power station monitoring and controlling

7/29/2019 power station monitoring and controlling

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POWER STATION MONITORING AND CONTROLING

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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


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


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
http://en.wikipedia.org/wiki/Solenoidhttp://en.wikipedia.org/wiki/Solenoid


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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

power station monitoring and controlling

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