The 12 vdc SPDT relay is a relay with one input and two outputs and five terminals, including the coil terminals. It has one input and two outputs. An SPDT relay does not only switch, it also routes signals. SPDT relays are used to control two electrical or electronic circuits. It has two contact positions.
We can consider output 1 and output 2 since the SPDT Relay has two outputs. In normal circumstances, one output is connected to the input while the other is disconnected. As soon as the relay is energized, the input terminal disconnects from the previous output and connects to another output. When the SPDT relay is off, the output terminal is connected to the common terminal, which is called the Normally Closed (NC) terminal. In the off condition of a relay, the output terminal that is disconnected from the common terminal is called Normally Open or NO Terminal.
When the coil of the relay is not energized or connected to any power supply, the NC contact remains NC and the NO contact remains NO. However, when the coil of the relay is energized or connected to power, the NC contact becomes NO and the NO contact becomes NC.
The SPDT relay symbol can be seen here:
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| spdt relay symbol |
Now, let’s see the connection diagram of SPDT Relay to understand its working principle easily.
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| spdt relay connection diagram |
As 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.
If the switch S1 is in the off position, the relay coil will not receive any power, and LED 1 will be off, while LED 2 will remain on. When the switch S1 is turned on, the relay coil will receive power and become energized. In this example, LED 1 will glow and LED 2 will remain off.
SPDT relays have a wide range of applications in electronic as well as electrical circuits.