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8/13/2019 Electro Mechanical Relay
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FROM-
RONAK JAISWAL - 05
DARSHIL VAGHANI - 44 MANANK SHASTRI - 62
AKSHAY PATEL- 63
GUIDE TEACHER- KRISHNA CHAUHAN
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Objectives
Upon completion of this presentation, we will be ableto:
Construct an experimental relay Describe how a relay works
Observe basic relay functions
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A relay is defined as an electrically controlled devicethat opens and closes electrical contacts, or activatesand deactivates operation of other devices in the sameor another electrical circuit. Two types of relaytechnology are available:-
1. Mechanical
2. Solid State.
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A relay may also be called an electromagnetic switch
Relays use a low amperage circuit to control a high
amperage circuit The low amperage circuit controls an electromagnetic
device
The electromagnetic device closes/opens the high
amperage circuit
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Latching Relay
Reed Relay
Mercury-wetted Relay
Mercury Relay PolarizedRelay
Machine Tool Relay
Ratchet Relay
Vacuum Relays
Coaxial Relay
Contactor Solid-stateRelay
Solid State ContactorRelay
Buchholz Relay
Forced-guided Contacts
Relay Overload Protection
Relay
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It is a reed switch enclosed in a solenoid. The switch has a set ofcontacts inside an evacuated or inert gas-filled glass tube whichprotects the contacts against atmospheric corrosion; the contactsare made of magnetic material that makes them move under theinfluence of the field of the enclosing solenoid or an externalmagnet.
Reed relays can switch faster than larger relays and require verylittle power from the control circuit. However, they haverelatively low switching current and voltage ratings. Though rare,the reeds can become magnetized over time, which makes themstick 'on' even when no current is present; changing the
orientation of the reeds with respect to the solenoid's magneticfield can resolve this problem. Sealed contacts with mercury-wetted contacts have longer
operating lives and less contact chatter than any other kind ofrelay.
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It is a form of reed relay in which the contacts arewetted with mercury. Such relays are used to switchlow-voltage signals (one volt or less) where themercury reduces the contact resistance and associated
voltage drop, for low-current signals where surfacecontamination may make for a poor contact, or forhigh-speed applications where the mercury eliminatescontact bounce.
Mercury wetted relays are position-sensitive and mustbe mounted vertically to work properly. Because of thetoxicity and expense of liquid mercury, these relays arenow rarely used.
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A machine tool relayis a type standardized for industrialcontrol of machine tools, transfer machines, and othersequential control.
They are characterized by a large number of contacts
(sometimes extendable in the field) which are easilyconverted from normally-open to normally-closed status,easily replaceable coils, and a form factor that allowscompactly 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 displacedthe machine tool relay from sequential controlapplications.
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A relay allows circuits to be switched by electricalequipment: for example, a timer circuit with a relay
could switch power at a preset time. For many years relays were the standard method of
controlling industrial electronic systems. A number ofrelays could be used together to carry out complex
functions (relay logic). The principle of relay logic is based on relays which
energize and de-energize associated contacts. Relaylogic is the predecessor of ladder logic, which iscommonly used in programmable logic controllers.
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Separation of AC and DC circuits
Interface between electronic control circuits and
power circuits
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A relay is an electromechanical device that uses smallelectrical currents and voltages to control largerelectrical currents and voltages. Relays have unlimitedpossibilities, ranging from industrial applications to
consumer electronics, such as microwave ovens andtelevision sets.
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Relay Symbol
Normally closed
(NC) contact
CR1-1 CR1-2
Normally open
(NO) contact
CR1
Coil
Contacts are open whenno current flows through
the coil but close as soon
as the coil is energized.
Contacts are closed whenno current flows through
the coil but open as soon
as the coil is energized.
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Coil Voltage Voltage required for switching
Contact Rating How much current the relay can
handle
Normally Open (NO) or Normally Closed (NC)
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Electromagnetic Relay Operation
An electromagnetic relay is a magnetic switch. It uses
electromagnetism to switch contacts.
A relay will usuallyhave only one coil
but may have any
number of different
contacts.
