What is SPDT Relay? Single Pole Double Throw Relay

An SPDT Relay or Single Pole Double Throw Relay is a type of relay that has one input and two outputs. Including the coil terminals, it has a total of five terminals. It not only provides the switching function even it can route the signal. An SPDT relay can control two electrical or electronic circuits. It operates in two contacts position.

As the SPDT Relay has two outputs so we can consider output 1 and output 2. In normal conditions, the input is connected to any of the output while another output is disconnected from the input. When the relay is energized, the input terminal disconnects from the previous output and connects to another output. In an SPDT relay, the input terminal is also known as the common terminal, and the output terminal is connected to the common terminal when the relay is in off condition, which is called Normally Closed or NC Terminal. The output terminal that is disconnected from the common terminal when the relay is in off condition is called Normally Open or NO Terminal.

When the coil of the relay is not energized or not connected to any power supply the NC contact remains NC and the NO contact remains NO. But when the coil of the relay is energized or connected to the power supply the NC contact becomes NO and the NO contact become NC

Here, you can see the symbol of Single Pole Double Through or SPDT Relay.

Now, let's see the connection diagram of SPDT Relay to understand its working principle easily.

Here, you can see the coil of the relay is connected to a battery in series with a switch S1. LED 1 is connected to the NO terminal and LED 2 is connected to the NC terminal.

When the switch S1 is in off condition, the coil of the relay will not get any power supply so LED 1 will be in off condition while LED 2 will glow. When the switch S1 is turned on the relay coil will get the power supply and it will be energized. So the LED 1 will be glow while LED 2 will be turned off.

The SPDT relays have a wide range of applications in both electrical and electronic circuits such as sensor circuit, Relay Boards, automation system, and many more.

Read Also: 

• What is SPST Relay? Single Pole Single Throw
• What is DPST Relay? Double Pole Single Throw
• What is DPDT Relay? Double Pole Double Throw
• What is Relay Switch? Circuit Diagram and Working Principle
• Undervoltage Relay Connection Diagram and Wiring Procedure

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