What is a relay? A relay is a small device that allows you to use the battery to power a high powered device, by using a a low-current electrical 'signal' usually hooked up to your fuse box or any other device within your car. EXAMPLE: a high powered off-road led light to turn on, by flicking your high beams.
Why do you need a relay? When installing something into your car's, you should avoid 'jumping' off of factory wiring/electronics. that causes an extra strain/drain upon the factory wires & electronics that are designed to carry the load of only the factory installed components. The fuse box is NOT a 'general use' power outlet, like the power outlets in your house. For example, the ignition (IGN) circuit is designed to power the car's ignition system and nothing else. Hooking up a high-current device to this circuit can create a fire hazard at worst, and you can start poping fuses and destroying electronics at best. By using a relay, you can use the IGN circuit to control a high-current device without directly sucking power from the IGN circuit itself. EXAMPLE: you want to install a subwoofer that turns on as soon as you turn the key to ON.
Is a relay hard to hook up? NO. Most relays require only 4 or 5 wires. (depending how you want it set up)
Is a relay expensive? NO. most relay (and its respect plug) cost less than $6 on amazon
NOTE: Not all relay's are the same. Some are 30amp, some are 50amp, etc. You want to generally know the amperage that your project will be, and buy the appropriate relay.
Where can i buy a relay? You can buy a spare relay, or you can buy a relay AND its respective plug. I suggest always spending the extra $1 and getting the respective plug and the wires that come off the end of it. even if you dont use the plug now, you can always save it for a new project. Here is a good deal on Amazon:
NOTE: There will be 4 or 5 numbers on top with some connecting lines/siggles: 30, 85, 86, 87, and a 5pin relay will have 87a in the middle.
What do does numbers mean?: the numbers are there to correspond to the proper terminals at the bottom of the relay. the numbers are like "name tags" for each pin, thats all. pin30: to the battery positive terminal (you should install an inline fuse here) pin85: directly to ground(anything metal) pin86: 'signal' wire. you can wire this to the fuse box. you should use a bussmann "add a fuse" to connect into the fusebox. pin87: this is the 12v (+) POSITIVE wire that will go to the item you want powered (dont forget to ground the negative wire of the item you are installing) pin87A: [ONLY FOUND ON A 5PIN RELAY] this is the pin that will get power, when the "switch"(pin86) power supply is turned OFF
I own a ford, and my fuse box is always 'ON' unlike many cars where the ignition is turned off via the key. Then you will need to tap into the ignition wire for your "signal"(pin86).
I bought a 5-pin relay, can I still use it as a simple on/off(4pin) switch instead of a changeover switch? YES. simply leave pin 87a disconnected
Does a relay take the place of a fuse? NO! a relay provides no protection from overload or short-circuits.
Do I need to use a fuse if I use a relay? YES! you must still fuse your power wires!
I found a great deal on amazon for 2 relay's, 2 harnesses, 2 inline fuses: http://www.amazon.com/gp/product/B00C0SATHE/ref=as_li_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=B00C0SATHE&linkCode=as2&tag=imstricken06-20&linkId=OHPL755B56MXFZWO
these are 5 pin relay's, so if you dont need the 5th pin (87a) just dont hook anything up to it. the relay will function just like a normal 4 pin relay otherwise.