What is a relay? : "Relay" is an electrical switch that allows another electrical circuit to open and close. This switch is controlled by an electromagnet. It was invented in 1835 by Joseph Henry.
The relay's contacts can be normally open (“Normally Open - NO”), normally closed (“Normally Closed - NC”) or the changing type of the contact.
Relays look like a transistor task, for example when you supply current on a simple 3 leg relay, the arm on the chassis opens the current on the other side, ie it can be used for control.
There is only one difference from transistors: no need to use with resistance. When the coil is magnetizing the two contacts, a contact of the relay closes a contact.
The relays are an electromagnetic switch that operates with low currents. When a voltage is applied to the electromagnetic coil on the electromagnetic coil, the coil gains the magnet feature and acts as a key by pulling a metal palette facing the opposite one by connecting one or more contacts to one another.
Relays, which lost their popularity after the production of Thyristors and Triacs, are still used in many fields. Its only advantage over thyristors and triacs is that it can have multiple switch contacts in a single body, it can open or close multiple loads at the same time, and even turn on and off some loads at the same time. This process is completely related to the design of the relay's contacts. Relays can be used to operate on both AC and DC. Generally; relays are divided into two as switch and measuring relays. Also; Relays are also grouped according to their operation, usage purpose and how they are connected to the circuit.