In this tutorial (relay types and applications) we will understand need of relay in various application and how we use it in the different circuitry for the different types of application. We will also know working of relay, how we select the relay for our circuit, practical consideration of relay, protection of relay, types of relay and comparison of relay and transistor.


It is an electro-mechanical switch which used to control high voltage from the low voltage digital logic. It is very important device for the controlling the AC appliance through the digital hardware. Digital hardware work on the high and low voltarelayge signal, whenever we required to control the high power AC appliances we need a hardware which interface these digital hardware to high power devices. A symbol of relay shown in figure in right side. Relay consist a coil and contacts, when the coil get energized from the supply then pole of relay switch to N/O from N/C. According to the connection switching of devices can be achieved through the contact to the N/O or N/C. The various types of relay available in the market according to the number of poles, N/O and N/C.

Need of a relay

 Now a question arise in mind that if different switches are available then why we used relay it increase the size of hardware as well as cost also. So a relay is used where isolation may be required between the input and output. There is no electrical connection inside the relay between the two circuits (coil and switch contact) they are completely separated from each other, the link is either magnetic or mechanical. For example a low voltage battery or digital logic circuit can use a relay to controlling 230V AC mains supply. By using proper arrangement of switch three phase supply can be also control.

Working Principle

 The relay work on the Lenz law according to them when a wire wound on a material in the form a coil and applies the current then this coil act as a magnet. When current flow in the coil magnetic field induce due to these magnetic field a lever attract toward the coil. Now the switching action performs by charging and discharging of the coil. A small normal relay consist three terminals that is pole (common), normally open (N/O) and normally close (N/C). The functioning of these three terminals as follow.

Pole/COM = Pole is movable part so it is connected with the lever.

NC = A terminal which is connected with the pole without and providing power supply is called N/C

NO = Normally Open terminal connected with pole when relay gets on.

According the application relay contact connected with hardware. Whenever we required that load gets on after the on condition of relay then we connected with the pole and NO or when we required load devices get on in off conditions of relay then we connected load between pole and NC.

Practical consideration to choosing a relay

For better functioning of a circuit we need to consider following features.

1. Physical size and pin arrangement

Relays are available in various size and different pin arrangement. To select appropriate relay we need to refer supplier’s datasheets and compare analysis of different relay, now which one best for our design existing PCB is choose.

2. Operating voltage

Relays are available in market by voltage and current rating, typically 6 V to 24 V relay easily available in market with 250 mA to 15 Amp. The operating voltage and current generally mentions on the catalogue and datasheets of Manufacture Company. Some relays may be operated perfectly with a little lower power supply of their rated value.

3.      Coil resistance

The design hardware must be able to supply the proper current of a relay. The value of current calculated by ohm’s law

Relay coil current = Supply Voltage/Coil Resistance

Generally to drive a relay minimum 30 mA current required. For example a 12 V relay with coil resistance is 125 ohm, and current will be 96 mA. If it drives from the any digital logic hardware then we need minimum 96 mA current rather we required transistor for the current amplification. Processor based hardware does not have more current so they need transistor for the amplification of the current or in other hand analog hardware like op-amp and timer based hardware consist these current that’s why they doesn’t required and transistor circuits. This is due to because most of the analog IC internally have transistor for the drive load.

 4.    Switch ratings (voltage and current)

The switching rating of relay decides which rating load may be drive from the selected relay.  The vendor mention on the relay of the load rating voltage and current. These rating may be high voltage and current like 1 Amp to 15 Amp with 24 volt to 220 V AC as well as DC.

 5.      Switch contact arrangement (SPDT, DPDT etc)

The switching arrangements of contact choose according to the application. Generally relay contact are SPDT or DPDT. SPDT relay is more popular in market.

Protection of relay

When it is used in the circuit then it does not connected directly the following additional component may be used for proper functioning.

Protection diode: – A protection diode connected in to the reverse biased of the relay. Reason behind the using of the diode is that relay design from the coil, when diode switching in high frequency high coil cannot charge and discharge fast, so providing a discharging path a diode is connected these diode come in pictures when relay gets off. This diode called free-wheeling diode or backwards diode. When it gets off condition then high voltage ‘spike’ also produce that damage the relay, transistors and IC which is connected the circuit. So diode also protect from these voltage.

Protection Capacitor: – A capacitor is connected parallel to the diode to protect it from high voltage.

Switching Transistor: – Transistor is used with relay coil and work as a switch. The use for transistor is two reasons.

  1. To drive load from the low or high voltage digital logic.
  2. For amplify the current

Types of relay

Relays are available in two types

  1. PCB mount
  2. Chassis mount
  3. Reed relays

PCB mount relays: these relays are mostly used because they easily mount on the hardware, small size light weight and long life.

Chassis mount relay: – this relay is mount on the chassis so we required additional space for mount the relay due to this they are rarely used in hardware and they are large size and small life.

Reed relays: – This type of relay come in the DIP package IC. It consist a coil which is surrounding with the read switch. When current flowing in it, coil gets magnetize and contact achieve.  Coil resistance of read switch have higher typically, approx 1 K . Reed relay comes is wide range operating voltage generally 9-20 V with low current rating, maximum current rating are available 500 mA. They are capable for high speed switching.

Comparison of Relays and transistors





Relay control AC appliances Transistors cannot  control AC appliance


Used for switching of large current typically greater than 5 Amp Used for switching of small DC current typically less than 1 Amp.


Electromechanical switch Electrical switch


Relay can switch many contacts at once Transistors can switch only one contact at once


Relay can switch AC as well as DC Transistor can switch on DC


It can switch with high voltage It cannot switch with high voltage


Switching speed are slow (except read relay) Switching speed is very fast.


It is bulkier It is small


It required more power to operate It required less power to operate


Relays require more current to operate Transistor doesn’t require more current to operate