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Electromagnetic Relay Operation
With no current flow
through the coil(coil de-energized),
the armature is held
away from the core
by spring tension.
When the coil is energized,
the electromagnetic field
moves the armature
causing the contact points
of the relay to open or
close.
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In a microwave oven, the push of a few tiny buttons onthe keypad gives commands to a microcontroller,
which can produce only very small output voltages.Those small voltages turn on a relay, which is capableof controlling the large voltages and currents requiredto produce the heating effect that takes place.
In a television, the tiny impulses from the hand-heldremote unit control a relay in the power supply.
Examples of Electromechanical Relays
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A relay is basically a switch that is controlled by anelectromagnet.
A relay is used when a large current needs to beturned on or off by a small current.
Relays are used in many industrial applications aswell as devices such as televisions, stereo systemsetc.
Manufactured Relay
Homemade Relay
Examples of Electromechanical Relays
All electromechanical relays have three basic parts:
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A relay is basically a switch that is controlled by anelectromagnet.
A relay is used when a large current needs to beturned on or off by a small current.
Relays are used in many industrial applications aswell as devices such as televisions, stereo systemsetc.
Electromagnet
Switching
contacts
Switching
contacts
Electromagnet
All electromechanical relays have three basic parts:
1. Electromagnetconsists of an iron core wrapped with turns of insulated wir
2. Armaturethis is the moving part of the relay.
3. Switching contactsat least one must be stationary and one fastened to
the armature. This is known as a single-pole single-throw arrangement.
Armature
Armature
Note: In the manufactured relay, the switching
contacts are insulated from the armature. Our
experimental relay uses the metal armature and
a 8d common nail as the switching contacts.
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This diagram shows the
basic parts of an
electromechanical relay: a
spring, moveable armature,
electromagnet, moveable
contact, and stationary
contact. The spring keeps
the two contacts separated
until the electromagnet is
energized, pulling the two
contacts together.
Moveable Armature
Moveable Contact
ElectromagnetSpring
Stationary Contact
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Spring
To Control Circuit
Moveable Armature Moveable Contact
Load
Power Supply
Electromagnet
This diagram shows how to wire
an electromechanical relay.
When the control circuit turns the
electromagnet on, the moveablearmature is drawn towards the
electromagnet and connects the
moveable contact and the
stationary contact. This
completes the circuit and delivers
power to the load.
Stationary Contact
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Relays are used for:
Amplifying a digital signal, switching a large amount ofpower with a small operating power. Some special cases are: A telegraph relay, repeating a weak signal received at the end
of a long wire Controlling a high-voltage circuit with a low-voltage signal, as
in some types of modems or audio amplifiers,
Controlling a high-current circuit with a low-current signal,
as in the starter solenoid of an automobile, Detecting and isolating faults on transmission and
distribution lines by opening and closing circuitbreakers (protection relays),
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Suppose, there is a need to control a solenoid valve for a waterdrain application. Control is to be accomplished with amicrocontroller. The solenoid valve requires 120 VAC to open.Assuming that a 120 VAC power supply is available, how cancontrol of the solenoid valve be accomplished using a
microcontroller that can only supply 5 VDC?
This problem is easily solved using a relay. There are many relaysthat are turned on and off with a 5 VDC coil. The relay providesthe interface between the microcontroller and the 120 VAC power
supply that is needed to open and close the valve.
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120 VAC
Ground
To Microcontroller
Solenoid Valve
Relay
Coil
Circuit for Control of a 120 VAC Solenoid Valve
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Contacts can switch AC or DC
Low initial cost
Very low contact voltage drop, thus no heat sink is
required High resistance to voltage transients
No Off-State leakage current through open contacts
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The contacts wear and thus have limited lifedepending on loads
Short contact life when used for rapid switchingapplications or high loads
Poor performance when switching high inrushcurrents
Package Size
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Electromechanical relays are an excellent solutionto separate electronic control circuitry and powercircuitry. Electromechanical relays are not the
best choice in high frequency switchingapplications and do have a limited life due to wearon the contacts inside the relay. When used in thea proper application, the electromechanical relay
provides safe and reliable integration betweenpower circuits and control circuits.