relay is a switch that is operated by electric current. This Switch may contain
many types of contact forms which we will look into further in to the article.
level controls or signals switch the relays and contractors to a higher voltage
or current supply using different arrangements on contact.
now we have seen a collection of input devices that be used to detect different
physical variables and signals. So they are known as sensors. Already,
Actuators can control or operate some external physical process and are classed
as output devices. Actuators convert an electrical signal into a corresponding
physical quantity such as sound. Its changes one type of quantity for another
and is commonly used by a low voltage command signal hence it is classified as
a transducer. Depending on the number of stable forms the output has, actuators
can be classified into either binary or continuous.
most common type of actuators are electrical relays and motors. Since a relay
has two stable forms we can then say it’s an actuator, either energized and
latched or de-energized and unlatched. m\a motor however can rotate 3600
can be used to open doors, close valves and open latches electrically through
robotic applications. A relay is used so that a solenoid plunger can operate in
one or more types of ways, the relay can be used in an infinite number of ways.
previously stated, a relay is known as an electrical switch that provides one
with a connection (electrical) between two points or more in response to the
application of any control signal. The most common and used kind of electrical
relay is the electromechanical relay.
ability to turn an equipment “on” or “off” is the most fundamental function of
the relay. Switches are the easiest way to control the power supply. The
ability to manually turn them “off” and “on” is its biggest disadvantage. They
are large and slow and posses small current.
the other hand, electrical relays are electrically operated switches that come
differently according to their types of applications. Contractors are single or
multiple contracts within a single package with the larger power relays power
relays used for high current switching applications.
fundamental operating principles of “light duty” electro-mechanical relays that
can be used in motor or robotic circuits is our main concern in the talk of
electrical relays. Relays are usually directly set up on PCB boards and/or
connected free standing. Load currents are usually fractions of ampere up to 20
+ amperes; they are usually used in general electrical and electronic control
electro-mechanical relay converts magnetic flux generated by the application of
a low voltage control signal (either DC or AC) across the relay
terminals, into useful mechanical energy. The mechanical force that is
generated can be used for the contol and manipulation of electrical contacts
build in the relay. The “primary circuit” is the most common form of
electro-mechanical relay consists of an energizing coil tied around a permeable
magnetic field circuit is completed by a fixed iron core potion called the yoke
and a move-able spring loaded part called the armature and closing the air gap
between them. The armature is pivoted. This allows it to move freely within the
generated magnetic field while the electrical contacts are closed. Springs are
joined between armature and the yoke for the return stroke to “reset” the
contacts back to their initial rest position when the relay coil is in the
“de-energized” condition, A.K.A. turning it off.
any relay, what can be noted is the presence of two sets of electrically
conductive contacts. Relays are known to be “Normally Closed”, or “Normally
Open”. The NO labeled relay initiates contact. On the other hand, the NC
labeled relay breaks contacts. The contacts are closed in the normally open
position if only the field current is “ON”.
contacts are permanently closed in the normally closed position when the field
current is “OFF” as the switch contacts return to its resting position. Note
that these terms, i.e. Normally Open and Normally Closed or Make and Break
Contacts discuss the state of the electrical contacts when the relay coil is
off. Contact elements may be of double or single make or break designs. An
example of this arrangement is given below.
relay contacts are a set of metals that are in contact in order to prevent a
short-circuit and permit the flow of current electricity. An example of a
device that uses this mechanism is a switch. Hence, they are said to be
electrically conductive devices. The resistance between the contacts is very
high in the Mega-Ohms when the contacts are open, producing no circuit current
flow and an open circuit condition.
The contact resistance should be zero when the
contacts are closed, a short circuit, but it is not always the case. When
closed all relay contacts have a certain amount of “contact resistance” and
which is called the “On-Resistance”.
ON-resistance will be very small with a new relay and contacts generally less
than 0.2Ωbecause the tips are new and clean, but over time the tip resistance
example, if the contacts are passing a load current of say 20A, then the
voltage drop across the contacts using Ohms Law is 0.2 x 20 = 4 volts. If we
assume that the value of the supply voltage is 16 volts, this means that the
value of the load voltage will be 20V. As the contact tips begin to wear, they
will start to show signs of arcing damage. This is because the circuit current
still wants to flow. As a gap begins to emerge between the contacts, the relay
coil loses energy due to excess load. The contact resistance of the tips to
increase further as the contact tips becomes damaged if they are not adequately
shielded from loads with high inductance and capacitance values. The contact
tips may become so burnt and damaged to the point that they are physically
closed but do not pass any or very little current If allowed to continue.
the gap between the contacts begin to reduce, a short-circuit condition arises.
This can also lead to destruction of the circuit if the arcing becomes
persistent. The volt drop across the contacts for the same load current
increases to 1 x 20 = 20 volts DC If the contact resistance has increased due
to arcing to say 1Ω. The faulty relay will have to be replaced if the drop in
voltage value is massive
Modern contact tips are made of, or coated
with, a variety of silver based alloys to extend their life span as given in
the following table to reduce the effects of contact arcing and high
Relay Contact Tip Materials
Electrical and thermal conductivity are the higher than all the other metals.
It shows low contact resistance, it is also inexpensive and popularly in use.
Contacts tarnish easily through sulphurisation influence.
Cadmium Oxide (AgCdO)
Very little possibility to arc and weld, good wear resistance and arc
Hardness and melting point are high, arc resistance is good.
It is not a precious metal.
High contact pressure is needed to reduce resistance.
Contact resistance is a little high, and resistance to corrosion is poor.
Equivalent to the electrical conductivity of silver, excellent arc resistance.
Gold and Silver Alloys
Excellent corrosion resistance, and used mainly for low-current circuits.
sheets display information about the rating of all load types whether DC or AC.
