temperature dependent dc fan speed controller withou using micrcontroller
1. CHAPTER 1
1.1 Project Background
Sometimes electric fan usage is wasting power because of human attitude.
Human also mostly demands something that easily to be used without wasting
energy. To minimize or reduce the power usage, this project developed an
automatic fan system where speed is controlled by the room temperature.
1.2 Problem Statement
Most human feels the inconvenient about changing the fan speed level
manually when the room temperature changes. So, the automatic fan system
that automatically changes the speed level according to temperature changes is
recommended to be built for solving this problem.
1.3 Project Objectives
The objectives of this project are to:
i. Enable the electric fan to automatically change the speed level according to
ii. Develop an automatic fan system that can change the speed level due to the
environment temperature changes.
iii. Develop an automatic fan system that can preview the status of the
temperature and the speed level by using Liquid Crystal Display(LCD).
2. 1.4 Project Scopes
The system is built using:
i. Temperature sensor LM35
ii. IC 3914
iii. Voltage regulator
iv. The DC motor as the output for the system.
4. 2.4 Working
This temperature dependent controller fan speed uses an LM35 temperature
sensor IC for precision sensing of the temperature. The output voltage of LM35 varies
linearly with the temperature changes in degree Celsius. Low output impedence,
linear output variation with input changes and precise inherent calibration of LM35
make interfacing of this device to read out or control circuitry easy. LM35 exhibits
extremely low self heating, as it draws only 60UA from the power supply and
operates over a wide temperature range of -50 to +150 C.
The output of LM35 is fed to LM3914 IC, which sense the analogue voltage
levels and drives five relay. Suddenly changes the fan speed corresponding to the rise
in temperature. The output pins of IC LM3914 are pulled high to Vcc. LM3914
contains its own adjustable reference and an accurate 10 step voltage divider network.
The buffer drives ten individual camparators referenced to the precision potential
When the power supply is applied to circuit, Its goes to bridge regtifier which
convert the ac cruuect into dc current. A voltage regulator is also connected across the
rectifier which regulate the voltage at 5 volt. Now we can heat the LM35 temperature
sensor, the output pin of the sensor is connected to the 5th pin of LM3914 IC. Finally
when the sensor temperature reaches 39 degree Celsius, pin 10 goes low to energies
relay RL5. As result the fan starts moving at the maximum speed because in this
position all the relays are energized. When the temperature decreases below 39 degree
Celsius the speed of fan is also decreases.
5. CHAPTER 3
An integrated circuit or monolithic integrated circuit (also referred to as
an IC, a chip, or a microchip) is a set of electronic circuits on one small plate ("chip")
of semiconductor material, normally silicon. This can be made much smaller than
a discrete circuit made from independent components. ICs can be made very compact,
having up to several billion transistors and other electronic components in an area the
size of a fingernail. The width of each conducting line in a circuit can be made smaller
and smaller as the technology advances; in 2008 it dropped below 100 nanometer, and
now is tens of nanometers.
ICs were made possible by experimental discoveries showing that semiconductor
devices could perform the functions of vacuum tubesand by mid-20th-century
technology advancements in semiconductor device fabrication. The integration of large
numbers of tinytransistors into a small chip was an enormous improvement over the
manual assembly of circuits using discrete electronic components. The integrated
circuit's mass production capability, reliability and building-block approach to circuit
design ensured the rapid adoption of standardized integrated circuits in place of designs
using discrete transistors.
ICs have two main advantages over discrete circuits: cost and performance. Cost is low
because the chips, with all their components, are printed as a unit
by photolithography rather than being constructed one transistor at a time.
Furthermore, packaged ICs use much less material than discrete circuits. Performance
is high because the IC's components switch quickly and consume little power
(compared to their discrete counterparts) as a result of the small size and close
proximity of the components. As of 2012, typical chip areas range from a few square
millimeters to around 450 mm2, with up to 9 million transistors per mm2.
Integrated circuits are used in virtually all electronic equipment today and have
revolutionized the world of electronics. Computers, mobile phones, and other
digital home appliances are now inextricable parts of the structure of modern societies,
made possible by the low cost of integrated circuits.
6. 3.2 LM3914 IC
The LM3914 is an integrated circuit (IC) designed by National
Semiconductor and used to operate displays that visually show the magnitude of
an analog signal.
One LM3914 can drive up to 10 LEDs, LCDs, or vacuum fluorescent displays on its
outputs. The linear scaling of the output thresholds makes the device usable, for
example, as a voltmeter. In the basic configuration it provides a ten step scale which is
expandable to over 100 segments with other LM3914 ICs in series.
This IC was introduced by National Semiconductor in 1980 and is still available as of
2013 from Texas Instruments. There are also two variants of it produced, the only
difference being using 3dB logarithmic (LM3915) or VU-meter (LM3916) scale.
7. 3.2.1 FEATURES
All the devices in this group operate with a range of voltages from 3-35 V, can
drive LED and VFD displays. They can provide a regulated output current between
2-30 mA to directly drive displays.
Internally, each device contains ten comparators and a resistor scaling network, as well
as a 1.25 volt reference source. As the input voltage increases, each comparator turns
on. The device can be configured for either a bar-graph mode, where all lower-output
terminals switch on, or "dot" mode in which only one output goes on. The device is
packaged in an 18 pin dual in-line package or in a surface mount leadless chip carrier.
