Monitoring and Controlling a Scientific Computing Infrastructure
power station monitoring and controlling
Transcript of power station monitoring and controlling
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CHAPTER 1
1.1INTRODUCTION
EMBEDDED SYSTEM:
An embedded system is a special-purpose system in which the computer is completely
encapsulated by or dedicated to the device or system it controls. Unlike a general-purpose
computer, such as a personal computer, an embedded system performs one or a few predefined
tasks, usually with very specific requirements. Since the system is dedicated to specific tasks,
design engineers can optimize it, reducing the size and cost of the product. Embedded systems
are often mass-produced, benefiting from economies of scale.
Personal digital assistants (PDAs) or handheld computers are generally considered
embedded devices because of the nature of their hardware design, even though they are more
expandable in software terms. This line of definition continues to blur as devices expand. With
the introduction of the OQO Model 2 with the Windows XP operating system and ports such as a
USB port both features usually belong to "general purpose computers", the line of
nomenclature blurs even more.
Physically, embedded systems ranges from portable devices such as digital watches and
MP3 players, to large stationary installations like traffic lights, factory controllers, or the systems
controlling nuclear power plants.
In terms of complexity embedded systems can range from very simple with a single
microcontroller chip, to very complex with multiple units, peripherals and networks mounted
inside a large chassis or enclosure
Fig1:Embedded System
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1.2 EXAMPLE OF EMBEDED SYSTEM:
Handheld computers Household appliances, including microwave ovens, washing machines, television sets,
Avionics, such as inertial guidance systems, flight control hardware/software and other
integrated systems in aircraft and missiles
Cellular telephones and telephone switches Engine controllers and antilock brake controllers for automobiles Home automation products, such as thermostats, air conditioners, sprinklers, and security
monitoring systems
Handheld calculatorsDVD players and recorders Medical equipment Personal digital assistant Videogame consoles Computer peripherals such as routers and printers. Industrial controllers for remote machine operation.
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CHAPTER 2
2.1 BLOCK DIAGRAM:
Fig 2.1 :BLOCK DIAGRAM
2.1.1 BLOCK DIAGRAM DISCRIPTION:
The block diagram shows the project of power station monitoring and controlling. Here in this
project we use micro controller, PC, RS 232, LEDS, relays and transformers. Here we consider 3 sub-
stations as 3 transformers in the section. if the load at the is high then automatically load can be
transferred to the another transformer..Likewise it handles. Thus we monitor and control the power
at the station.
2.2 SCHEMATIC DIAGRAM:
Fig 2.2:SCHEMATIC DIAGRAM
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2.2.1 SCHEMATIC DESCRIPTION :
Firstly, the required operating voltage for Microcontroller 89C51 is 5V. Hence the
5V D.C. power supply is needed by the same. This regulated 5V is generated by first
stepping down the 230V to 9V by the step down transformer.
The step downed ac. voltage is being rectified by the Bridge Rectifier. The diodes
used are 1N4007. The rectified a.c voltage is now filtered using a C filter. Now the
rectified, filtered D.C. voltage is fed to the Voltage Regulator. This voltage regulator
allows us to have a Regulated Voltage which is +5V.
The rectified; filtered and regulated voltage is again filtered for ripples using an
electrolytic capacitor 100F. Now the output from this section is fed to 40th pin of 89c51
microcontroller to supply operating voltage.
The microcontroller 89c51 with Pull up resistors at Port0 and crystal oscillator of
11.0592 MHz crystal in conjunction with couple of capacitors of is placed at 18th
& 19th
pins of 89c51 to make it work (execute) properly.
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CHAPTER 3
3.HARDWARE COMPONENTS:
3.1 MICRO CONTROLLER (AT89S51)
3.1.1 INTRODUCTION
A Micro controller consists of a powerful CPU tightly coupled with memory, various I/O
interfaces such as serial port, parallel port timer or counter, interrupt controller, data acquisition
interfaces-Analog to Digital converter, Digital to Analog converter, integrated on to a single
silicon chip.
