Relay working and types

A relay is electro mechanical switch which is controlled electrically.  It is used for isolation of two circuits having different operating voltages.  For example we want to isolate 220 volt AC power supply from 5 volt dc power source, in such case relay is used to separate them.  This type of relay is called electromagnetic relay (EMR).

Applications of relays

In many industrial applications and in control rooms where we have to control several circuits with one signal for this purpose relay is the best device to be used, because of its switching capability it can be used in different circuits. It is basically an electrical operating switch. Most of the times, they are used to switch the state of the circuit. It is also used as protection or isolation. Sometimes it is also used for time delay. They are aslo used in pic microcontroller projects.

Components of electromagnetic RELAY

There are three most important parts:

  1. Electromagnet

In electromagnet there are coil of wires wounded on an iron core. It becomes active when there is presence of current or voltage in coil.

  1. Contacts

There are two contacts NO (normally open) and NC (normally closed). When there is no current or voltage in the coil NC path will be followed but when voltage or current pass through the coil it will follow NO path.

  1. Spring

It is used to make normally close and normally open contact when current passes through a coil.


Relay switching circuit (coil) based upon the DC or AC voltage. For example when 6v given across the coil, current start flowing through it, energized and switched from NC path to NO path. Before energizing NC path followed but after switching NO path will be followed.


In above figure 1 and 2 shows contacts which are open while 3 and 4 shows coil. When coil become energized it become closed and change its state. As given in below figure:


Relays falls into major three categories. These are:

  1. General purpose (in which simple switching done).
  2. Machine control  (used on large scale such as in industries when we dealing with large motor with high ratings).
  3. Reed  (this type of relay very fast and sensitive. Free from any kind of humidity etc).

Other types of relays also used know as solid state relay in which DC voltage used as input.

Relay interfacing with microcontroller

Microcontroller do not have considerable amount of current to operate relay. Relay coil needs minimum 10mA current to energize. But microcontroller output pint can provide maximum current up to 1-2mA.  To resolve this problem relay driver circuits are used between microcontroller and relay

Solid state relay

Solid states are also used in industrial applications. Solid state relay doesn’t have any coil, contact and spring.  It is basically made up of semi conductor materials.  Its switching speed is much higher than electromechanical relays, because there are no mechanical movements happened inside them. Its life is also longer than EMR. They can be directly interfaced with microcontrollers due to very low input current requirement.  Therefore solid state relays are ideal choice for microcontrollers and digital circuits.  They are also come with phase control options. Different current and voltage rating solid state relays are available in market.  Like 143058CP SSR have control volts of 4-32 volt DC and contact current of 3 Ampere.

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