3.3 ADVANTAGES OF IC’S
1. Very small size.
2. Low Cost.
3. Reduced Power Consumption.
4. Higher reliable.
5. Higher operating speed.
6. Reduced external wiring connections.
8. CHAPTER 4
Temperature sensors are vital to a variety of everyday products. For example,
household ovens, refrigerators, and thermostats all rely on temperature maintenance
and control in order to function properly. Temperature control also has applications in
chemical engineering. Examples of this include maintaining the temperature of a
chemical reactor at the ideal set-point, monitoring the temperature of a possible
runaway reaction to ensure the safety of employees, and maintaining the temperature of
streams released to the environment to minimize harmful environmental impact.
While temperature is generally sensed by humans as “hot”, “neutral”, or “cold”,
chemical engineering requires precise, quantitative measurements of temperature in
order to accurately control a process. This is achieved through the use of temperature
sensors, and temperature regulators which process the signals they receive from
From a thermodynamics perspective, temperature changes as a function of the average
energy of molecular movement. As heat is added to a system, molecular motion
increases and the system experiences an increase in temperature. It is difficult,
however, to directly measure the energy of molecular movement, so temperature
sensors are generally designed to measure a property which changes in response to
temperature. The devices are then calibrated to traditional temperature scales using a
standard (i.e. the boiling point of water at known pressure). The following sections
discuss the various types of sensors and regulators.
Temperature sensors are devices used to measure the temperature of a medium.
There are 2 kinds on temperature sensors: 1) contact sensors and 2) noncontact sensors.
However, the 3 main types are thermometers, resistance temperature detectors, and
thermocouples. All three of these sensors measure a physical property (i.e. volume of a
liquid, current through a wire), which changes as a function of temperature. In addition
to the 3 main types of temperature sensors, there are numerous other temperature
sensors available for use.
9. Contact Sensors
Contact temperature sensors measure the temperature of the object to which the sensor
is in contact by assuming or knowing that the two (sensor and the object) are in thermal
equilibrium, in other words, there is no heat flow between them.
Examples (further description of each example provide below)
Resistance Temperature Detectors (RTDs)
Full System Thermometers
Most commercial and scientific noncontact temperature sensors measure the thermal
radiant power of the Infrared or Optical radiation received from a known or calculated
area on its surface or volume within it.
An example of noncontact temperature sensors is a pyrometer, which is described into
further detail at the bottom of this section.
4.2 lm35 Sensor
The LM35 is an integrated circuit sensor that can be used to measure
temperature with an electrical output proportional to the temperature (in oC). The
LM35 temperature sensor measure temperature more accurately than using a
thermistor. The sensor circuitry is sealed and not subject to oxidation, etc. The LM35
generates higher output voltage than thermocouples and may not require that the outpur
voltage be amplified. It has an output voltage that is proportional to the Celsius
temperature. The scale factor of LM35 is 0.1 V/ oC. The LM35 draws only 60 micro
amps from its supply and possesses a low self heating capability. The sensor self
heating causes less than 0.1 oC temperature rise in still air.
11. CHAPTER 5
A rectifier is an electrical device that converts alternating current (AC), which
periodically reverses direction, to direct current (DC), which flows in only one
direction. The process is known as rectification. Physically, rectifiers take a number of
forms, including vacuum tube diodes, mercury-arc valves, copper and selenium oxide
rectifiers, semiconductor diodes, silicon-controlled rectifiers and other silicon-based
semiconductor switches. Historically, even synchronous electromechanical switches
and motors have been used. Early radio receivers, called crystal radios, used a "cat's
whisker" of fine wire pressing on a crystal of galena (lead sulfide) to serve as a
point-contact rectifier or "crystal detector".
Rectifiers have many uses, but are often found serving as components of DC power
supplies and high-voltage direct current power transmission systems. Rectification
may serve in roles other than to generate direct current for use as a source of power. As
noted,detectors of radio signals serve as rectifiers. In gas heating systems flame
rectification is used to detect presence of a flame.
Because of the alternating nature of the input AC sine wave, the process of rectification
alone produces a DC current that, though unidirectional, consists of pulses of current.
Many applications of rectifiers, such as power supplies for radio, television and
computer equipment, require a steady constant DC current (as would be produced by
a battery). In these applications the output of the rectifier is smoothed by an electronic
filter (usually a capacitor) to produce a steady current.
Before the development of silicon semiconductor rectifiers, vacuum
tube thermionic diodes and copper oxide- or selenium-based metal rectifier stacks
were used. With the introduction of semiconductor electronics, vacuum tube rectifiers
12. became obsolete, except for some enthusiasts of vacuum tube audio equipment. For
power rectification from very low to very high current, semiconductor diodes of
various types (junction diodes, Schottky diodes, etc.) are widely used.
Other devices that have control electrodes as well as acting as unidirectional current
valves are used where more than simple rectification is required—e.g., where variable
output voltage is needed. High-power rectifiers, such as those used in high-voltage
direct current power transmission, employ silicon semiconductor devices of various
types. These are thyristors or other controlled switching solid-state switches, which
effectively function as diodes to pass current in only one direction.
5.2 RECTIFIER CIRCUITS
Rectifier circuits may be single-phase or multi-phase (three being the most
common number of phases). Most low power rectifiers for domestic equipment are
single-phase, but three-phase rectification is very important for industrial applications
and for the transmission of energy as DC (HVDC).
