Relay

A RELAY is an Electromagnetic switch that opens & closes when sufficient current & voltage is applied to it.

Why are Relay Used?

Relays are basically used for

·         To TURN ON/OFF high voltage or high current circuit using a low voltage signals e.g. turning ON/OFF motors using a micro controller.

·         To provide isolation between the control circuit & the output e.g. operating a solenoid.

·         As circuit Breakers in case of overload or shot circuit.

OPERATION

A Relay consists of four parts:

1.       Electromagnet also called as coil.

2.       Armature.

3.       Spring.

4.       Electrical Contacts.

When current flows through the coil a magnetic field is generated, if the current is sufficient i.e. if it is equal to or greater than the “PULL IN” current then the magnetic field generated is strong enough to attract the armature which is linked mechanically to a moving contact. This movement of armature makes or breaks the contact. When the current is turned off the spring pulls back the armature to its default position. The Armature is connected to a POLE; a relay can have more than one pole & the contacts of these poles can be thrown in two ways:

·         Normally Closed (NC) contact is made when the relay is deactivated i.e. when there is no current flowing through the coil.

·         Normally Open (NO) contact is made when the relay is activated.

The COMMON (COM) is connected to the ARMATURE & is in contact with either NO or NC.

DESIGNATION

Relays are commonly designated on basis of number of poles & throws e.g.

·         SPST:  Single Pole Single Throw. Other that two terminals for coil, there are two more terminals which are either of Normally Open or Normally Closed type.

·         SPDT: Single Pole Double Throw. Total five terminals are there which includes two for coil & one common, one normally open & a normally closed contact.

·         DPST: Double Pole Single Throw. It is equivalent to TWO SPST relays which are controlled by a single coil.

·         DPDT: Double Pole Double Throw. It is equivalent to TWO DPDT relays which are controlled by a single coil.

For higher number of poles the “S” or “D” is replaced by the number of poles.

Other variables to be considered while purchasing a relay:

·         Voltage and Current required for activating the coil.

·         The Maximum rating of the contacts.

·         The Switching Time i.e. time required to TURN ON/ACTIVATE the relay.

·         Type of packaging/enclosure.

·         Type of Mounting.

·         Insulation Resistance.

·         Temperature Range.

 

Advantages of Relay

Disadvantages of Relay

RELAY DRIVER

Digital IC’s cannot provide the necessary current & voltage to TURN ON a Relay. So a Relay Driver Circuit is required. The basic function of the driver circuit is to provide the necessary current to energize the relay coil.

Generally relay coils operate from 5V to 24V & require about 25mA to 100mA current to energize the coil, the current required to Turn ON a relay is called as the “PULL IN” or “HOLDING” current. This PULL IN Current depends upon the Relay used.

The simplest of relay driver circuit consist of a NPN or a PNP transistor. Almost all the digital circuits can

provide enough base current to TURN ON a Transistor. In the adjoining figure you can see an NPN transistor BC 547 is being used to drive the relay. The relay has a 12V coil i.e. it can be turned ON only when the operating voltage i.e. the V_cc is 12V. The Resistor R1 is used to set the base current for the transistor, the value of R1 should be such that when V_in is applied the transistor is driven into saturation i.e. it is fully turned ON & the Relay is energized. It’s important that the transistor is driven into saturation so that the voltage drop across the transistor is minimum thereby dissipating very little power.

Now we have to calculate the value of R1. Suppose the relay requires a PULL IN current of 80mA. So the collector current has to be at least 80mA.  The minimum DC current gain of BC 547 is 100, so the minimum base current should be

                                I_B= I_C/H_fe

i.e.                       I_B=80mA/100

So the minimum base current is 0.8mA. But to be on the safer side i.e. just to make sure that the transistor is in saturation region, we approximately double this value to say 1.5mA. Now if the V_in is switching from 0V to 12V. Then the base resistor

 R1 = V_in/I_B

V_IN= 12V & I_B=1.5mA So R1= 8KΩ

So we can use 8KΩ resistor to provide base current of 1.5mA, which is sufficient to TURN ON the relay & activate the Relay.

Most of the digital circuits require 5V to operate in that case V_IN will be 5v so the base resistance R1 will be 3.33KΩ.

A diode (1N4007) is connected across the relay coil; this is done so as to protect the transistor from damage due to the BACK EMF generated in the relay's inductive coil. When the transistor is switched OFF the energy stored in the inductor is dissipated through the diode & the internal resistance of the relay’s coil.