Makeup of a Relay
Relays are simple electromechanical switches that operate on electromagnetic principles. Each relay contains a few simple parts that all work together to allow the relay to function.
Each relay contains an electromagnet, which controls the opening and closing of the relay, and an armature that actually opens and closes. Relays also contain a spring, which forces the relay back to its original state after each cycle, and a set of electrical contacts to transfer power.
Relays are used most often when it's necessary to switch from a small amount of power to a larger amount, and they are used to efficiently regulate the flow of energy in an application. Relays also can be placed in tandem, in order to "step up" to very high voltages.
How a Relay Works
A relay is made up of two circuits. One circuit is the switch, which controls power to the electromagnet that controls the relay.
With the electromagnet turned on, the armature is attracted to the contact point, and upon contact the armature completes the circuit. The completed circuit allows the current to flow freely, activating whatever device the relay is designed to operate.
When the first switch is turned off, the electromagnet loses power and stops attracting the armature. The spring pulls the armature away, and it is released from the contact point, and the second circuit opens.
This is most easily seen in a light switch. Flip the switch, and the light comes on; flip it the other way, and you have darkness.
What to Know About Relays
There are a few things that you need to know before you can purchase the correct relay for a task. Not all relays are created equal, and small differences can create large mistakes if you don't plan carefully First, figure out whether you want your device to be "always on" or "always off".
This also might be known as "normally on" or "normally off" If you want the device on constantly, you'll need to select a relay where the armature is always in contact with the contact point and the electromagnet is used to pull it away and break the circuit. Normally off is the opposite.
In order to avoid electrical mishaps, you'll also need to know the maximum voltage that you can put through the armature before it fails. If you burn out the armature, you'll have to replace the entire unit.