5.2.1 Single-phase rectifiers
In half wave rectification of a single-phase supply, either the positive or
negative half of the AC wave is passed, while the other half is blocked. Because only
one half of the input waveform reaches the output, mean voltage is lower. Half-wave
rectification requires a single diode in a single-phase supply, or three in a three-phase
supply. Rectifiers yield a unidirectional but pulsating direct current; half-wave
rectifiers produce far more ripple than full-wave rectifiers, and much more filtering is
needed to eliminate harmonics of the AC frequency from the output.
13. Half-wave rectifier
The no-load output DC voltage of an ideal half wave rectifier for a sinusoidal input
Vdc, Vav - the DC or average output voltage,
Vpeak, the peak value of the phase input voltages,
Vrms, the root-mean-square value of output voltage.
A full-wave rectifier converts the whole of the input waveform to one of
constant polarity (positive or negative) at its output. Full-wave rectification converts
both polarities of the input waveform to pulsating DC (direct current), and yields a
higher average output voltage. Two diodes and a center tapped transformer, or four
diodes in a bridge configuration and any AC source (including a transformer without
center tap), are needed. Single semiconductor diodes, double diodes with common
cathode or common anode, and four-diode bridges, are manufactured as single
Graetz bridge rectifier: a full-wave rectifier using 4 diodes.
For single-phase AC, if the transformer is center-tapped, then two diodes
back-to-back (cathode-to-cathode or anode-to-anode, depending upon output polarity
required) can form a full-wave rectifier. Twice as many turns are required on the
transformer secondary to obtain the same output voltage than for a bridge rectifier, but
the power rating is unchanged.
14. 5.2.2 Three-phase rectifiers
3-phase AC input, half and full-wave rectified DC output waveforms
Single-phase rectifiers are commonly used for power supplies for domestic equipment.
However, for most industrial and high-power applications, three-phase rectifier circuits
are the norm. As with single-phase rectifiers, three-phase rectifiers can take the form of
a half-wave circuit, a full-wave circuit using a center-tapped transformer, or a full-wave
Thyristors are commonly used in place of diodes to create a circuit that can regulate the
output voltage. Many devices that provide direct current actually generate three-phase
AC. For example, an automobile alternator contains six diodes, which function as a
full-wave rectifier for battery charging.
Three-phase, half-wave circuit
An uncontrolled three-phase, half-wave circuit requires three diodes, one
connected to each phase. This is the simplest type of three-phase rectifier but suffers
from relatively high harmonic distortion on both the AC and DC connections. This type
of rectifier is said to have a pulse-number of three, since the output voltage on the DC
side contains three distinct pulses per cycle of the grid frequency.
15. Three-phase, full-wave circuit using center-tapped transformer
If the AC supply is fed via a transformer with a center tap, a rectifier
circuit with improved harmonic performance can be obtained. This rectifier now
requires six diodes, one connected to each end of each transformer secondary winding.
This circuit has a pulse-number of six, and in effect, can be thought of as a six-phase,
Before solid state devices became available, the half-wave circuit, and the full-wave
circuit using a center-tapped transformer, were very commonly used in industrial
rectifiers using mercury-arc valves. This was because the three or six AC supply inputs
could be fed to a corresponding number of anode electrodes on a single tank, sharing a
With the advent of diodes and thyristors, these circuits have become less popular and
the three-phase bridge circuit has become the most common circuit.
Three-phase half-wave rectifier circuit using
thyristors as the switching elements, ignoring
Three-phase full-wave rectifier circuit using
thyristors as the switching elements, with a
center-tapped transformer, ignoring supply
16. 5.3 APPLICATION’S
The primary application of rectifiers is to derive DC power from an AC supply (AC
to DC converter). Virtually all electronic devices require DC, so rectifiers are used
inside the power supplies of virtually all electronic equipment.
Converting DC power from one voltage to another is much more complicated. One
method of DC-to-DC conversion first converts power to AC (using a device called
an inverter), then uses a transformer to change the voltage, and finally rectifies power
back to DC. A frequency of typically several tens of kilohertz is used, as this requires
much smaller inductance than at lower frequencies and obviates the use of heavy,
bulky, and expensive iron-cored units.
Rectifiers are also used for detection of amplitude modulated radio signals. The signal
may be amplified before detection. If not, a very low voltage drop diode or a diode
biased with a fixed voltage must be used. When using a rectifier for demodulation the
capacitor and load resistance must be carefully matched: too low a capacitance makes
the high frequency carrier pass to the output, and too high makes the capacitor just
charge and staying charged.
Rectifiers supply polarised voltage for welding. In such circuits control of the output
current is required; this is sometimes achieved by replacing some of the diodes in
a bridge rectifier with thyristors, effectively diodes whose voltage output can be
regulated by switching on and off with phase fired controllers.
Thyristors are used in various classes of railway rolling stock systems so that fine
control of the traction motors can be achieved. Gate turn-off thyristors are used to
produce alternating current from a DC supply, for example on the Eurostar Trains to
power the three-phase traction motors.
17. CHAPTER 6
A voltage regulator is designed to automatically maintain a constant
voltage level. A voltage regulator may be a simple "feed-forward" design or may
include negative feedback control loops. It may use an electromechanical mechanism, or
electronic components. Depending on the design, it may be used to regulate one or
more AC or DC voltages.
Electronic voltage regulators are found in devices such as computer power supplies where
they stabilize the DC voltages used by the processor and other elements. In
automobile alternators and central power station generator plants, voltage regulators
control the output of the plant. In an electric power distribution system, voltage regulators
may be installed at a substation or along distribution lines so that all customers receive
steady voltage independent of how much power is drawn from the line.
