLTAGE REGULATOR

A zener diode can be used to make a simple voltage regulation circuit as

pictured above. The output voltage is fixed at the zener voltage of the zener

diode used and so can be used to power devices requiring a fixed voltage.

Click here to find out more about the zener diode voltage regulator and how

you go about selecting the resistor and zener diode.

Advantages

3.6.1 Control

The ability of the Zener diode to control and reverse part of the current flowing through

it means it can be used to regulate and stabilize the voltage in a circuit and prevent prob-

lems that can occur when the supply or load voltage varies. Circuit designers can use the

Zener voltage of the diode to exactly regulate and stabilize the voltage in the circuit.

3.6.2 Size and Expense

Zener diodes are very small, so they can be used in many small electronic devices, such

as handheld devices and cell phones. They also can be used in smaller circuits that would

not work with any larger form of current regulation. Zener diodes are less expensive than

other diodes used in these devices. This can cut the cost to produce a device such as a

television or computer, lowering consumer costs.

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3.6.3 Performance

Zener diodes have a very high performance standard; often more than the electronic de-

vice they are placed in needs to operate at maximum efficiency. These diodes are

equipped to handle a higher voltage than the standard operating voltage, so they are able

to handle more power. These diodes will still operate at standard voltage, but will not

blow out if the voltage is still under their threshold. They are also small enough to allow

current to flow quickly through their circuits.

3.6.4 Compatibility and Obtainability

Zener diodes, due to their lower cost and greater control, are commonly used in electric

devices. They are also compatible with most systems, so they are a preferred method to

regulate voltage. They are also used in other applications, such as in solar panels. Though

these diodes don't often get damaged due to their current controls, they can still blow out

if the current exceeds what they are equipped to handle. If this were to happen, the SCR

would also likely blow out and both elements would need to be replaced. Luckily, Zener

diodes are fairly easy to obtain due to their common use and low cost.

3.6.5 Uses of Zener Diodes

Since the voltage dropped across a Zener Diode is a known and fixed

value, Zener diodes are typically used to regulate the voltage in electric

circuits. Using a resistor to ensure that the current passing through the Zener

diode is at least 5mA (0.005 Amps), the circuit designer knows that the voltage

drop across the diode is exactly equal to the Zener voltage of the diode.

3.7 Resistor

A resistor is a passive two-terminal electrical component that

implements electrical resistance as a circuit element.

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FIG 3.23: RESISTOR

The current through a resistor is in direct proportion to the voltage across

the resistor's terminals. This relationship is represented by Ohm's law:

Where I is the current through the conductor in units of amperes, V is the

potential difference measured across the conductor in units of volts, and R is

the resistance of the conductor in units of ohms.

The ratio of the voltage applied across a resistor's terminals to the intensity

of current in the circuit is called its resistance, and this can be assumed to be a

constant (independent of the voltage) for ordinary resistors working within

their ratings.

Resistors are common elements of electrical networks and electronic

circuits and are ubiquitous in electronic equipment. Practical resistors can be

made of various compounds and films, as well as resistance wire (wire made of

a high-resistivity alloy, such as nickel-chrome). Resistors are also implemented

within integrated circuits, particularly analog devices, and can also be

integrated into hybrid and printed circuits.

The electrical functionality of a resistor is specified by its resistance:

common commercial resistors are manufactured over a range of more than nine

orders. When specifying that resistance in an electronic design, the required

precision of the resistance may require attention to the manufacturing of the

chosen resistor, according to its specific application. The temperature

coefficient of the resistance may also be of concern in some precision

applications. Practical resistors are also specified as having a

maximum power rating which must exceed the anticipated power dissipation

of that resistor in a particular circuit: this is mainly of concern in power

electronics applications. Resistors with higher power ratings are physically

larger and may require heat sinks. In a high-voltage circuit, attention must

sometimes be paid to the rated maximum working voltage of the resistor.