If a system is developed with a microprocessor, the designer has to go for external
memory such as RAM, ROM, EPROM and peripherals. But controller is provided all these
facilities on a single chip. Development of a Micro controller reduces PCB size and cost of
design.
One of the major differences between a Microprocessor and a Micro controller is that a
controller often deals with bits not bytes as in the real world application.
Intel has introduced a family of Micro controllers called the MCS-51.
Fig 3.1: MICRO CONTROLLER
3.1.2 FEATURE:
Compatible with MCS-51 Products
4K Bytes of In-System Programmable (ISP) Flash Memory
Endurance: 1000 Write/Erase Cycles
4.0V to 5.5V Operating Range
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Fully Static Operation: 0 Hz to 33 MHz
Three-level Program Memory Lock
128 x 8-bit Internal RAM
32 Programmable I/O Lines
Two 16-bit Timer/Counters
Six Interrupt Sources
Full Duplex UART Serial Channel
Low-power Idle and Power-down Modes
3.1.3 DESCRIPTION:
The AT89S51 is a low-power, high-performance CMOS 8-bit microcontroller with 4K
bytes of in-system programmable Flash memory. The device is manufactured using Atmels
high-density non-volatile memory technology and is compatible with the industry- standard
80C51 instruction set and pinout. The on-chip Flash allows the program memory to be
reprogrammed in-system or by a conventional non-volatile memory programmer. By combining
a versatile 8-bit CPU with in-system programmable Flash on a monolithic chip, the AtmelAT89S51 is a powerful microcontroller which provides a highly-flexible and cost-effective
solution to many embedded control applications.
3.1.4 BLOCK DIAGRAM:
Fig 3.2:BLOCK DIAGRAM OF MICROCONTROLLER
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3.1.5 PIN DIAGRAM:
Fig 3.3: PIN DIAGRAM OF MICRO CONTROLLER
3.1.6 PIN DESCRIPTION:
VCC - Supply voltage.
GND - Ground.
Port 0:
Port 0 is an 8-bit open drain bidirectional I/O port. As an output port, each pin can sink eight
TTL inputs. When 1s are written to port 0 pins, the pins can be used as high-impedance inputs. Port 0
can also be configured to be the multiplexed low-order address/data bus during accesses to external
program and data memory. In this mode, P0 has internal pull-ups. Port 0 also receives the code bytes
during Flash programming and outputs the code bytes during program verification. External pull-ups
are required during program verification.
Port 1:
Port 1 is an 8-bit bidirectional I/O port with internal pull-ups. The Port 1 output buffers can
sink/source four TTL inputs. When 1s are written to Port 1 pins, they are pulled high by the internal
pull-ups and can be used as inputs. As inputs, Port 1 pins that are externally being pulled low will
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source current (IIL) because of the internal pull-ups. Port 1 also receives the low-order address bytes
during Flash programming and verification.
Table 3.1:FUNCTIONS OF PORT - 1
Port 2:
Port 2 is an 8-bit bidirectional I/O port with internal pull-ups. The Port 2 output buffers cansink/source four TTL inputs. When 1s are written to Port 2 pins, they are pulled high by the internal
pull-ups and can be used as inputs. As inputs, Port 2 pins that are externally being pulled low will
source current (IIL) because of the internal pull-ups. Port 2 also receives the high-order address bits
and some control signals during Flash programming and verification.
Port 3:
Port 3 is an 8-bit bidirectional I/O port with internal pull-ups. The Port 3 output buffers can
sink/source four TTL inputs. When 1s are written to Port 3 pins, they are pulled high by the internal
pull-ups and can be used as inputs. As inputs, Port 3 pins that are externally being pulled low will
source current (IIL) because of the pull-ups. Port 3 receives some control signals for Flash
programming and verification. Port 3 also serves the functions of various special features of the
AT89S51, as shown in the following table.
Table 3.2:FUNCTIONS OF PORT-2
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RST:
Reset input. A high on this pin for two machine cycles while the oscillator is running resets the
device. This pin drives High for 98 oscillator periods after the Watchdog times out. The DISRTO bit
in SFR AUXR (address 8EH) can be used to disable this feature. In the default state of bit DISRTO,
the RESET HIGH out feature is enabled.