18. 6.2 Electronic Voltage Regulators
A simple voltage regulator can be made from a resistor in series with a diode (or
series of diodes). Due to the logarithmic shape of diode V-I curves, the voltage across
the diode changes only slightly due to changes in current drawn or changes in the input.
When precise voltage control and efficiency are not important, this design may work
Feedback voltage regulators operate by comparing the actual output voltage to some
fixed reference voltage. Any difference is amplified and used to control the regulation
element in such a way as to reduce the voltage error. This forms a negative
feedback control loop; increasing the open-loop gain tends to increase regulation
accuracy but reduce stability. (Stability is avoidance of oscillation, or ringing, during
step changes.) There will also be a trade-off between stability and the speed of the
response to changes. If the output voltage is too low (perhaps due to input voltage
reducing or load current increasing), the regulation element is commanded, up to a
point, to produce a higher output voltage–by dropping less of the input voltage (for
linear series regulators and buck switching regulators), or to draw input current for
longer periods (boost-type switching regulators); if the output voltage is too high, the
regulation element will normally be commanded to produce a lower voltage. However,
many regulators have over-current protection, so that they will entirely stop sourcing
current (or limit the current in some way) if the output current is too high, and some
regulators may also shut down if the input voltage is outside a given range.
19. 6.3 Electromechanical Regulators
In electromechanical regulators, voltage regulation is easily
accomplished by coiling the sensing wire to make an electromagnet. The magnetic
field produced by the current attracts a moving ferrous core held back under spring
tension or gravitational pull. As voltage increases, so does the current, strengthening
the magnetic field produced by the coil and pulling the core towards the field. The
magnet is physically connected to a mechanical power switch, which opens as the
magnet moves into the field. As voltage decreases, so does the current, releasing spring
tension or the weight of the core and causing it to retract. This closes the switch and
allows the power to flow once more.
If the mechanical regulator design is sensitive to small voltage fluctuations, the motion
of the solenoid core can be used to move a selector switch across a range of resistances
or transformer windings to gradually step the output voltage up or down, or to rotate the
position of a moving-coil AC regulator.
Early automobile generators and alternators had a mechanical voltage regulator using
one, two, or three relays and various resistors to stabilize the generator's output at
slightly more than 6 or 12 V, independent of the engine's rpm or the varying load on the
vehicle's electrical system. Essentially, the relay(s) employed pulse width
modulation to regulate the output of the generator, controlling the field current reaching
the generator (or alternator) and in this way controlling the output voltage produced.
The regulators used for DC generators (but not alternators) also disconnect the
generator when it was not producing electricity, thereby preventing the battery from
discharging back into the generator and attempting to run it as a motor. The
rectifier diodes in an alternator automatically perform this function so that a specific
relay is not required; this appreciably simplified the regulator design.
20. 6.4 Automatic Voltage Regulator
To control the output of generators (as seen in ships and power stations, or
on oil rigs, greenhouses and emergency power systems) automatic voltage regulators
are used. This is an active system. While the basic principle is the same, the system
itself is more complex. An automatic voltage regulator (or AVR for short) consist of
several components such as diodes, capacitors, resistors and potentiometers or even
microcontrollers, all placed on a circuit board. This is then mounted near the generator
and connected with several wires to measure and adjust the generator.
How an AVR works: In the first place the AVR monitors the output voltage and
controls the input voltage for the exciter of the generator. By increasing or decreasing
the generator control voltage, the output voltage of the generator increases or decreases
accordingly. The AVR calculates how much voltage has to be sent to the exciter
numerous times a second, therefore stabilizing the output voltage to a predetermined
setpoint. When two or more generators are powering the same system (parallel
operation) the AVR receives information from more generators to match all output.
21. CHAPTER 7
In electronics, a diode is a two-terminal electronic component with
asymmetric conductance; it has low (ideally zero) resistance to current 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 has two electrodes, aplate (anode) and a heated
cathode. Semiconductor 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 selenium or germanium are sometimes used.
22. 7.2 Vacuum tube diodes
In 1873, Frederick Guthrie discovered the basic principle of operation
of thermionic diodes. Guthrie discovered that a positively charged electroscope could
be discharged by bringing a grounded piece of white-hot metal close to it (but not
actually touching it). The same did not apply to a negatively charged electroscope,
indicating that the current flow was only possible in one direction.
Thomas Edison independently rediscovered the principle on February 13,
1880. At the time, Edison was investigating why the filaments of his carbon-filament
light bulbs nearly always burned out at the positive-connected end. He had a special
bulb made with a metal plate sealed into the glass envelope. Using this device, he
confirmed that an invisible current flowed from the glowing filament through
the vacuum to the metal plate, but only when the plate was connected to the positive
Edison devised a circuit where his modified light bulb effectively replaced the resistor
in a DC voltmeter. Edison was awarded a patent for this invention in 1884. Since there
was no apparent practical use for such a device at the time, the patent application was
most likely simply a precaution in case someone else did find a use for the
so-called Edison effect.
About 20 years later, John Ambrose Fleming (scientific adviser to the Marconi
Company and former Edison employee) realized that the Edison effect could be used as
a precision radio detector. Fleming patented the first true thermionic diode, the Fleming
valve, in Britain on November 16, 1904 (followed by U.S. Patent 803,684 in
23. 7.3 Solid-state diodes
In 1874 German scientist Karl Ferdinand Braun discovered the
"unilateral conduction" of crystals. Braun patented the crystal rectifier in 1899. Copper
oxide and selenium rectifiers were developed for power applications in the 1930s.
