A relay is usually
an electromechanical device that is actuated by an electrical current. The
current flowing in one circuit causes the opening or closing of another circuit.
Relays are the
devices that detect conditions in circuits and cause the contacts to be ON or
OFF to CLOSE or OPEN the circuit as required with suitable arrangements.
HOW RELAY WORKS?
The diagram shows an inner section diagram of a relay. An iron core is surrounded by a control coil. As shown, the power source is given to the electromagnet through a control switch and through contacts to the load. When current starts flowing through the control coil, the electromagnet starts energizing and thus intensifies the magnetic field. Thus the upper contact arm starts to be attracted to the lower fixed arm and thus closes the contacts causing a short circuit for the power to the load. On the other hand, if the relay was already de-energized when the contacts were closed, then the contact move oppositely and make an open circuit.
As soon as the coil current is off, the movable
armature will be returned by a force back to its initial position. This force
will be almost equal to half the strength of the magnetic force. This force is
mainly provided by two factors. They are the spring and also gravity.
Relays are mainly made for two
basic operations. One is low voltage application and the other is high voltage.
For low voltage applications, more preference will be given to reduce the noise
of the whole circuit. For high voltage applications, they are mainly designed
to reduce a phenomenon called arcing.
Relay Basics
The basics for all the relays are the same. Take a look at a 4 – pin relay shown below. There are two colours shown. The green colour represents the control circuit and the red colour represents the load circuit. A small control coil is connected onto the control circuit. A switch is connected to the load. This switch is controlled by the coil in the control circuit. Now let us take the different steps that occour in a relay.
ENERGIZED
RELAY (ON)
As shown in the circuit, the current flowing through the coils represented
by pins 1 and 3 causes a magnetic field to be aroused. This magnetic field
causes the closing of the pins 2 and 4. Thus the switch plays an important role
in the relay working. As it is a part of the load circuit, it is used to
control an electrical circuit that is connected to it. Thus, when the relay in
energized the current flow will be through the pins 2 and 4.
DE
– ENERGIZED RELAY (OFF)
As soon as the current flow stops through pins 1 and 3, the switch opens and
thus the open circuit prevents the current flow through pins 2 and 4. Thus the
relay becomes de-energized and thus in off position.
Pole and Throw
Relays have the exact working of a switch. So, the same concept is also applied. A relay is said to switch one or more poles. Each pole has contacts that can be thrown in mainly three ways. They are- Normally Open Contact (NO) – NO contact is also called a make contact. It closes the circuit when the relay is activated. It disconnects the circuit when the relay is inactive.
- Normally Closed Contact (NC) – NC contact is also known as break contact. This is opposite to the NO contact. When the relay is activated, the circuit disconnects. When the relay is deactivated, the circuit connects.
- Change-over (CO) / Double-throw (DT) Contacts – This type of contacts are used to control two types of circuits. They are used to control a NO contact and also a NC contact with a common terminal. According to their type they are called by the names break before make and make before break contacts.
CLASSIFICATION
OF RELAYS;
There are two basic classifications of relays:
·
Electromechanical
·
Solid
State
Electromechanical
relays have moving parts, whereas solid state relays have no moving parts.
ELECTROMECHANICAL RELAYS:
As their name implies, Electromechanical Relays are
Electro-Magnetic devices that convert a magnetic flux generated by the
application of an electrical control signal either AC or DC current, into a
pulling mechanical force which operates the electrical contacts within the
relay. The most common form of electromechanical relay consist of an energizing
coil called the "Primary Circuit" wound around a permeable iron core.
It
has
both a fixed portion called the Yoke, and a moveable spring loaded part
called the Armature, that completes the magnetic field circuit by
closing the air gap between the fixed electrical coil and the moveable
armature. This armature is hinged or pivoted and is free to move within the
generated magnetic field closing the electrical contacts that are attached to
it. Connected between the yoke and armature is normally a spring (or springs)
for the return stroke to "Reset" the contacts back to their initial
rest position when the relay coil is in the "de-energized" condition,
ie. Turned "OFF".
SOLID STATE RELAY:
One of the main disadvantages of an Electromechanical
Relay (EMR) is that it is a "mechanical device", that is it has
moving parts. Over a period of time these parts will wear out and fail, or that
the contact resistance through the constant arcing and erosion may make the
relay unusable and it will therefore need to be replaced. Also, they are
electrically noisy with the contacts suffering from contact bounce which may
affect any electronic circuits to which they are connected.
There is another type of relay called a Solid
State Relay or (SSR) for short which is a solid state
contactless, pure electronic relay. It has no moving parts with the contacts
being replaced by transistors, thyristors or triacs. The electrical separation
between the input control signal and the output load voltage is accomplished
with the aid of an opto-coupler type Light Sensor.
The Solid State Relay provides a high degree of
reliability, long life and reduced electromagnetic interference (EMI), (no arcing
contacts or magnetic fields), together with a much faster response, as compared
to the conventional electromechanical relay. Also the input control power
requirements of the solid state relay are generally low enough to make them
compatible with most IC logic families without the need for additional buffers,
drivers or amplifiers. However, being a semiconductor device they must be
mounted onto suitable heatsinks to prevent the output switching semiconductor
device from over heating.
