Control Relay Tutorial - How a Relay Works

This basic relay is constructed of 5 main parts:
The Coil
The Armature
The Contacts
The Base (which consists of the socket pins)
The Molded Plastic Frame

The relay works on the principle of electromagnetic force. When the coil is energized, it becomes magnetized. The armature (made of a ferromagnetic material and in close proximity to the coil) - is attracted to the coil by this magnetic force and moves towards it until it comes to rest against the coil's iron core.

Attached to the pivoting end of the armature is a spring. The purpose of the spring is to return the armature to its original position (away from the coil) when the coil is de-energized. Also attached to the armature are the arms of the movable set of contacts (the common contacts). See the illustration below:

In an 8 pin relay (as shown here) there are 2 common contacts, 2 normally open contacts, and 2 normally closed contacts. In an 11 pin relay, there are 3 of each of the aforementioned. These contacts are made of an electrically conductive material such as copper. The common contacts have this material embedded on both sides of the movable arms. The relationships of all of the contacts is explained as follows:

The common contacts carry the supply voltage that is to be connected to another electrical device(s). In the de-energized state of the relay, these common contacts are in contact with (touching) the normally closed contacts.

Before we go to far, it's important to think of a relay as an electrical switch. That is, a remote controlled switch, designed to direct the current path from one part of the circuit to another. You must also understand that although there are (in this case) 2 of each type of contact (common, normally open and normally closed), each is designed to complete a path independent and separate from the other contact of similar type.

Also, when we use the term "normally closed" it means that in the normal state of the relay (de-energized) the common contacts are providing conductive paths to their normally closed contact partners and only to these contacts. At the same time, there is no completed paths to the "normally open" contacts.

So then, when the relay is de-energized, the common contact #1 is making contact with the normally closed contact #1 and the common contact #2 is completing a circuit path with the normally closed contact #2. When the relay is energized, the situation is reversed. Now the common contact #1 is completing a conductive path to the normally open contact #1 and at the same time, the common contact #2 is making contact with the normally open contact #2. The electrical conductive paths that did exist with the "normally closed" contacts have now been "opened". The paths no longer exist.

The Pin Out connections on the base of the relay are as follows:

Pin #1 = Wired to Common Contact #1
Pin #2 = Wired to One end of the Relay Coil (electricity flowing through both ends is required to energize)
Pin #3 = Wired to Normally Open Contact #1
Pin #4 = Wired to Normally Closed Contact #1
Pin #5 = Wired to Normally Closed Contact #2
Pin #6 = Wired to Normally Open Contact #2
Pin #7 = Wired to the Other end of the Relay Coil
Pin #8 = Wired to Common Contact #2

Base of Octal Pin Relay

For other views of the relay, click here.

To further illustrate how a relay works, let's look at it as it functions within an electrical circuit.

First, we need to know the symbols for the parts of a relay. The illustration below shows the 3 symbols used in an electrical schematic to represent the 3 parts: The coil, a normally open contact and a normally closed contact.

You'll note that there is no symbol representing the common contact of a relay. That's because it's generally understood that one side of each of the normally closed and normally open contact symbols is to be the common contact side. It can be either side and it depends on how you reference the pin connections in the drawing. Look at the illustration below as we examine a pushbutton, relay coil, 2 contacts and 2 lights in a simple control circuit.

NOTE: ALL ELECTRICAL SCHEMATIC DIAGRAMS CREATED USING EZ SCHEMATICS

L1 and L2 represents our supply voltage which could be 120 VAC or 24 VDC or something else... it really doesn't matter in this case. The symbol labeled PB1 represents our push button. It is shown in the open position. As a footnote, all electrical schematics are drawn in the de-energized state.

The devices labeled GRN and RED represent our lamps. CR1 is the relay coil, CR1.1 is one of the normally closed contacts belonging to the relay CR1 and CR1.2 is one of the normally open contacts. Note that the pin numbers for the relay parts have been referenced (refer to the pin out table above for clarity).

RELAY UN-ENERGIZED (as shown above)

When the push button (PB1) is not pressed, current (shown in purple) is able to flow from L1 through the normally closed contact CR1.1, through the green lamp (GRN) and back to L2, thus energizing and illuminating our green lamp.

RELAY ENERGIZED (as shown below)

NOTE: ALL ELECTRICAL SCHEMATIC DIAGRAMS CREATED USING EZ SCHEMATICS

When the push button (PB1) is pressed, current is able to flow from L1 through PB1, through the relay coil (CR1) and back to L2, energizing our relay. At this point, our contacts switch. All of the contacts that were closed, now open and all of the contacts that were open, now close.

So, with the relay CR1 energized, the contact CR1.1 opens, blocking current flow on that path and de-energizing our green lamp. The contact CR1.2 closes, allowing current to flow from L1, through the contact CR1.2 and the lamp (RED) and back to L2, thus energizing and illuminating our red lamp.

Note that the symbols used in the latter drawing (and the colors as well, for that matter) are for illustrative purposes only. Remember, electrical schematics are always drawn in the un-energized state.

These illustrative drawings were created with the software application EZ Schematics. A description of the program can be found here.

Learn more about relay logic here.