Indian scientist Jagadish Chandra Bose was the first to use a crystal for
detecting radio waves in 1894. The crystal detector was developed into a practical
device for wireless telegraphy by Greenleaf Whittier Pickard, who invented
a silicon crystal detector in 1903 and received a patent for it on November 20, 1906.
Other experimenters tried a variety of other substances, of which the most widely used
was the mineral galena (lead sulfide). Other substances offered slightly better
performance, but galena was most widely used because it had the advantage of being
cheap and easy to obtain. The crystal detector in these early crystal radio sets consisted
of an adjustable wire point-contact (the so-called "cat's whisker"), which could be
manually moved over the face of the crystal in order to obtain optimum signal. This
troublesome device was superseded by thermionic diodes by the 1920s, but after high
purity semiconductor materials became available, the crystal detector returned to
dominant use with the advent of inexpensive fixed-germanium diodes in the
1950s. Bell Labs also developed a germanium diode for microwave reception, and
AT&T used these in their microwave towers that criss-crossed the nation starting in the
late 1940s, carrying telephone and network television signals. Bell Labs did not
develop a satisfactory thermionic diode for microwave reception.
7.4 Point-contact diodes
A point-contact diode works the same as the junction diodes described
below, but their construction is simpler. A block of n-type semiconductor is built, and a
conducting sharp-point contact made with some group-3 metal is placed in contact with
the semiconductor. Some metal migrates into the semiconductor to make a small region
of p-type semiconductor near the contact. The long-popular 1N34 germanium version
is still used in radio receivers as a detector and occasionally in specialized analog
24. 7.5 Junction diodes
7.5.1 p–n junction diode
A p–n junction diode is made of a crystal of semiconductor, usually
silicon, but germanium and gallium arsenide are also used. 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. This region is called
the depletion region due to the absence of charge carriers (electrons and holes in this
case). The diode's terminals are attached to the n-type and p-type regions. The boundary
between these two regions, called a p–n junction, is where the action of the diode takes
place. The crystal 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.
7.5.2 Schottky 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.
25. CHAPTER 8
A capacitor (originally known as a condenser) is
a passive two-terminal electrical component used to store energyelectrostatically in
an electric field. The forms of practical capacitors vary widely, but all contain at least
two electrical conductors (plates) separated by a dielectric (i.e. insulator). The
conductors can be thin films, foils or sintered beads of metal or conductive electrolyte,
etc. The "nonconducting" dielectric acts to increase the capacitor's charge capacity. A
dielectric can be glass, ceramic, plastic film, air, vacuum, paper, mica, oxide layer etc.
Capacitors are widely used as parts of electrical circuits in many common electrical
devices. Unlike a resistor, an ideal capacitor does not dissipate energy. Instead, a
capacitor stores energy in the form of an electrostatic field between its plates.
When there is a potential difference across the conductors (e.g., when a capacitor is
attached across a battery), an electric field develops across the dielectric, causing
positive charge +Q to collect on one plate and negative charge −Q to collect on the
other plate. If a battery has been attached to a capacitor for a sufficient amount of time,
no current can flow through the capacitor. However, if a time-varying voltage is applied
across the leads of the capacitor, a displacement current can flow.
An ideal capacitor is characterized by a single constant value for its capacitance.
Capacitance is expressed as the ratio of the electric charge Q on each conductor to the
potential difference V between them. The SI unit of capacitance is the farad (F), which
is equal to one coulomb per volt (1 C/V). Typical capacitance values range from about
1 pF (10−12 F) to about 1 mF (10−3 F).
The capacitance is greater when there is a narrower separation between conductors and
when the conductors have a larger surface area. In practice, the dielectric between the
plates passes a small amount of leakage current and also has an electric field strength
limit, known as the breakdown voltage. The conductors and leads introduce an
undesired inductance and resistance.
26. 8.2 OPERATION
A capacitor consists of two conductors separated by a non-conductive
region. The non-conductive region is called the dielectric. In simpler terms, the
dielectric is just an electrical insulator. Examples of dielectric media are glass, air,
paper, vacuum, and even a semiconductor depletion region chemically identical to the
conductors. A capacitor is assumed to be self-contained and isolated, with no
net electric charge and no influence from any external electric field. The conductors
thus hold equal and opposite charges on their facing surfaces, and the dielectric
develops an electric field. In SI units, a capacitance of one farad means that one
coulomb of charge on each conductor causes a voltage of one volt across the device.
An ideal capacitor is wholly characterized by a constant capacitance C, defined as the
ratio of charge ±Q on each conductor to the voltage V between them:
Because the conductors (or plates) are close together, the opposite charges on the
conductors attract one another due to their electric fields, allowing the capacitor to
store more charge for a given voltage than if the conductors were separated, giving
the capacitor a large capacitance.
Sometimes charge build-up affects the capacitor mechanically, causing its
capacitance to vary. In this case, capacitance is defined in terms of incremental
8.3 Hydraulic analogy
In the hydraulic analogy, charge carriers flowing through a wire are
analogous to water flowing through a pipe. A capacitor is like a rubber membrane
27. sealed inside a pipe. Water molecules cannot pass through the membrane, but some
water can move by stretching the membrane. The analogy clarifies a few aspects of
The current alters the charge on a capacitor, just as the flow of water changes
the position of the membrane. More specifically, the effect of an electric current is
to increase the charge of one plate of the capacitor, and decrease the charge of the
other plate by an equal amount. This is just as when water flow moves the rubber
membrane, it increases the amount of water on one side of the membrane, and
decreases the amount of water on the other side.
