A relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays.
Relays are used where it is necessary to control a circuit by a separate low-power signal, or where several circuits must be controlled by one signal.
“A relay is a device designed to cause a sudden predicted change in a single
or multiple electrical output circuits when certain conditions are satisfied by
the electrical circuit that contains the relay device”.
Symbol
The above figure shows the most commonly
used symbol of a relay. A1 and A2 represent its coil and 11, 12 & 14
represent its contacts.
Relay Symbol
Where do we use relays?
Relays have a wide range of application. You can find
them everywhere: home appliances, automobiles, industries, and even in copy
machines. In some applications, they are used for switching or control of
circuit (as in timer-based lighting control) whereas in others they are used to
sense and protect circuits (as in the case of earth fault protection relays).
Therefore, it is difficult to specify their area of application area.
Principle of operation
They are basically classified into two types based on their
working principle as electro-mechanical and solid-state relays. Let us discuss
the principle of operation of each one of them in detail.
Operation of electromechanical relays
Electromechanical relays transfer signals between its
contact through a mechanical motion. It consists of two sections: the first is
the electromagnet section and the other is the armature and mechanical contacts
section. The electromagnet section consists of a set of coil wound over a
magnetic core.
When an input voltage (almost equal to the rated voltage
of the coil) is applied to the coil, it gets magnetized and attracts the
armature. The mechanical contacts are attached to the armature. Hence, when the
armature is pulled towards the electromagnet, the contact closes. When the
input voltage applied to the coil is removed, the armature is brought back to
its original position by the spring release.
Operation of Solid-state relays
Solid-state relays are commonly known as SSRs. Unlike electromechanical type, they do not posses any mechanically moving parts. On the other hand, it consists of semiconductor and electronic components within. In solid-state relays, the electromagnetic section is replaced by optocoupler and required driver circuits and the output contact section is replaced by a TRIAC or transistor plus snubber and driver circuits.
When the rated voltage is applied to the input section,
current flows through the optocoupler. The output of the optocoupler is used to
operate the switching circuit of TRIAC or transistor. Switching circuit applies
a gate pulse to the TRIAC and the TRIAC starts conducting. Similarly, when the
applied input voltage is removed, the optocoupler turns off the TRIAC switching
circuit and which, in turn, stops the gate pulse to the TRIAC and the TRAIC
stops conducting.
In the following sections, we shall discuss in detail the
parts and operation of electromechanical and solid-state relays in detail.
Parts of an electromechanical relay
A typical electromechanical relay consists of the
following components:
- Electromagnetic coil
- Armature
- Core
- Movable contacts
- Spring return arrangement
Electromagnetic coil
The electromagnetic coil is the most important part of an
electromechanical relay. It consists of a set of copper windings over a
magnetic core. As you know, the flow of current through the coil produces a
magnetic field. Therefore, when voltage is applied to the coil, it becomes an
electromagnet and attracts the armature.
Armature
An armature is a movable piece of metal, balanced using a
pivot.
Core
Core is the metallic part over which the coil is wound.
Movable contacts and fixed contact
Contacts are the conducting parts inside the relay, that
open or close when voltage is applied to its coil. The contact that is
attracted by the electromagnet is called movable contact and that is stationary
and connected to the terminals are called fixed contacts.
Spring arrangement
Spring arrangements are also present in them, such as to
bring the armature and the contacts back to the original position when the coil
is de-energized.
Parts of a solid-state relay
As discussed earlier, solid-state relays do not have any
movable parts within. In order to explain their internal parts, here, we have
split it to the following sections:
- Input circuit section
- Electrical isolation
- Driver circuitry
- Output section
- Input Circuit Section The input circuit consists of diodes transistors gates and resistors required to drive the optocoupler.
- Electrical Isolation Unlike electromagnetic relays, the input and output sections of an SSR do not have any physical contacts. Galvanic separation is provided between them using optocouplers.
- Driver Circuit Driver circuits consist of components required to turn on the TRIAC or transistors or thyristors in the output circuit. The o/p of the optocoupler is conditioned and the gate pulse needed to trigger the transistor is generated.
- Output Section The output section consists of semiconductor devices such as transistor or TRIAC or thyristor as an alternative to relay contacts.
Application
Relays
have a wide range of applications starting from washing machines at homes to
the telecommunication systems at the International space station, they can be
found everywhere. The following are a few key applications:
- They are used in electronic circuits and home appliances for isolating low voltage or DC circuits from high voltage AC circuits.
- They are the backbone of industrial process automation systems. They are used in combination with PLCs for process control. They are one of the key components in an automation cabinet.
- Used for signaling and control in railway networks.
- In motor control circuits for motor switching, protection as well as control.
- In substations and power distribution centers for sensing various faults and operating the circuit breaker.
Relay
selection considerations
The following factors must be considered while selecting a relay for any application.
- Nominal Voltage: The voltage at which the coil is designed to operate.
- Rated Power: The power consumed by the coil at normal room temperature.
- Contact Rating: The current carrying capacity and voltage rating of their contacts
- Contact Mechanism: The number of contacts required and the contact configuration (NO/NC/changeover).
- Environmental Protection: the degree of sealing required, meaning, whether the external casing of relay is necessary or not?
- Insulation Resistance: Insulation resistance between any two sets of contacts and that between the contacts and the coil.
