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RELAY 

WORKING PRINCIPLE

OPERATIN OF NORMAL RELAY 

TRANSISTOR DRIVING CKIRCUIT

REALY INTERFACIG WITH MICRO

CONTROLLER

RELAYS

  A relay is an electrical switch that opens and closes under the control of another electrical

circuit. In the original form, the switch is operated by electromagnet to open or close one or many sets of contact, because of the relay is able to control an output circuit higher power 

than the input circuit. It can be considered to be, in a broad sense, a form of an electrical

amplifier.

An electrical current through conductor will produce a magnetic field at right angles to the

direction of electron flow. If that conductor is wrapped into a coil shape, the magnetic field

 produced will be oriented along the length of the coil. The greater the current, the greater the

strength of the magnetic field, all other factor being equal. Relays are extremely useful when

we have a need to control a large amount of current and/or voltage with small electrical

signal. The relay coil which produces the magnetic field may only consume fractions watt of 

 power, while the contacts closed or opened by that magnetic field may be able to conduct

hundreds of times that amount of power to a load. In effect, a relay acts as a binary on or 

off! amplifier. "ust as with transistors, the relays ability to control one electrical signal with

another misapplication in the construction of logic functions. #or now, the relay$s

%amplifying& ability will be explored.

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WORKING PRINCIPLE: 

In the above schematic, the relays coil is energi'ed by the low(voltage )*v! dc source, while

the single(throw ++T! contact interrupts the high voltage -v! ac circuit. it is quite li0ely

that the current required to energi'es the relay coil will be hundreds of times less then current

rating of the contact. Typical relay coil currents are well below )amp, while typical contact

ratings for industrial relays are at least )amp. 1ne relays coil/armature assembly may be

used to actuate more then one set of contacts. Those contacts may be normally(open,

normally(close, or any combination of the two. As with switches, the %normal& state of relays

contacts is that state when the coil is de(energi'ed, 2ust as you wounded the relay sitting on a

self, not connected to any circuit. Relay contacts may be open(air pads of metal alloy,

mercury tubes, or even magnetic reeds, 2ust as the other type of switches. The choice of contacts in a relays depends on the same factors which dictate contact choice in other 

types of switches. 1pen(air contacts are the best for high current application.

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Operation:

3hen current flows through the coil, a magnetic field are created around the coil i.e.,

the coil is energi'ed. This causes the armature to be attracted to the coil. The

armature$s contact acts li0e a switch and closes or opens the circuit. 3hen the coil is

not energi'ed, a spring pulls the armature to its normal state of open or closed. There

are all types of relays for all 0inds of applications.

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  Fig 3.4.2: Rela Operation an! "#e o$ prote%tion !io!e#

Transistors and I4s must be protected from the brief high voltage 5spi0e5 produced

when the relay coil is switched off. The above diagram shows how a signal diode eg

)6-)-! is connected across the relay coil to provide this protection. The diode is

connected 5bac0wards5 so that it will normally not conduct. 4onduction occurs only

when the relay coil is switched off, at this moment the current tries to flow

continuously through the coil and it is safely diverted through the diode. 3ithout the

diode no current could flow and the coil would produce a damaging high voltage

5spi0e5 in its attempt to 0eep the current flowing.

In choosing a relay, the following characteristics need to be considered7

). The contacts can be normally open 61! or normally closed 64!. In the 64 type,

the contacts are closed when the coil is not energi'ed. In the 61 type, the contacts are

closed when the coil is energi'ed.

*. There can be one or more contacts. i.e., different types li0e ++T single pole single

throw!, +8T single pole double throw! and 88T double pole double throw! relay.

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9. The voltage and current required to energi'e the coil. The voltage can vary from a

few volts to : volts, while the current can be from a few milliamps to *milliamps.

The relay has a minimum voltage, below which the coil will not be energi'ed. This

minimum voltage is called the %pull(in& voltage.

-. The minimum 84/A4 voltage and current that can be handled by the contacts. This

is in the range of a few volts to hundreds of volts, while the current can be from a few

amps to -A or more, depending on the relay.

&RANSIS&OR 'RI(ER CIRC)I&:

An +8T relay consists of five pins, two for the magnetic coil, one as the common

terminal and the last pins as normally connected pin and normally closed pin. 3hen

the current flows through this coil, the coil gets energi'ed. Initially when the coil is

not energi'ed, there will be a connection between the common terminal and normally

closed pin. ;ut when the coil is energi'ed, this connection brea0s and a new

connection between the common terminal and normally open pin will be established.

Thus when there is an input from the microcontroller to the relay, the relay will be

switched on. Thus when the relay is on, it can drive the loads connected between the

common terminals and normally open pin. Therefore, the relay ta0es :< from the

microcontroller and drives the loads which consume high currents. Thus the relay acts

as an isolation device.

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  Vcc

 

RELAY

  GROUND

8igital systems and microcontroller pins lac0 sufficient current to drive the circuits

li0e relays and bu''er which consume high powers. 3hile the relay$s coil needs

around )milli amps to be energi'ed, the microcontroller$s pin can provide a

maximum of )(*milli amps current. #or this reason, a driver such as a power transistor is placed in between the microcontroller and the relay.

AT89C51

 

P1.0

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  Fig 3.4.3: &ran#i#tor 'ri*er Cir%"it

The operation of this circuit is as follows7

The input to the base of the transistor is applied from the microcontroller port pin

).. The transistor will be switched on when the base to emitter voltage is greater 

than .=< cut(in voltage!. Thus when the voltage applied to the pin ). is high i.e.,

).>) ?.=

the load will be operated.

3hen the voltage at the pin ). is low i.e., ).> @.=

off state and the relay will be 1##. Thus the transistor acts li0e a current driver to

operate the relay accordingly.

RELAY IN&ERFACING WI&+ &+E ,ICROCON&ROLLER:

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  DRIVER

CIRCUIT

  RELAY

 LOAD

 

Fig 3.4.4: Rela Inter$a%ing -it /0C1

 

AT 4:)