The more a capacitor is charged, the larger its voltage drop; i.e., the more it
"pushes back" against the charging current. This is analogous to the fact that the
more a membrane is stretched, the more it pushes back on the water.
Charge can flow "through" a capacitor even though no individual electron can
get from one side to the other. This is analogous to the fact that water can flow
through the pipe even though no water molecule can pass through the rubber
membrane. Of course, the flow cannot continue in the same direction forever; the
capacitor will experience dielectric breakdown, and analogously the membrane
will eventually break.
The capacitance describes how much charge can be stored on one plate of a
capacitor for a given "push" (voltage drop). A very stretchy, flexible membrane
corresponds to a higher capacitance than a stiff membrane.
A charged-up capacitor is storing potential energy, analogously to a stretched
8.4 Energy of electric field
Work must be done by an external influence to "move" charge between the
conductors in a capacitor. When the external influence is removed, the charge
separation persists in the electric field and energy is stored to be released when the
charge is allowed to return to its equilibrium position. The work done in establishing
the electric field, and hence the amount of energy stored, is
28. Here Q is the charge stored in the capacitor, V is the voltage across the capacitor,
and C is the capacitance.
In the case of a fluctuating voltage V(t), the stored energy also fluctuates and
hence power must flow into or out of the capacitor. This power can be found by
taking the time derivative of the stored energy:
8.5 Current–voltage relation
The current I(t) through any component in an electric circuit is defined as
the rate of flow of a charge Q(t) passing through it, but actual
charges—electrons—cannot pass through the dielectric layer of a capacitor. Rather,
one electron accumulates on the negative plate for each one that leaves the positive
plate, resulting in an electron depletion and consequent positive charge on one
electrode that is equal and opposite to the accumulated negative charge on the other.
Thus the charge on the electrodes is equal to the integral of the current as well as
proportional to the voltage, as discussed above. As with any antiderivative, a constant
of integration is added to represent the initial voltage V(t0). This is the integral form of
the capacitor equation:
Taking the derivative of this and multiplying by C yields the derivative form:
The dual of the capacitor is the inductor, which stores energy in a magnetic
field rather than an electric field. Its current-voltage relation is obtained by
exchanging current and voltage in the capacitor equations and replacing C with
the inductance L.
29. 8.6 APPLICATION
8.6.1 Energy storage
A capacitor can store electric energy when disconnected from its
charging circuit, so it can be used like a temporary battery, or like other types
of rechargeable energy storage system. 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.
8.6.2 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-bridgewire detonators or slapper detonators in nuclear weapons and
other specialty weapons. Experimental work is under way using banks of capacitors as
power sources for electromagneticarmour and electromagnetic railguns and coilguns.
8.6.3 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 generation of higher voltages than the input voltage.
30. Capacitors are connected in parallel with the power circuits of most electronic devices
and larger systems (such as factories) to shunt away and conceal current fluctuations
from the primary power source to provide a "clean" power supply for signal or control
circuits. Audio equipment, for example, uses several capacitors in this way, to shunt
away power line hum before it gets into the signal circuitry. The capacitors act as a
local reserve for the DC power source, and bypass AC currents from the power supply.
This is used in car audio applications, when a stiffening capacitor compensates for the
inductance and resistance of the leads to the lead-acid car battery.
31. CHAPTER 9
A transistor is a semiconductor device used
to amplify and switch electronic signals and electrical power. It is composed
of semiconductor material with at least three terminals for connection to an external
circuit. A voltage or current applied to one pair of the transistor's terminals changes the
current through another pair of terminals. Because the controlled (output) power can be
higher than the controlling (input) power, a transistor can amplify a signal. Today,
some transistors are packaged individually, but many more are found embedded
in integrated circuits.
The transistor is the fundamental building block of modern electronic devices, and is
ubiquitous in modern electronic systems. Following its development in 1947 by
American physicists John Bardeen, Walter Brattain, and William Shockley, the
transistor revolutionized the field of electronics, and paved the way for smaller and
cheaper radios, calculators, and computers, among other things. The transistor is on the
list of IEEE milestones in electronics, and the inventors were jointly awarded the
1956 Nobel Prize in Physics for their achievement.
32. 9.2 Importance
The transistor is the key active component in practically all
modern electronics. Many consider it to be one of the greatest inventions of the 20th
century. Its importance in today's society rests on its ability to be mass-produced using
a highly automated process (semiconductor device fabrication) that achieves
astonishingly low per-transistor costs. The invention of the first transistor at Bell
Labs was named an IEEE Milestone in 2009.
Although several companies each produce over a billion individually packaged (known
as discrete) transistors every year, the vast majority of transistors are now produced
in integrated circuits (often shortened to IC, microchips or simply chips), along
with diodes, resistors, capacitors and other electronic components, to produce
complete electronic circuits. A logic gate consists of up to about twenty transistors
whereas an advanced microprocessor, as of 2009, can use as many as 3 billion
transistors (MOSFETs). "About 60 million transistors were built in 2002 ... for [each]
man, woman, and child on Earth."
The transistor's low cost, flexibility, and reliability have made it a ubiquito us device.