The AC type Solid State Relay turns "ON"
at the zero crossing point of the AC sinusoidal waveform, prevents high inrush
currents when switching inductive or capacitive loads while the inherent turn
"OFF" feature of thyristors and triacs provides an improvement over
the arcing contacts of the electromechanical relays. Like EMR's an RC
(Resistor-Capacitor) snubber network is generally required across the output
terminals of the SSR to protect the semiconductor output switching device from
noise and voltage transient spikes when used to switch highly inductive or
capacitive loads and in most modern SSR's this RC snubber network is built as
standard into the relay itself. Non-zero detection switching (instant
"ON") type SSR's are also available for phase controlled applications
such as the dimming or fading of lights at concerts, shows, disco lighting etc,
or for motor speed control type applications.
As the output switching device of a solid state
relay is a semiconductor device (Transistor for DC switching applications, or a
Triac/Thyristor combination for AC switching), the voltage drop across the
output terminals of an SSR when "ON" is much higher than that of the
electromechanical relay, typically 1.5 - 2.0 volts. If switching large currents
for long periods of time an additional heat sink will be required.
TYPES OF RELAYS
A latching relay has two relaxed states
(bistable). 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 for an instant, while it is being switched, and it retains
its last setting across a power outage. A remanent core latching relay requires
a current pulse of opposite polarity to make it change state.
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.
POLARIZED RELAY
A polarized relay placed the armature
between the poles of a permanent magnet to increase sensitivity. Polarized
relays were used in middle 20th Century telephone exchanges to detect faint
pulses and correct telegraphic distortion. The poles were on screws, so a
technician could first adjust them for maximum sensitivity and then apply a
bias spring to set the critical current that would operate the relay.
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-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.
CONTACTOR RELAY
A contactor is a very heavy-duty relay used
for switching electric motors and lighting loads. Continuous current ratings
for common contactors range from 10 amps to several hundred amps. High-current
contacts are made with alloys containing silver. The unavoidable arcing causes
the contacts to oxidize; however, silver oxide is still a good conductor. Such
devices are often used for motor starters. A motor starter is a contactor with
overload protection devices attached. The overload sensing devices are a form
of heat operated relay where a coil heats a bi-metal strip, or where a solder
pot melts, releasing a spring to operate auxiliary contacts. These auxiliary
contacts are in series with the coil. If the overload senses excess current in
the load, the coil is de-energized. Contactor relays can be extremely loud to
operate, making them unfit for use where noise is a chief concern.
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 falsely triggered by
transients.
SOLID STATE CONTACTOR RELAY
A solid state contactor is a very heavy-duty
solid state relay, including the necessary heat sink, used for switching
electric heaters, small electric motors and lighting loads; where frequent
on/off cycles are required. There are no moving parts to wear out and there is
no contact bounce due to vibration. They are activated by AC control signals or
DC control signals from Programmable logic controller (PLCs), PCs, Transistor-transistor
logic (TTL) sources, or other microprocessor and microcontroller controls.
BUCHHOLZ RELAY
A Buchholz relay is a safety device sensing
the accumulation of gas in large oil-filled transformers, which will alarm on
slow accumulation of gas or shut down the transformer if gas is produced
rapidly in the transformer oil.
FORCED-GUIDED CONTACTS RELAY
A forced-guided contacts relay has relay
contacts that are mechanically linked together, so that when the relay coil is
energized or de-energized, all of the linked contacts move together. If one set
of contacts in the relay becomes immobilized, no other contact of the same
relay will be able to move. The function of forced-guided contacts is to enable
the safety circuit to check the status of the relay. Forced-guided contacts are
also known as "positive-guided contacts", "captive
contacts", "locked contacts", or "safety relays".
OVERLOAD PROTECTION RELAY
Electric motors need overcurrent protection to
prevent damage from over-loading the motor, or to protect against short
circuits in connecting cables or internal faults in the motor windings. One
type of electric motor overload protection relay is operated by a heating
element in series with the electric motor. The heat generated by the motor
current heats a bimetallic strip or melts solder, releasing a spring to operate
contacts. Where the overload relay is exposed to the same environment as the
motor, a useful though crude compensation for motor ambient temperature is
provided.
RELAYS AND TRANSISTORS COMPARED
Like relays, transistors can be
used as an electrically operated switch. For switching small DC currents (<
1A) at low voltage they are usually a better choice than a relay. However,
transistors cannot switch AC (such as mains electricity) and in simple circuits
they are not usually a good choice for switching large currents (> 5A).
In these cases a relay will be needed, but note that a low power transistor may
still be needed to switch the current for the relay's coil! The main advantages
and disadvantages of relays are listed below:
ADVANTAGES OF RELAYS: - Relays can switch AC and DC, transistors can only switch DC.
- Relays can switch higher voltages than standard transistors.
- Relays are often a better choice for switching large currents (> 5A).
- Relays can switch many contacts at once.
DISADVANTAGES OF RELAYS:
- Relays are bulkier than transistors for switching small currents.
- Relays cannot switch rapidly (except reed relays), transistors can switch many times per second.
- Relays use more power due to the current flowing through their coil.
- Relays require more current than many ICs can provide, so a low power transistor may be needed to switch the current for the relay's coil.