ALE/PROG:
Address Latch Enable (ALE) is an output pulse for latching the low byte of the address during
accesses to external memory. This pin is also the program pulse input (PROG) during Flash
programming. In normal operation, ALE is emitted at a constant rate of 1/6 the oscillator frequency
and may be used for external timing or clocking purposes. Note, however, that one ALE pulse is
skipped during each access to external data memory. If desired, ALE operation can be disabled by
setting bit 0 of SFR location 8EH. With the bit set, ALE is active only during a MOVX or MOVC
instruction. Otherwise, the pin is weakly pulled high. Setting the ALE-disable bit has no effect if the
microcontroller is in external execution mode.
PSEN:
Program Store Enable (PSEN) is the read strobe to external program memory. When the
AT89S51 is executing code from external program memory, PSEN is activated twice each machine
cycle, except that two PSEN activations are skipped during each access to external data memory.
EA/VPP:
External Access Enable. EA must be strapped to GND in order to enable the device to fetch
code from external program memory locations starting at 0000H up to FFFFH. Note, however, that if
lock bit 1 is programmed, EA will be internally latched on reset. EA should be strapped to VCC for
internal program executions. This pin also receives the 12-volt programming enable voltage (VPP)
during Flash programming.
XTAL1:
Input to the inverting oscillator amplifier and input to the internal clock operating circuit.
XTAL2:
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Output from the inverting oscillator amplifier.
Oscillator Characteristics:
XTAL1 and XTAL2 are the input and output, respectively, of an inverting amplifier
which can be configured for use as an on-chip oscillator, as shown in Figs 6.2.3. Either a quartz
crystal or ceramic resonator may be used. To drive the device from an external clock source,
XTAL2 should be left unconnected while XTAL1 is driven as shown in Figure 6.2.4.There are
no requirements on the duty cycle of the external clock signal, since the input to the internal
clocking circuitry is through a divide-by-two flip-flop, but minimum and maximum voltage high
and low time specifications must be observed.
Fig 3.4 Oscillator Connection Fig 3.5 External Clock Drive Configuration
3.2 POWER SUPPLY:
The power supplies are designed to convert high voltage AC mains electricity to a
suitable low voltage supply for electronic circuits and other devices. A power supply can by
broken down into a series of blocks, each of which performs a particular function. A d.c power
supply which maintains the output voltage constant irrespective of a.c mains fluctuations or load
variations is known as Regulated D.C Power Supply
For example a 5V regulated power supply system as shown below:
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Fig 3.6: REGULATED POWER SUPPLY
3.3 TRANSFORMER:
A transformer is an electrical device which is used to convert electrical power from one
Electrical circuit to another without change in frequency.
Transformers work only with AC and this is one of the reasons why mains electricity is
AC. Step-up transformers Transformers convert AC electricity from one voltage to another with
little loss of power. increase in output voltage, step-down transformers decrease in output
voltage. Most power supplies use a step-down transformer to reduce the dangerously high mains
voltage to a safer low voltage.
The input coil is called the primary and the output coil is called the secondary. There is
no electrical connection between the two coils; instead they are linked by an alternating magnetic
field created in the soft-iron core of the transformer. The two lines in the middle of the circuit
symbol represent the core. Transformers waste very little power so the power out is (almost)
equal to the power in. Note that as voltage is stepped down current is stepped up. The ratio of
the number of turns on each coil, called the turns ratio, determines the ratio of the voltages. A
step-down transformer has a large number of turns on its primary (input) coil which is connected
to the high voltage mains supply, and a small number of turns on its secondary (output) coil to
give a low output voltage.
