A relay is essentially a switch that works electrically rather than mechanically. Although various relay designs exist, the ones most commonly found in low voltage auto and marine applications are electromechanical relays that work by activating an electromagnet to pull a set of contacts to open or close a circuit. These are widely used in all vehicle electrical systems.
Why is that useful? Many sensors are incredibly sensitive electronic pieces of equipment and produce only small electrical currents. But we often need them to drive larger appliances that use larger currents. The relays close the “gate”, making it possible for small currents to activate larger ones. That means relays switches can work as (turning things on and off) or as amplifiers (turning small currents into larger ones).
How do they work
When the coil is energized, a magnetic field is generated around it which pulls the articulated armature into contact. This completes the "high" current circuit between the terminals and the relay is said to be energized. When the voltage is removed from the coil terminal, the spring pulls the armature back to its "rest" position and breaks the circuit between the terminals. So by applying or removing power to the coil (the low current circuit), we turn the high current circuit on or off.
Why use a Relay?
There are several reasons why we may need to use a relay:
Change a high current circuit using a lower current circuit
This is the most common and useful reason when an existing line breaker or circuit is not capable of handling the required current. For example, if you want to install some high powered work lights that turn on with your headlights but there is a risk that they will exceed the capacity of your existing installation.
Cost savings
High current capacity wiring and switches cost more than lower current capacity versions, so using relays minimizes the requirement for more expensive components.
Activate more than one circuit from a single input
You can use a single input from one part of an electrical system (eg central locking output, manual switch, etc.) to activate one or more relays which then complete one or more circuits and thus perform multiple functions from an input signal.
Realization of logical functions.
Electromagnetic relays can be put into some pretty clever (and complex) applications when they are wired to perform logic operations based on certain inputs (e.g. open and close a +12V output from a momentary input , alternating left and right flashing lights, etc.) Although these logic functions have now been superseded by electronic modules for OEM designs, it can still be useful, fun, and often more cost-effective to use relays to realize them in some after-market projects (particularly when you have a bespoke application) .
Advantages of relays
The great advantage of electromagnetic relays is the complete electrical separation between the driving current (the one that circulates through the coil of the electromagnet) and the circuits controlled by the contacts. This makes it possible to handle high voltages or high powers with low control voltages.
They also offer the possibility of remote control of a device through the use of small control signals, making consumption low.
On the other hand, the voltage drop across the shorted contacts is very small, because their resistance is milliohms.