Some form of arc suppression or filtering is required across the relay contacts
in order to achieve long life and consistency when load with high inductance
and capacitance rating is placed on it.
improve the longevity of a relay tip, it is a good idea to connect it in
parallel with an RC network. This reduces the rate of arcing. The voltage peak,
which occurs at the instant the contacts open, will be safely short circuited
by the RC network, hence suppressing any arc generated at the contact tips.
Electrical Relay Contact Types
contacts can be classed by their actions as well as the usual Normally Closed,
(NC) and Normally Open, (NO) (which basically describes the way relays contacts
are connected). These relays can be made up of one or more original switch
contacts. Each of these contacts is known as a “Pole”. A relay coil is used
to connect the poles and these contact types are usually shown as:
– Single Pole Single Throw
– Single Pole Double Throw
– Double Pole Single Throw
– Double Pole Double Throw
(B) or “Make” (M) would be the action sdiscussed above. An example of
a simple relay with one set of contacts would be seen as:
Pole Single Throw – (Break before Make)”, or SPST – (B-M)
are many examples with more common diagrams used to explain and showcase the
relays are also denoted by the combinations of their contacts or switching
elements and the number of contacts combined within a single relay. For
instance, a contact which is normally open in the de-energized position “OFF”
of the relay is called a “Form A contact” or make contact. Whereas a contact
which is normally closed in the de-energized position of the relay is called a
“Form B contact” or break contact.
set of contacts are referred to as “Form C contacts” or change-over contacts
when both a make and break set of contact elements are present at the same time
so that the two contacts are electrically connected to produce a common point
(identified by three connections).
relay contacts in parallel so these contacts handle higher load currents is not
seen as a very smart move. For example, one should never try to provide a 20A
load with two relay contacts in parallel that have 10A contact ratings each, as
the mechanically operated relay contacts don’t ever open or close at the exact
same time. The end result is usually that one of the contacts will overload
even for a briefly resulting in premature relay failure over time. This is a
final and important point about electrical relays.
load voltages should not be mixed with contacts from a sister relay or from the same relay. Though, separate relays
are utilized for safety while electrical relays can also be used to allow low
power electronic type circuits to switch high currents or voltages both “Off”
most significant part of any electrical relay is the “Coil“.
The coil refers to the part which converts all electrical current to
electromagnetic flux that is used to mechanically operate all relay contacts.
The main problem relay coils have is the possession of highly inductive loads
made from wire coils. Any coil of wire has an impedance value made up of
Inductance (L) and Resistance (R) in series (i.e. LR Series Circuit).
magnetic field is generated around the current flows through the coil which is
self induced. When magnetic flux of the coil drops, a back emf with high value
is generated. This induced reverse
voltage value could be very high in comparison to the changing voltage, and may
damage any semiconductor device. By
connecting the reverse biased diode across the coil of the relay, we can secure
the protection of the transistor and semiconductor device.
induced back E.M.F is generated as the magnetic flux collapses in the coil when
the current flowing through the coil is switched “OFF”.
diode, responsible for the dissipation of energy is triggered by the reverse
electromotive force. This protects the semiconductor transistor from developing
the method of application of the diode is defined, a free-wheeling diode or
fly-back diode can be used. Other types of inductive loads which require a
flywheel diode for protection are solenoids, inductive coils and motors.
devices used for protection include a combined resistor-capacitor network and
Zener Diodes. Semiconductor components can also be secured through the use of a
The Solid State Relay
of the major shortfalls of an electromechanical relay is that it is a
“mechanical device”, i.e. it has moving
parts so their switching speed due to physical movement of the metal contacts
using a magnetic field is slow even though the electro-mechanical relay (EMR)
are inexpensive, easy to use and permit a load circuit to be switched by an
longevity and efficiency of the relay may be affected by erosion and continuous
arcing. Also, they are electrically noisy with the contacts suffering from
contact bounce could affect any electronic circuits to which they are
Another type of relay called a Solid State
Relay or (SSR) for short. It is an electronic relay that does not require
solid state relay is only electronic and therefore has no moving parts within
its design as thyristors, and power transistors have taken the place of
opto-coupler type Light Sensor helps us achieve the gap between the output
voltage and the input control signal. Together with a much faster and almost
instant response time, as compared to the conventional electromechanical relay
the Solid State Relay is more reliable and durable. Also, there is no
interference from an external signal or arcing.
prevent overheating of the output semiconductor, it can be strategically placed
on an heatsink. 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.
Solid State Relay
solid state relay performs several functions. It prevents overcurrent and
prevents arcing that may be caused by load with high inductive and capacitive
(RC) snubber network is generally required across the output terminals of the
SSR. It shields the semiconductor from noise from an external source. It also
helps to prevent destruction when a voltage spike takes place.
RC snubber network is built as standard into the relay itself reducing the need
for additional external components is used In most modern SSR’s .
regulation of the brightness of light in pubs and club houses is done by using
the detection ability of an SSR.
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. For switching
large currents for long periods of time an additional heat sink will be
required because the output device is a semiconductor.
Input/output Interface Modules
modules are another type of solid state relay specifically created for computer
interface and as micro-controllers to “real world” loads and switches. Currently,
there are different types of I/O modules:
- AC or DC Input voltage
- TTL or CMOS logic level output
- AC or DC Output voltage
module contains all the necessary circuitry requirements to provide a complete
interface and isolation within one small device. They are available as
individual solid state modules or integrated into 4, 8 or 16 channel devices.
Modular Input/output Interface System Solid state relays are usually more expensive than electromechanical relays. Also, it is difficult for a solid state relay to switch currents emanating from small load.
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