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Fig 3.7: AN ELECTRICAL TRANSFORMER
3.4 RECTIFIER:
A circuit which is used to convert ac to dc is known as RECTIFIER. The process of
conversion ac to dc is called rectification
3.4.1 TYPES OF RECTIFIER
Half wave Rectifier Full wave rectifier
1. Centre tap full wave rectifier.
2. Bridge type full bridge rectifier.
Comparison of rectifier circuits:
Table 3.3: COMPARISION OF RECTIFIER CIRCUIT
Parameter
Type of Rectifier
Half wave Full wave Bridge
Number of diodes
1 2 4
PIV of diodes
Vm 2Vm Vm
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D.C output voltage Vm/ 2Vm/ 2Vm/
Vdc,at
no-load
0.318Vm 0.636Vm 0.636Vm
Ripple factor 1.21 0.482 0.482
Ripple
frequency f 2f 2f
Rectification
efficiency 0.406 0.812 0.812
Transformer
Utilization
Factor(TUF)
0.287 0.693 0.812
RMS voltage Vrms Vm/2 Vm/2 Vm/2
Full-wave Rectifier:
From the above comparison we came to know that full wave bridge rectifier as more
advantages than the other two rectifiers. So, in our project we are using full wave bridge rectifier
circuit.
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Bridge Rectifier:
A bridge rectifier makes use of four diodes in a bridge arrangement to achieve full-wave
rectification. This is a widely used configuration, both with individual diodes wired as shown
and with single component bridges where the diode bridge is wired internally.
A bridge rectifier makes use of four diodes in a bridge arrangement as shown in fig (3.8)
to achieve full-wave rectification. This is a widely used configuration, both with individual
diodes wired as shown and with single component bridges where the diode bridge is wired
internally.
Fig 3.8:BRIDGE RECTIFIER WITH FOUR DIODES
Operation:
During positive half cycle of secondary, the diodes D2 and D3 are in forward biased
while D1 and D4 are in reverse biased as shown in the fig(3.9). The current flow direction is
shown in the fig (3.9) with dotted arrows.
Fig 3.9:CURRENT FLOW DURING POSITIVE HALF CYCLE
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During negative half cycle of secondary voltage, the diodes D1 and D4 are in forward
biased while D2 and D3 are in reverse biased as shown in the fig(3.10). The current flow
direction is shown in the fig (3.10) with dotted arrows.
Fig 3.10:CURRENT FLOW DURING NEGATIVE HALF CYCLE
3.5 FILTER:
A Filter is a device which removes the a.c component of rectifier output but allows the
d.c component to reach the load
Capacitor Filter:
We have seen that the ripple content in the rectified output of half wave rectifier is 121% or
that of full-wave or bridge rectifier or bridge rectifier is 48% such high percentages of ripples is
not acceptable for most of the applications. Ripples can be removed by one of the following
methods of filtering.
(a) A capacitor, in parallel to the load, provides an easier by pass for the ripples voltage though
it due to low impedance. At ripple frequency and leave the d.c.to appears the load.
(b) An inductor, in series with the load, prevents the passage of the ripple current (due to high
impedance at ripple frequency) while allowing the d.c (due to low resistance to d.c)
Various combinations of capacitor and inductor, such as L-section filter section filter,
multiple section filter etc. which make use of both the properties mentioned in (a) and (b) above.
Two cases of capacitor filter, one applied on half wave rectifier and another with full wave
rectifier.
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Filtering is performed by a large value electrolytic capacitor connected across the DC
supply to act as a reservoir, supplying current to the output when the varying DC voltage from
the rectifier is falling. The capacitor charges quickly near the peak of the varying DC, and then
discharges as it supplies current to the output. Filtering significantly increases the average DC
voltage to almost the peak value (1.4 RMS value).
To calculate the value of capacitor,
C = *3*f*r*Rl
Where,
f = supply frequency,
r = ripple factor,
Rl = load resistance
Note: In our circuit we are using 1000F. Hence large value of capacitor is placed to reduce
ripples and to improve the DC component.
3.6 REGULATOR:
3.6.1 INTRODUCTION
Voltage regulator ICs is available with fixed (typically 5, 12 and 15V) or variable output
voltages. The maximum current they can pass also rates them. Negative voltage regulators are
available, mainly for use in dual supplies. Most regulators include some automatic protection
from excessive current ('overload protection') and overheating ('thermal protection'). Many of
the fixed voltage regulators ICs have 3 leads and look like power transistors, such as the 7805
+5V 1A regulator shown on the right. The LM7805 is simple to use. You simply connect the
positive lead of your regulated.
