The humble relay in control panels is probably the component you're most likely to have worked with. But it's surprising how often some of its functions are misunderstood. So if you're one of those people, don't worry - you're not alone!
One way to think about a relay is that it is like a light switch, where there's a button/switch you can push with your finger to allow electricity to flow through it. The difference is that a relay has an 'electronic finger', in the form of a magnetic coil. For the most basic relays this electronic finger has to stay pushing down for all the time that the relay is on.
The reason they're useful is for separating circuits. So for example if you have a controls circuit in a panel that's 24V AC, and need to use it to switch on power to a 24V DC solenoid, then you'll need to put 24V DC through a relay and out to the solenoid, and then use the controls circuit to switch that relay on.
The coil
The 'coil' is an electromagnetic coil, so when electricity is passed through it then it becomes magnetic. This pulls a metal armature towards it which is the electronic finger. The two terminals to connect live and neutral on either side of the coil are almost always called A1 and A2. These coils can come in any standard voltage, such as 24V DC or 110V AC. Remember that if it's AC power than live can go to A1 and neutral to A2 or vice versa, but if it's DC power then it must be connected the right way round.
The poles
Relays have a number of 'poles', usually one to four. A pole is a single circuit, so on a 2-pole relay you have two entirely separate circuits. The only thing that they share is the same coil, so the two poles are switched at the same time. So this is different from a 2-pole light switch, which usually has two switches as well as two circuits - a 2-pole relay only has one switch.
The throws
Each pole has a 'throw', so the switch is either open by default ('normally open' or 'NO') and 'thrown' closed, or closed by default ('normally closed' or 'NC') and thrown open. In fact it will often have a contact on both sides, so you can have one contact live when the relay is off and the other contact live when the relay is on. This is called a 'double throw', each pole has a 'common' ('C') terminal which then connects to a normally closed terminal when the relay is off and connects to a normally open terminal when the relay is on.
The physical parts of a relay are usually:
Relay base (for mounting on the din rail in a control panel)
Relay
Retaining clip
Label
The bases
These come in various standard shapes and sizes, most commonly;
8-pin rectangular
8-pin round ('octal')
11-pin round (also 'octal', for some reason!)
14-pin rectangular
The pins
Each 'pin' on the relay goes into a hole in the relay base which then corresponds to a screw terminal, or 'contact'. So a 14-pin relay is likely to be 4-pole double three, with these standard pins;
Coil contacts - A1 & A2
Pole 1 - C, NO, NC
Pole 2 - C, NO, NC
Pole 3 - C, NO, NC
Pole 4 - C, NO, NC
The current
One of the most important things when working on relays is to make sure that they're rated for the amount of current (and voltage) that you're passing through them. Sometimes they'll have a rating per pole as well as an overall rating, so it might be 6A per pole on a 4-pole relay but only 10A overall, so you can't put 6A through all four poles.
The bells and whistles
There's various features on the relay which come in useful, they're optional so not all of these are on every relay;
They'll usually have a diagram on the side showing what each terminal number does
Often there's a 'test flag', which is a small lever to manually turn the relay on without putting power to the coil
An LED lamp indicates whether the coil is energised or not
And the rest
All of the above relates to the most basic relays. There's a myriad of different types, but the language used to describe them is largely outlined above. Other types of relay include;
Latching relay - needs a permanent power supply and a separate trigger input, then the relay will stay on after the trigger has stopped, and need another trigger to turn it off again
Contactor - a relay with another name, which is designed to handle higher current
Timer - adds the ability to delay the relays reactions, or cycle on and off, often with several different functions selectable by a dial on the front
