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A linear regulated power supply will be constructed to give 12v regulated DC output at 1A. The controller will work in the CV mode at a load of 0-1A and will work in the CC mode if load resistance is decreased further. The project will deal with the design aspects of a linear power supply. The mode of changing of the power supply depends on the critical resistance.
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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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CV_CC DC POWERSUPPLY
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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CV_CC DC POWERSUPPLY
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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CV_CC DC POWERSUPPLY
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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CV_CC DC POWERSUPPLY
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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CV_CC DC POWERSUPPLY
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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CV_CC DC POWERSUPPLY
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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CV_CC DC POWERSUPPLY
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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CV_CC DC POWERSUPPLY
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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CV_CC DC POWERSUPPLY
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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