masterwacker

I have a decent understanding of wiring in a vehicle, but I am now starting some wiring and re-wiring projects in my truck and I need to further expand my knowledge.

To my current understanding the use of a relay is so the power needed to operate something does not go to through the switch. The switch turns "on" or closes the relay so that power will go through the relay and turn on whatever it is you want turned on. Is this right?

Lets say I want to power four aux lights run off of the same switch. Each light draws 7.5 amps. Therefore I will need a relay that is rated for more than 30 amps (7.5 x 4 = 30) Is this right?

When should you use a relay and when is it not necessary?

Oh, and when a relay has 5 terminals what are all the terminals for. I know one is for the switch, ground, power from battery, power to accessory, but what is the fifth terminal for?

Ironmike4x4

If the lights have the same amperages then why run a relay when you can use just a switch rated to whatever amps total?

masterwacker

Because the switches I want to use in all of my applications, contura switches from waytekwire, will not handle some of the loads. This way everything is fused, relayed, and ran properly to reduce teh chance of problems. When I wire a house I am analy neat, lable everything, and do everything surpassing code. This way if someone was to come in and do something they would know exactly which wire was for what, which circuit is for what and since everything was overbuilt it also makes troubleshooting easier.

Robinhood150

Yup, always use a relay. You get less voltage drop and you can use smaller (better looking) switches.

Here's a good diagram of what the 5 different contacts do:
http://www.the12volt.com/relays/relays.asp

Basically it's so you can wire the system to be "on" when the relay is energized or "on" when the relay is not energized.

Mr.mud

The 5th terminal may be a N.C contact. Some relays have a power in terminal with a hot terminal and a dead terminal on the output side, when the relay is turned on the hot terminal goes dead and the dead terminal goes hot. In electrical terms this is called normaly open "N.O" and normaly closed "N.C". To turn lights on and off you would use the N.O. side. Good Luck!

masterwacker

then what would you hook up to the normally closed side then?

4Crawler

iTrader: (3)

You use the NC contact for things you want to be on when the relay is off. One example might be that you have two circuits that you don't want to be on at the same time. Feed the power for one relay coil through the NC contact of the second relay and likewise feed the second relay coil power from the NC contact on the first relay. This way, if #1 relay is on, it's NC contact is now open and there is no way to turn on #2.

Note that sometimes the 87a contact is not NC but a second NO contact in parallel with 87, so this gives you 2 NO contacts instead of one NC and one NO. This lets you control two circuits off of one relay.