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Practical resistors have a series inductance and a small parallel capacitance;

these specifications can be important in high-frequency applications. In a low-

noise amplifier or pre-amp, the noise characteristics of a resistor may be an

issue. The unwanted inductance, excess noise, and temperature coefficient are

mainly dependent on the technology used in manufacturing the resistor. They

are not normally specified individually for a particular family of resistors

manufactured using a particular technology.[1] A family of discrete resistors is

also characterized according to its form factor, that is, the size of the device

and the position of its leads (or terminals) which is relevant in the practical

manufacturing of circuits using them.

The amount of resistance offered by a resistor is determined by its physical

construction. A carbon composition resistor has resistive carbon packed into a

ceramic cylinder, while a carbon film resistor consists of a similar ceramic

tube, but has conductive carbon film wrapped around the outside. Metal film or

metal oxide resistors are made much the same way, but with metal instead of

carbon. A wire wound resistor, made with metal wire wrapped around clay,

plastic, or fiberglass tubing, offers resistance at higher power levels. Those

used for applications that must withstand high temperatures are typically made

of materials such as cermet, a ceramic-metal composite, or tantalum, a rare

metal, so that they can endure the heat.

Resistors are coated with paint or enamel, or covered in molded plastic to

protect them. Because they are often too small to be written on, a standardized

color-coding system is used to identify them. The first three colors represent

ohm value, and a fourth indicates the tolerance, or how close by percentage the

resistor is to its ohm value. This is important for two reasons: the nature of its

construction is imprecise, and if used above its maximum current, the value

can change or the unit itself can burn up.

Every resistor falls into one of two categories: fixed or variable. A fixed

resistor has a predetermined amount of resistance to current, while a variable

one can be adjusted to give different levels of resistance. Variable resistors are

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also called potentiometers and are commonly used as volume controls on audio

devices. A rheostat is a variable resistor made specifically for use with high

currents. There are also metal-oxide varistors, which change their resistance in

response to a rise in voltage; thermistors, which either raise or lower resistance

when temperature rises or drops; and light-sensitive resistors.

The property of a substance, which opposes the flow of an electric

current through it, is called a resistance. It is measured in ohms and is

represented by letter ‘R’.

Each resistor has two main characteristics

1) Its resistance value in ohms and 2) its power dissipating capacity in

watts

Resistors are employed for many purposes such as electric heaters,

telephone equipments, and electric and electronic circuit elements and in

current limiting devices. As resistors are used in wide applications there values

like power rating value, tolerance etc vary. Resistors of resistance value

ranging from .1ohms to many mega ohms are manufactured. Acceptable

tolerance levels range from +/- 20% to as low as +/-.001%. The power rating

may be as low as 1/10 watts and can be in several hundred watts. These all

vary in range and type of application a particular resistor is used.

3.7.1 Classification of Resistors

From operating conditions point of view, resistors can be classified into

two.

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1) 3.7.2 Fixed resistors

FIG 3.24: FIXED RESISTORS

1) Fixed resistors are further classified into:

a) Carbon composition type resistors b) Metalized type resistors c) Wire wound type resistors

a) Carbon composition type resistors:

This is the most common type of low wattage resistor. The resistive material is of carbon-clay composition and the leads are made of tinned copper. These resistors are cheap and reliable and stability is high.

b) Wire wound resistors:

These resistors are a length of wire wound an insulating cylindrical core. Usually wires of material such as constantan (60% copper and 40% nickel) and managing which have high resistivity’s and low temperature coefficients are employed. The completed wire wound resistor is coated with an insulating material such as baked enamel.

c) Metalized resistors

It is constructed using film deposition techniques of depositing a thick film

of resistive material onto an insulating substrate. Only approximate values of

resistance can be had by this method.

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2) Adjustable/ variable resistors

Fig 3.25: VARIABLE RESISTORS

For circuits requiring a resistance that can be adjusted while it remains

connected in the circuit (for egg: volume control on radio), variable resistors

are required. They usually have 3 lead two fixed and one movable.

There are 2 types of general purpose variable resistors.

-Rheostat

-Potentiometers

Adjustable resistors

A resistor may have one or more fixed tapping points so that the resistance can

be changed by moving the connecting wires to different terminals. Some wire

wound power resistors have a tapping point that can slide along the resistance

element, allowing a larger or smaller part of the resistance to be used.