Transistorized mechatronic circuits have replaced electromechanical devices in
controlling appliances and machinery. It is often easier and cheaper to use a
standard microcontroller and write a computer program to carry out a control function
than to design an equivalent mechanical control function.
33. 9.3 Operation
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
another pair of terminals. This property is called gain. It can produce a stronger output
signal, a voltage or current, that is proportional to a weaker 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.
There are two types of transistors, which have slight differences in how they are used in
a circuit. A bipolar transistor has terminals labeled base, collector, and emitter. A
small current at the base terminal (that is, flowing between the base and the emitter) can
control or switch a much larger current between the collector and emitter terminals. For
a field-effect transistor, the terminals are labeled gate, source, and drain, and a
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.
Because 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.
9.3.1 Transistor as a switch
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 rise exponentially. The collector
voltage drops because of reduced resistance from collector to emitter. If the voltage
difference between the collector and emitter were zero (or near zero), the collector
current would be limited only by the load resistance (light bulb) and the supply voltage.
This is called saturation because current is flowing from collector to emitter freely.
When saturated, the switch is said to be on.
34. 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
In any switching circuit, values of input voltage would be chosen such that the output is
either completely off, 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
9.3.2 Transistor as an amplifier
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 Vout.
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 milli 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
The key advantages that have allowed transistors to replace their vacuum tube
predecessors in most applications are
No power consumption by a cathode heater; the characteristic orange glow of
vacuum tubes is due to a simple electrical heating element, much like a light bulb
35. Small size and minimal weight, allowing the development of miniaturized
Low operating voltages compatible with batteries of only a few cells.
No warm-up period for cathode heaters required after power application.
Lower power dissipation and generally greater energy efficiency.
Higher reliability and greater physical ruggedness.
Extremely long life. Some transistorized devices have been in service for more
than 50 years.
Complementary devices available, facilitating the design
of complementary-symmetry circuits, something not possible with vacuum tubes.
Greatly reduced sensitivity to mechanical shock and vibration, thus reducing
the problem of microphonics in sensitive applications, such as audio.
A resistor is a passive two-terminal electrical component that
implements electrical resistance as a circuit element. Resistors act to reduce current
flow, and, at the same time, act to lower voltage levels within circuits. In electronic
circuits resistors are used to limit current flow, to adjust signal levels, bias active
elements, terminate transmission lines among other uses. High-power resistors that can
dissipate many watts of electrical power as heat may be used as part of motor controls,
in power distribution systems, or as test loads for generators. Resistors may have fixed
resistances that only change a little with temperature, time or operating voltage.
Variable resistors can be used to adjust circuit elements (such as a volume control or a
lamp dimmer), or as sensing devices for heat, light, humidity, force, or chemical
Resistors are common elements of electrical networks and electronic circuits and are
ubiquitous in electronic equipment. Practical resistors as discrete components can be
36. composed of various compounds and forms. Resistors are also implemented within
The electrical function of a resistor is specified by its resistance: common commercial
resistors are manufactured over a range of more than nine orders of magnitude. The
nominal value of the resistance will fall within a manufacturing tolerance.
10.2.1 Ohm's law
The behavior of an ideal resistor is dictated by the relationship specified
by Ohm's law:
Ohm's law states that the voltage (V) across a resistor is proportional to the curre nt
(I), where the constant of proportionality is the resistance (R). For example, if a
300 ohm resistor is attached across the terminals of a 12 volt battery, then a current
of 12 / 300 = 0.04 amperes flows through that resistor.
Practical resistors also have some inductance and capacitance which will also
affect the relation between voltage and current in alternating current circuits.
The ohm (symbol: Ω) is the SI unit of electrical resistance, named after Georg
Simon Ohm. An ohm is equivalent to a volt per ampere. Since resistors are
specified and manufactured over a very large range of values, the derived units of
milliohm (1 mΩ = 10−3 Ω), kilohm (1 kΩ = 103 Ω), and megohm (1 MΩ = 106 Ω)
are also in common usage.
10.2.2 Series and parallel resistors
37. The total resistance of resistors connected in series is the sum of their
individual resistance values.
The total resistance of resistors connected in parallel is the reciprocal of the
sum of the reciprocals of the individual resistors.
So, for example, a 10 ohm resistor connected in parallel with a 5 ohm
resistor and a 15 ohm resistor will produce the inverse of
1/10+1/5+1/15 ohms of resistance, or 1/(.1+.2+.067)=2.725 ohms.
A resistor network that is a combination of parallel and series
connections can be broken up into smaller parts that are either one or
the other. Some complex networks of resistors cannot be resolved in
this manner, requiring more sophisticated circuit analysis. Generally,
the Y-Δ transform, or matrix methods can be used to solve such
38. The value of a resistor can be measured with an ohmmeter, which may be one
function of a multimeter. Usually, probes on the ends of test leads connect to the
resistor. A simple ohmmeter may apply a voltage from a battery across the unknown
resistor (with an internal resistor of a known value in series) producing a current which
drives a meter movement. The current, in accordance with Ohm's law, is inversely
proportional to the sum of the internal resistance and the resistor being tested, resulting
in an analog meter scale which is very non- linear, calibrated from infinity to 0 ohms. A
digital multimeter, using active electronics, may instead pass a specified current
through the test resistance. The voltage generated across the test resistance in that case
is linearly proportional to its resistance, which is measured and displayed. In either case
the low-resistance ranges of the meter pass much more current through the test leads
than do high-resistance ranges, in order for the voltages present to be at reasonable
levels (generally below 10 volts) but still measurable.
