What's the real function of a RELAY ?

There are a few functions. Yes one of them is to switch a higher voltage. Computer control systems often operate at 24v dc so a relay is used to switch the higher voltage and/or current of the power side.

They are also used because you can have more than one contact so it lets you switch many devices from one output. You can also have "normally closed" contacts that open when you energize the relay. They used to be used in large quantities for these properties until PLC's etc pretty much took over.

They provide an electrical isolation so they also get used for this.

Yes, its just a fancy switch but an extremely useful one.

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RE:

What's the real function of a RELAY ?

Ok, i know some basics of how does a relay functions, it has a coil that moves an iron bar when current is applied and that iron bar closes a circuit allowing the conection.

Now... the relay has a switch that sends the current to the coil so the iron bar closes the main relay switch, so...

Function Of Relay

They're not for isolation that much in vehicles. They're usually to allow a small current to control a large current. That way, a switch to turn on the headlights, start the engine or honk the horn doesn't have to carry the full current of the device. The switch can be small, with small wiring and only has to deliver a small current to control the relay. They also keep arcing down, since the relay switches much faster than a manual switch.

I use relays every day in my Low Voltage Protection controllers for pumps. The coils in the relay keep the power switch contacts closed as long as a sufficient voltage exest to maintain the magnetic field. If voltage drops off, the magnetic field is not sufficient to keep the contacts closed, so they open, protecting the motor from an undervoltage condition. They can also be used for overload protection by using a current to open the contact when the current flow is to high, causing a strong enough magnetic field to open the contacts.

Relays are also used in applications where you need to control something remotely. It is a lot safer and cheaper to wire a low voltage(or current) remote switch to power a relay for a high voltage (or current) application.

Relays on automobiles control a variety of functions such as signal lights. They are of a compact design and changeout is simply unplug the old and plug in the new.

"it has a coil that moves an iron bar when current is applied and that iron bar closes a circuit allowing the connection."

correct. But you have a few misconceptions:

"the relay has a switch that sends the current to the coil"

NO, there is no switch in the relay that switches coil current. The coil wires/terminals are directly connected to two of the pins of the relay

"or is it that the switch that the relay closes is a higher voltage switch than the main relay switch that sends the signal to the coil ?"

see above, there is no switch to the coil

Relays have lots of uses.

Power gain. A 10 watt relay (it takes 10 watts to the coil to cause the contacts to close) can switch a 1000 watts with the contacts. Or more.

Isolation. There is no connection between the coil and the contact. And between one set of contacts and a second set of contacts. Isolation resistance is very high.

switching: Very low ON resistance, very high OFF resistance, better than you can get with transistors.

Latching. A relay can be wired so that it latches like a memory cell.

well yes the relay is a fancy switch. Not always just an electro magnetic one, but mostly. The idea is to sop arcing when the blades of the normal switch move apart or together. Becuase of the lag on a normal switch you get burning if there are moderately high loads.

Electronic or electro magnetic ones allow you to use a small curent to throw a switch of much higher current with not so much, or no, lag.

We wouldnt be able to do such things as start a car reliably without a relay switch. We would also not be able to have multifunction switches on the gearshift or column

Basically "To convert one action or signal to another".

You speak of a 'solenoid switch`, an electrical switch,

that is one kind of relay. (There are also pneumatic, and

hydraulic relays.)

It is most commonly used to switch multiple circuits with

a single input, or to control a high energy circuit with a

low energy one.

A motor starter is often a relay combined with over current protection and other accessories.

When combined with sensors, simple automatic

logic 'control' circuits can be created.

A single thermostat closing can initiate the start of an AC system, (Fans, Compressor, etc.).

It is an electrically isolated switch, so the switching power doesn't get mixed up with the switched power. It can also typically handle higher amounts of power than say a transistor. Negatives are that it's very slow and consumes a bit of current to hold the switched state, and also doesn't amplify like a transistor (it's either on or off) and also the contacts can bounce which makes the switched signal unclean.

Basically the relay is a fancy switch, like you said. A major use of them is to control high voltage or current circuits with a lower voltage or current switch. You operate the lower voltage/current switch - the relay operates the higher voltage/current. Like a car's engine starter.

There are many more/different uses for relays, and many different types of relays - too many to go into here (please see link below for a ton of info).