Where continuous adjustment of the resistance value during operation of

equipment is required, the sliding resistance tap can be connected to a knob

accessible to an operator. Such a device is called a rheostat and has two

terminals.

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3.7.3 RESISTOR (1K)

FIG 3.26:1K RESISTOR

3.7.4 RESISTOR (100E)

FIG 3.27 100 OHM RESISTOR

Rheostat

The most common way to vary the resistance in a circuit is to use a  rheostat, a

two-terminal variable resistor. For low-power applications (less than about 1

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watt) a three-terminal potentiometer is often used, with one terminal

unconnected or connected to the wiper.

Where the rheostat must be rated for higher power (more than about 1 watt),

they may be built with a resistance wire wound around a semicircular insulator,

with the wiper sliding from one turn of the wire to the next. Sometimes a

rheostat is made from resistance wire wound on a heat-resisting cylinder, with

the slider made from a number of metal fingers that grip lightly onto a small

portion of the turns of resistance wire. The "fingers" can be moved along the

coil of resistance wire by a sliding knob thus changing the "tapping" point.

Wire-wound rheostats made with ratings up to several thousand watts are used

in applications such as DC motor drives, electric welding controls, or in the

controls for generators. The rating of the rheostat is given with the full

resistance value and the allowable power dissipation is proportional to the

fraction of the total device resistance in circuit.

FIG 3.28: RHEOSTAT

3.8 POTENTIOMETER

A common element in electronic devices is a three-terminal resistor with a

continuously adjustable tapping point controlled by rotation of a shaft or knob.

These variable resistors are known as potentiometers when all three terminals

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are present, since they act as a continuously adjustable voltage divider. A

common example is a volume control for a radio receiver.

Accurate, high-resolution panel-mounted potentiometers (or "pots") have

resistance elements typically wire wound on a helical mandrel, although some

include a conductive-plastic resistance coating over the wire to improve

resolution. These typically offer ten turns of their shafts to cover their full

range. They are usually set with dials that include a simple turn’s counter and a

graduated dial. Electronic analog computers used them in quantity for setting

coefficients, and delayed-sweep oscilloscopes of recent decades included one

on their panels.

FIG 3.29: POTENTIOMETER

Potentiometers are commonly used to control electrical devices such as volume

controls on audio equipment. Potentiometers operated by a mechanism can be

used as position transducers, for example, in a joystick. Potentiometers are

rarely used to directly control significant power (more than a watt), since the

power dissipated in the potentiometer would be comparable to the power in the

controlled load.

3.8.1 POTENTIOMETER CONSTRUCTION

Potentiometers comprise a resistive element, a sliding contact (wiper) that

moves along the element, making good electrical contact with one part of it,

electrical terminals at each end of the element, a mechanism that moves the

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wiper from one end to the other, and a housing containing the element and

wiper.

Many inexpensive potentiometers are constructed with a resistive element

formed into an arc of a circle usually a little less than a full turn, and a wiper

rotating around the arc and contacting it. The resistive element, with a terminal

at each end, is flat or angled. The wiper is connected to a third terminal,

usually between the other two. On panel potentiometers, the wiper is usually

the center terminal of three. For single-turn potentiometers, this wiper typically

travels just under one revolution around the contact. The only point of ingress

for contamination is the narrow space between the shaft and the housing it

rotates in.

Another type is the linear slider potentiometer, which has a wiper which slides

along a linear element instead of rotating. Contamination can potentially enter

anywhere along the slot the slider moves in, making effective sealing more

difficult and compromising long-term reliability. An advantage of the slider

potentiometer is that the slider position gives a visual indication of its setting.

While the setting of a rotary potentiometer can be seen by the position of a

marking on the knob, an array of sliders can give a visual impression of, for

example, the effect of a multi-channel equaliser.

The resistive element of inexpensive potentiometers is often made of graphite.

Other materials used include resistance wire, carbon particles in plastic, and a

ceramic/metal mixture called cermet. Conductive track potentiometers use

conductive polymer resistor pastes that contain hard-wearing resins and

polymers, solvents, and lubricant, in addition to the carbon that provides the

conductive properties. Others are enclosed within the equipment and are

intended to be adjusted to calibrate equipment during manufacture or repair,

and not otherwise touched. They are usually physically much smaller than

user-accessible potentiometers, and may need to be operated by a screwdriver

rather than having a knob. They are usually called "preset potentiometers".