Measuring low-value resistors, such as fractional-ohm resistors, with acceptable
accuracy requires four-terminal connections. One pair of terminals applies a known,
calibrated current to the resistor, while the other pair senses the voltage drop across the
resistor. Some laboratory quality ohmmeters, especially milliohmmeters, and even
some of the better digital multimeters sense using four input terminals for this purpose,
which may be used with special test leads. Each of the two so-called Kelvin clips has a
pair of jaws insulated from each other. One side of each clip applies the measuring
current, while the other connections are only to sense the voltage drop. The resistance is
again calculated using Ohm's Law as the measured voltage divided by the applied
39. 11.1 INTRODUCTION
A transformer is an electrical device that transfers energy between two or
more circuits through electromagnetic induction.
A varying current in the transformer's primary winding creates a varying magnetic
flux in the core and a varying magnetic field impinging on the secondary winding. This
varying magnetic field at the secondary induces a varying electromotive force (emf) or
voltage in the secondary winding. Making use of Faraday's Law in conjunction with
high magnetic permeability core properties, transformers can thus be designed to
efficiently change AC voltages from one voltage level to another within power
Transformers range in size from RF transformers less than a cubic centimetre in
volume to units interconnecting the power grid weighing hundreds of tons. A wide
range of transformer designs is encountered in electronic and electric power
applications. Since the invention in 1885 of the first constant potential transformer,
transformers have become essential for the AC transmission, distribution, and
utilization of electrical energy.
11.2.1 Ideal transformer
It is very common, for simplification or approximation purposes, to analyze
the transformer as an ideal transformer model as represented in the two images. An
ideal transformer is a theoretical ,linear transformer that is lossless and
perfectly coupled; that is, there are no energy losses and flux is completely confined
41. within the magnetic core. Perfect coupling implies infinitely high core magnetic
permeability and winding inductances and zero net magnetomotive force.
A varying current in the transformer's primary winding creates a varying magnetic flux
in the core and a varying magnetic field impinging on the secondary winding. This
varying magnetic field at the secondary induces a varying electromotive force (emf) or
voltage in the secondary winding. The primary and secondary windings are wrapped
around a core of infinitely high magnetic permeability[e] so that all of the magnetic flux
passes through both the primary and secondary windings. With a voltage source
connected to the primary winding and load impedance connected to the secondary
winding, the transformer currents flow in the indicated directions.
According to Faraday's law of induction, since the same magnetic flux passes through
both the primary and secondary windings in an ideal transformer, a voltage is induced
in each winding, according to eq. (1) in the secondary winding case, according to eq. (2)
in the primary winding case. The primary emf is sometimes termed counter emf. This
is in accordance with Lenz's law, which states that induction of emf always opposes
development of any such change in magnetic field.
Ideal transformer equations (eq.)
By Faraday's law of induction
. . . (1)[a]
. . . (2)
Combining ratio of (1) & (2)
Turns ratio . . . (3) where
for step-down transformers, a > 1
for step-up transformers, a < 1
By law of Conservation of Energy, apparent,real and reactive power are each
conserved in the input and output
. . . (4)
Combining (3) & (4) with this endnote yields the ideal transformer identity
42. . (5)
By Ohm's Law and ideal transformer identity
. . . (6)
Apparent load impedance Z'L (ZL referred to the primary)
11.2.2 Real transformer
The ideal transformer model assumes that all flux generated by the primary
winding links all the turns of every winding, including itself. In practice, some flux
traverses paths that take it outside the windings. Such flux is termed leakage flux, and
results in leakage inductance in series with the mutually coupled transformer windings.
Leakage flux results in energy being alternately stored in and discharged from the
magnetic fields with each cycle of the power supply. It is not directly a power loss, but
results in inferior voltage regulation, causing the secondary voltage not to be directly
proportional to the primary voltage, particularly under heavy load. Transformers are
therefore normally designed to have very low leakage inductance.
In some applications increased leakage is desired, and long magnetic paths, air gaps, or
magnetic bypass shunts may deliberately be introduced in a transformer design to limit
the short-circuit current it will supply. Leaky transformers may be used to supply loads
that exhibit negative resistance, such as electric arcs, mercury vapor lamps, and neon
signs or for safely handling loads that become periodically short-circuited such
as electric arc welders.
Air gaps are also used to keep a transformer from saturating, especially
audio-frequency transformers in circuits that have a DC component flowing in the
Knowledge of leakage inductance is also useful when transformers are operated in
parallel. It can be shown that if the percent impedance[l] and associated winding leakage
reactance-to-resistance (X/R) ratio of two transformers were hypothetically exactly the
43. same, the transformers would share power in proportion to their respective volt-ampere
ratings (e.g. 500 kVA unit in parallel with 1,000 kVA unit, the larger unit would carry
twice the current). However, the impedance tolerances of commercial transformers are
significant. Also, the Z impedance and X/R ratio of different capacity transformers
tends to vary, corresponding 1,000 kVA and 500 kVA units' values being, to illustrate,
respectively, Z ~ 5.75%, X/R ~ 3.75 and Z ~ 5%, X/R ~ 4.75.
ADVANTAGES & DISADVANTAGES &APPLICATION
44. 1. Low cost
2. Easy to use.
3. Implement in single door.
It is used only when one single purson cuts the rays of the sessor hence it
cannot be used when two person cross simultaneously.
1. For counting process.
2. For automatic room light control.