﻿ How to Connect a Single Pole Double Throw (SPDT) Relay in a Circuit

# How to Connect a Single Pole Double Throw (SPDT) Relay in a Circuit

In order to know how to connect a single pole double throw(SPDT) relay, you must know what each pin terminal represents and how the relay works.

### Terminal Pins

A Single Pole Double Throw Relay comes with five terminal points.

The terminals are COIL, COIL, COM, and NO, and NC.

This correlates to the following in the relay:

### Terminal Descriptions

COIL- This is one end of the coil.
COIL- This is the other end of the coil. These are the terminals where you apply voltage to in order to give power to the coils (which then will close the switch). Polarity does not matter. One side gets positive voltage and the other side gets negative voltage. Polarity only matters if a diode is used.
NO- This is Normally Open switch. This is the terminal where you connect the device that you want the relay to power when the relay is powered, meaning when the COIL receives sufficient voltage. The device connected to NO will be off when the relay has no power and will turn on when the relay receives power.
NC- This is the Normally Closed Switch. This is the terminal where you connect the device that you want powered when the relay receives no power. The device connected to NC will be on when the relay has no power and will turn off when the relay receives power.
COM- This is the common of the relay. If the relay is powered and the switch is closed, COM and NO have continuity. If the relay isn't powered and the switch is open, COM and NC have continuity. This is the terminal of the relay where you connect the first part of your circuit to.

Now that we know what each terminal pin represents, we now wire it to a circuit for it to do a real-world function. We're going to connect a single pole double throw relay to a circuit to light up a LED. When the relay isn't powered, the red LED is lit and stays on. When the relay is powered, the red LED shuts off and the green LED lights up.

This is the circuit below:

Since the relay is rated for 12V, it should receive 12 volts in order to power on. It may work with less voltage, but 12V is really what it should receive. This goes into either side of the COIL terminals. Even if you switched the positive and negative voltage of the power supply, it should work exactly the same.

The COM terminal of the relay gets connected to the first part of the circuit. If there is no first part of the circuit, this terminal can be left open. In this case, the first part of the circuit is a 5-volt power supply to light the red LED.

The NC terminal of the relay gets power even when the relay isn't powered. This means that as long as the 5-volt power supply is on, the red LED will be lit.

The NO terminal of the relay gets power only when the relay is powered. When the relay receives 12 volts of power, the relay's snaps from the NC position to the NO position. The red LED now shuts off and the green LED turns on.

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