Some presets are accessible by a small screwdriver poked through a hole in the

case to allow servicing without dismantling.

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Multiturn potentiometers are also operated by rotating a shaft, but by several

turns rather than less than a full turn. Some multiturn potentiometers have a

linear resistive element with a slider which moves along it moved by a worm

gear; others have a helical resistive element and a wiper that turns through 10,

20, or more complete revolutions, moving along the helix as it rotates.

Multiturn potentiometers, both user-accessible and preset, allow finer

adjustments; rotation through the same angle changes the setting by typically a

tenth as much as for a simple rotary potentiometer.

A string potentiometer is a multi-turn potentiometer operated by an attached

reel of wire turning against a spring, enabling it to convert linear position to a

variable resistance.

User-accessible rotary potentiometers can be fitted with a switch which

operates usually at the anti-clockwise extreme of rotation. Before digital

electronics became the norm such a component was used to allow radio and

television receivers and other equipment to be switched on at minimum

volume with an audible click, then the volume increased, by turning a knob.

Multiple resistance elements can be ganged together and controlled by the

same shaft, for example, in stereo audio amplifiers for volume control.

3.8.2 10K POTENTIOMETER

Potentiometers are incredibly useful, whether you're controlling the volume on your

stereo or the 'mood lighting' in your room. The problem with traditional potentiometers is

the fact that your microcontroller doesn't have an easy way to interface with them. Digital

Potentiometers solve that problem by allowing you to control a voltage splitter with

digital signals.

Wire it up just like a potentiometer and use serial signals to 'turn the knob.' Another

handy feature of digital potentiometers is that because they aren't controlled

mechanically, they don't have a pre-determined sweep profile. In other words, depending

on the way you write your code the potentiometer can 'sweep' in a linear fashion, a

logarithmic fashion, or according to any other profile you like. Digital potentiometers can

also be used in conjunction with rotary encoders to consolidate large banks of

potentiometers into one 'smart' rotary control.

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CHAPTER 4

CIRCUIT DIAGRAM

FIG 4.1 CIRCUIT DIAGRAM OF CV-CC DC POWER SUPPLY

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4.1 WORKING PRINCIPLE

Unregulated power supply is derived from a step down transformer, a

diode bridge rectifier consisting of diodes D1.D2, D3 and D4 and a filter

capacitor. The unregulated DC input is connected to transistor Q1 which acts as

series regulator element, the voltage drop across this depends upon the input

voltage and the regulated output voltage. Q1 is biased through 100 ohms

resistor. Collectors of transistors Q2 and Q3 which act as control elements are

connected to base of Q1.While resistor R sets the current limit, Potentiometer

and resistor voltage divider arrangement connected to the base of Q2 regulates

the output voltage. Zener Z limits the lowest output voltage which is given by

zener voltage + 0.7 volts. The current limit is set by value fixed by quotient

(0.7/R).

Output voltage is set by adjusting the potentiometer. When the base voltage

exceeds 5.4 volts the regulating action of transistor Q2 starts which bypasses

the base current of Q1. Similarly the current limiting action of Q3 starts when

voltage across R exceeds 0.7 Volts. By selecting suitable value of R the desired

current limit can be set.

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CHAPTER 5

RESULT

The power supply operates thus in the CV mode as. However, if one

decreases the resistance such that the current exceeds the maximum allowable

value is, the current will be limited to be and the power supply operates in

the CC mode. The resistance RC=Vs/is is called the critical resistance and

determines whether one operates in the CV (RL>RC) or CC (RL>RC) mode.

FUTURE SCOPE

In future all the electronic equipments working with the specific voltage

and current may be fixed with this constant voltage and constant current circuit

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CHAPTER 6

ADVANTAGES

Protection against over voltages and over currents

Less ripples and noise

Provides limited amount of voltage and current

BIBLIOGRAPHY

www.wikipedia.org

www.seminars.com

www.123seminars.com

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