What is a Relay Switch
and How Do You Wire It?
By John Hamlin, Guest Author

What is a Relay Switch? -

A relay switch is an electromechanical device that is used to make or break an electrical connection. It is an electrically operated switch that uses an electromagnet to operate manually. A relay switch consists of many flexible mechanical parts that can be controlled by an electronic signal without human involvement. Relay switches are used to control high-powered circuits by using a low-powered signal. Relay switches are of many types, and each performs a different function.

Parts of a Relay Switch -

An electromechanical relay switch is encased in a copper or plastic material. It consists of mechanical parts such as an electromagnet, contacts, yoke, spring, and a moveable armature.

     Electromagnet -

    An electromagnet plays a pivotal role in relay switch working. It is a metal that is not magnetic itself but can be made magnetic with the help of an electric signal.

     Armature -

   An armature is a simple moveable piece of metal that is balanced on a stand or a pivot. The function of this part is to break and make connections when the contacts are connected to it. 

     Contacts -

   Contacts are the conductors that are connected to the terminals inside the device.


    A yoke is a small metal piece that is present on the core. When the coil is stimulated, the yoke attracts and holds the armature.

     Spring -

   The spring is optional and is not present in every relay switch. One end of the armature is connected to the spring. It helps to ensure that it is easily and freely moveable.

How Relay Switches Work -Relay in NORMALLY CLOSED Condition-

When the core has no voltage, it cannot produce a magnetic field. Without being magnetic, it cannot attract the movable armature. Therefore, the initial position is normally closed.


Relay in NORMALLY OPENED Condition-A magnetic field is produced when a sufficient amount of voltage is applied to the core. This makes the core magnetic and thus attracts the armature and changes its position. The external circuit is connected and the armature is connected to the opened pin of the relay.Difference Between 4- and 5-Pin Relay -4-Pin Relay-

   A 4-pin relay√ controls only one circuit. There are two types that are accessible: normally open and normally closed.

   For coil regulation, two types of pins (85 and 86) are used. To switch power on a single circuit board, two pins (30 and 87) are used.

   When the coil is energized to normally open, the relay switch will power ON for the circuit.

   When the coil is stimulated to be normally closed, it will shut the power OFF for the circuit.

5-Pin Relay-

   A 5-pin relay√ works as a power switch between two circuits.

   To power up the coil, two pins (85 and 86) are needed. Three pins (30, 87 and 87A) are required for transmitting power among both circuits.

   When the coil is stimulated, the power will shift from a normally closed pin to normally open pin immediately.

Wiring a Relay Switch -

      When pins 85 and 86 are connected to the coil of any voltage, the SPDT switch will start working immediately.

      Therefore, the coil is not powered, and the pins 30 and 87A are connected. On applying voltage, the pins 85 and 86 and 30 and 87 connect.

      With the positive terminal of the current source, pin 30 connects with a fuse.

      On the negative terminal of the power source, the load connects with the help of pin 87.

      After that, pin 86 is connected to the positive terminal, and pin 85 connects to one of the posts of the push-type switch.

      The remaining post of the switch should be grounded.

Applications of Relay Switches -

      The applications of the relay switch are limitless, so they are often used to control high voltage circuits with a low voltage power supply.

      Relay switches are used in computer circuits for arithmetic functions.

      Relay switches are used in electric motors and automatic stabilizers.

      Relay switches are used in television sets in order to turn on picture tubes with a DC supply.

      Relay switches are used in traffic signals, temperature controllers, and working railroad models.

      Relay switches can be recycled and reused. The coil winding is used as a jumper wire in various circuits.

      The internal parts such as screws, nuts, bolts, and washers can also be reused in various circuits.

[ I have used the Atlas Relay Switch shown above for wiring model railroad signals, so that when you hit the momentary switch to change the direction of a turnout, the lights on the signals change from red to green or vice versa. That way when your train is approaching the turnout, you know whether the turnout is lined up properly for your approach or not. - Ed.]

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