Relay Connections | Pin Details | Working Principle

A relay is an electromagnetic switch used to isolate and control high power devices from a low power device like Microcontrollers. It can also be used with IC555 to make different DIY Projects. There are different types of relay used according to the application needs. In our electronic circuits, the most commonly used relay is an SPDT Relay. SPDT Stands for Single Pole Double Through, which means a common terminal that connects to either of two other terminals (either NO/NC). There is another type of relay that is used in High power devices like 3 Phase motors and it is called Contactors. But we are not going to cover this topic here. We will see how to use the SPDT Relays used in our small electronic projects.

As you look at the symbol, you might have understood what is Normally Open and Normally Closed. A Normally Closed terminal always contacted with COM Terminal. Only if you give supply to the coil it latches towards ‘Normally Open’ terminal. If the power supply to the coil is removed then it latches back to the NC Terminal.

The SPDT Relay has 5 Terminals. Two for controlling the coil (input) and other three for controlling high voltage devices(an external device like fan, light, etc), The output terminals are NO, COM and NC.

• COIL (T1) – Coil 1 terminal (input low voltage)
• COIL (T2) – Coil 2 Terminal (input low voltage)
• COM – Common terminal for output
• NO – Normally Open Terminal
• NC – Normally Closed Terminal

The Coil T1 and Coil T2 are input terminals for the relay. In a 5V Relay, you need to supply 5V to the coil terminals that will energize the coil.

At the output terminal COM, NO, NC. We will be connecting High power devices like AC Light, Fan, DC Motor, etc.

A Simple example of connecting an AC Light with Relay is shown below.

The output side of the terminals only shown here (NO, COM, NC). A Neutral wire from the power socket is connected via relay to the light’s neutral terminal. The phase wire is directly connected to the light.

To turn ON the Light, we need Phase and Neutral to its terminals. We have connected the neutral terminal of light to ‘Normally Open‘ Terminal of the Relay.

So, in an idle state, the light won’t be turned ON. Only if we give power supply to the Coils of Relay, the relay will latch it terminal towards ‘Normally Open’ terminal and then the Power Supply flows through it. If the power to the coil is removed the Light will be turned OFF.

Similarly, if you connected the light in NC Terminal, by default the light will be in ON State. When you apply a voltage to the coil the Light will be turned OFF.