Relays

What is Relay

Relays are electromagnetic switches which are frequently used in electrical circuits. In a relay there is an electromagnetic coil and set of contacts. The coil is wound with thin copper wire so it has a high resistance due to which it takes or consumes very low current. Current taken by the electromagnetic coil is used to produce electromagnetic force which attracts the armature in the relay. The points or contacts are attached to the armature hence they state change when armature is attracted by the coil electromagnetic force. There are two types of contracts used in a relay. They are normally opened and normally closed. They are mentioned when the circuit is at rest and there is no power applied to the relay coil. When power is applied to the relay coil the contacts changes their state. Means the normally opened then becomes closed contact and normally close becomes opened. The armature is pivoted with a spring. When power is disconnected, the spring brings the armature back to its rest condition and the contacts also change their state accordingly. In a relay there are two circuits. One is the coil circuit and the other is the contacts circuit. The coil circuit controls the contacts circuit. Contacts are design to take heavy current. By applying small current to the relay coil could switch on heavy contacts which could pass large current. Relays are very frequently applied in automotive and cars electrical circuit application. They are used in horn circuit, fuel pump circuit, starter circuit, fan circuit, lights circuit, air conditioner circuit and many more.

A typical relay circuit diagram and working principle

In industry relays are used to translate status of machines to plc or dcs system and also for interlocking. Industrial relays are more robust and strong compared to other ordinary relay. In industry 24v, 110v and 220v coil relays are common. They could be 4 pin to 24 pin. 14 pin relays are very common which have four sets of normally open and closed contacts set.

Typical relay application in automotive horn circuit.

ISO Standards for relays:

ISO International Standards Organization had given specification for most common relays in automotive. They specify terminals as 30,87a, 87,86 and 85. Normally terminal 30 is connected to battery positive voltage or BT+. This applied voltage could come from some switch for control or they can directly come from battery through a suitable fuse. Terminal 30 and 87a forms normally closed and they are connected together when power is not applied to the relay coil. When power is applied to the coil terminal 30 and 87 are connected. So terminal 87 is NO or normally open terminal. Terminal 85 is connected to ground to provide ground or return path to the coil. Terminal 86 is connected to battery positive.

Automotive relay iso standard and pin configuration