Fig 3.11: Three Terminal Voltage Regulator
78XX:
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The Bay Linear LM78XX is integrated linear positive regulator with three terminals. The
LM78XX offer several fixed output voltages making them useful in wide range of applications.
When used as a zener diode/resistor combination replacement, the LM78XX usually results in an
effective output impedance improvement of two orders of magnitude, lower quiescent current.
The LM78XX is available in the TO-252, TO-220 & TO-263packages,
3.6.2 FEATURES
Output Current of 1.5A Output Voltage Tolerance of 5% Internal thermal overload protection Internal Short-Circuit Limited No External Component Output Voltage 5.0V, 6V, 8V, 9V, 10V, 12V, 15V, 18V, 24V Offer in plastic TO-252, TO-220 & TO-263 Direct
3.7 RELAY:
3.7.1 INTRODUCTION
A relay is used to isolate one electrical circuit from another. It allows a low current
control circuit to make or break an electrically isolated high current circuit path. The basic relay
consists of a coil and a set of contacts. The most common relay coil is a length of magnet wire
wrapped around a metal core. When voltage is applied to the coil, current passes through the
wire and creates a magnetic field. This magnetic field pulls the contacts together and holds them
there until the current flow in the coil has stopped. The diagram below shows the parts of a
simple relay.
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Fig 3.12: RELAY
OPERATION:
When a current flows through the coil, the resulting magnetic field attracts an armature
that is mechanically linked to a moving contact. The movement either makes or breaks aconnection with a fixed contact. When the current is switched off, the armature is usually
returned by a spring to its resting position shown in figure 6.6(b). Latching relays exist that
require operation of a second coil to reset the contact position.
By analogy with the functions of the original electromagnetic device, a solid-state relay operates
a thyristor or other solid-state switching device with a transformer or light-emitting diode to
trigger it.
POLE AND THROW
SPST
SPST relay stands for Single Pole Single Throw relay. Current will only flow through the
contacts when the relay coil is energized.
Fig 3.13: SPST RELAY
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SPDT Relay
SPDT Relay stands for Single Pole Double Throw relay. Current will flow between the
movable contact and one fixed contact when the coil is De-energized and between the movable
contact and the alternate fixed contact when the relay coil is energized. The most commonly used
relay in car audio, the Bosch relay, is a SPDT relay.
Fig3.14: SPDT RELAY
DPST Relay
DPST relay stands for Double Pole Single Throw relay. When the relay coil is energized,
two separate and electrically isolated sets of contacts are pulled down to make contact with their
stationary counterparts. There is no complete circuit path when the relay is De-energized.
Fig 3.15: DPST RELAY
DPDT Relay
DPDT relay stands for Double Pole Double Throw relay. It operates like the SPDT relay
but has twice as many contacts. There are two completely isolated sets of contacts.
Figure: DPDT Relay
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This is a 4 Pole Double Throw relay. It operates like the SPDT relay but it has 4 sets of
isolated contacts.
Fig 3.16: 4 POLE DOUBLE THROW RELAY
3.7.2 TYPES OF RELAY
1. Latching Relay2. Reed Relay3. Mercury Wetted Relay4. Machine Tool Relay5. Solid State Relay (SSR)
Latching relay
Latching relay, dust cover removed, showing pawl and ratchet mechanism. The ratchet
operates a cam, which raises and lowers the moving contact arm, seen edge-on just below it. The
moving and fixed contacts are visible at the left side of the image.
A latching relay has two relaxed states (bi-stable). These are also called "impulse",
"keep", or "stay" relays. When the current is switched off, the relay remains in its last state. This
is achieved with a solenoid operating a ratchet and cam mechanism, or by having two opposing
coils with an over-center spring or permanent magnet to hold the armature and contacts in
position while the coil is relaxed, or with a remanent core.
In the ratchet and cam example, the first pulse to the coil turns the relay on and the second
pulse turns it off. In the two coil example, a pulse to one coil turns the relay on and a pulse to the
opposite coil turns the relay off. This type of relay has the advantage that it consumes power only
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for an instant, while it is being switched, and it retains its last setting across a power outage. A
remnant core latching relay requires a current pulse of opposite polarity to make it change state.
Fig3.17: LATCHING RELAY
REED RELAY:
A reed relay has a set of contacts inside a vacuum or inert gas filled glass tube, which
protects the contacts against atmospheric corrosion. The contacts are closed by a magnetic field
generated when current passes through a coil around the glass tube. Reed relays are capable of
faster switching speeds than larger types of relays, but have low switch current and voltage
ratings.
MERCURY-WETTED RELAY:
A mercury-wetted reed relay is a form of reed relay in which the contacts are wetted with
mercury. Such relays are used to switch low-voltage signals (one volt or less) because of their
low contact resistance, or for high-speed counting and timing applications where the mercury
eliminates contact bounce. Mercury wetted relays are position-sensitive and must be mounted
vertically to work properly. Because of the toxicity and expense of liquid mercury, these relays
are rarely specified for new equipment. See also mercury switch.
MACHINE TOOL RELAY:
A machine tool relay is a type standardized for industrial control of machine tools, transfer
machines, and other sequential control. They are characterized by a large number of contacts
(sometimes extendable in the field) which are easily converted from normally-open to normally-
http://en.wikipedia.org/wiki/Reed_relayhttp://en.wikipedia.org/wiki/Vacuumhttp://en.wikipedia.org/wiki/Inert_gashttp://en.wikipedia.org/wiki/Corrosionhttp://en.wikipedia.org/wiki/Coilhttp://en.wikipedia.org/wiki/Mercury_%28element%29http://en.wikipedia.org/wiki/Mercury_switchhttp://en.wikipedia.org/wiki/File:LatchingRelay_tn.jpghttp://en.wikipedia.org/wiki/Mercury_switchhttp://en.wikipedia.org/wiki/Mercury_%28element%29http://en.wikipedia.org/wiki/Coilhttp://en.wikipedia.org/wiki/Corrosionhttp://en.wikipedia.org/wiki/Inert_gashttp://en.wikipedia.org/wiki/Vacuumhttp://en.wikipedia.org/wiki/Reed_relay -
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closed status, easily replaceable coils, and a form factor that allows compactly installing many
relays in a control panel. Although such relays once were the backbone of automation in such
industries as automobile assembly, the programmable logic controller (PLC) mostly displaced
the machine tool relay from sequential control applications.
SOLID-STATE RELAY
A solid state relay (SSR) is a solid state electronic component that provides a similar
function to an electromechanical relay but does not have any moving components, increasing
long-term reliability. With early SSR's, the tradeoff came from the fact that every transistor has a
small voltage drop across it. This voltage drop limited the amount of current a given SSR could
handle. As transistors improved, higher current SSR's, able to handle 100 to 1,200 Amperes,
have become commercially available. Compared to electromagnetic relays, they may be falselytriggered by transients.
Fig3.18: SOLID RELAY, WHICH HAS NO MOVING PARTS
SPECIFICATION
Number and type of contactsnormally open, normally closed, (double-throw) Contact sequence"Make before Break" or "Break before Make". For example, the old
style telephone exchanges required Make-before-break so that the connection didn't get
dropped while dialing the number.
Rating of contactssmall relays switch a few amperes, large contactors are rated for upto 3000 amperes, alternating or direct current
Voltage rating of contacts typical control relays rated 300 VAC or 600 VAC,automotive types to 50 VDC, special high-voltage relays to about 15 000 V
http://en.wikipedia.org/wiki/Form_factorhttp://en.wikipedia.org/wiki/Programmable_logic_controllerhttp://en.wikipedia.org/wiki/Solid_state_relayhttp://en.wikipedia.org/wiki/Solid_state_%28electronics%29http://en.wikipedia.org/wiki/Electromechanicalhttp://en.wikipedia.org/wiki/Amperehttp://en.wikipedia.org/wiki/File:Solid_state_relay.jpghttp://en.wikipedia.org/wiki/Amperehttp://en.wikipedia.org/wiki/Electromechanicalhttp://en.wikipedia.org/wiki/Solid_state_%28electronics%29http://en.wikipedia.org/wiki/Solid_state_relayhttp://en.wikipedia.org/wiki/Programmable_logic_controllerhttp://en.wikipedia.org/wiki/Form_factor -
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Coil voltage machine-tool relays usually 24 VAC, 120 or 250 VAC, relays forswitchgear may have 125 V or 250 VDC coils, "sensitive" relays operate on a few milli-
amperes
3.7.3 APPLICATIONS:
RELAYS ARE USED:
To control a high-voltage circuit with a low-voltage signal, as in some types of modems, To control a high-current circuit with a low-current signal, as in the starter solenoid of an
automobile,
To detect and isolate faults on transmission and distribution lines by opening and closingcircuit breakers (protection relays),
To isolate the controlling circuit from the controlled circuit when the two are at differentpotentials, for example when controlling a mains-powered device from a low-voltage
switch. The latter is often applied to control office lighting as the low voltage wires are
easily installed in partitions, which may be often moved as needs change. They may also
be controlled by room occupancy detectors in an effort to conserve energy,
To perform logic functions. For example, the 23oolean AND function is realized byconnecting relay contacts in series, the OR function by connecting contacts in parallel.
Due to the failure modes of a relay compared with a semiconductor, they are widely used
in safety critical logic, such as the control panels of radioactive waste handling
machinery.
As oscillators, also called vibrators. The coil is wired in series with the normally closedcontacts. When a current is passed through the relay coil, the relay operates and opens the
contacts that carry the supply current. This stops the current and causes the contacts to
close again. The cycle repeats continuously, causing the relay to open and close rapidly.
Vibrators are used to generate pulsed current.
To generate sound. A vibrator, described above, creates a buzzing sound because of therapid oscillation of the armature. This is the basis of the electric bell, which consists of a
vibrator with a hammer attached to the armature so it can repeatedly strike a bell.
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To perform time delay functions. Relays can be used to act as an mechanical time delaydevice by controlling the release time by using the effect of residual magnetism by means
of a inserting copper disk between the armature and moving blade assembly.
3.8 LED:
3.8.1 INTRODUCTION
A light-emitting diode (LED) is a semiconductor light source. LEDs are used as indicator
lamps in many devices, and are increasingly used for lighting. Introduced as a practical
electronic component in 1962, early LEDs emitted low-intensity red light, but modern versions
are available across the visible, ultraviolet and infrared wavelengths, with very high brightness.
The LED is based on the semiconductor diode. When a diode is forward
biased, electrons are able to recombine with holes within the device, releasing energy in the form
of photons. This effect is called electroluminescence and the color of the light (corresponding to
the energy of the photon) is determined by the energy gap of the semiconductor. An LED is
usually small in area (less than 1 mm2), and integrated optical components are used to shape its
radiation pattern and assist in reflection.
LEDs present many advantages over incandescent light sources including lower energy
consumption, longer lifetime, improved robustness, smaller size, faster switching, and greater
durability and reliability. However, they are relatively expensive and require more
precise current and heat management than traditional light sources. Current LED products for
general lighting are more expensive to buy than fluorescent lamp sources of comparable output.
3.8.2 WORKING :
Charge carrier junction from electrodes with different voltages. When an electron meets
a hole, it falls into a lower energy level, and releases energy in the form of a photon.
The wavelength of the light emitted, and therefore its color, depends on the band gap energy of
the materials forming the p-njunction. In silicon or germanium diodes, the electrons and holes
recombine by a non-radioactive
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transitions which produces no optical emission, because these are indirect band gap materials.
The materials used for the LED have a direct band gap with energies corresponding to near-
infrared, visible or near-ultraviolet light.
Fig 3.20: INNER WORKING OF AN LED
Table 3.4:COLOURS AND MATERIALS OF LED
COLORS AND MATERIALS
Color Wavelength (nm) Voltage (V) Semiconductor Material
Infrared > 760 V< 1.9Gallium arsenide (GaAs)
Aluminum gallium arsenide (AlGaAs)
Red 610 < < 7601.63 < V