Clearing Up Electric Questions

[puckhead]

So I have been wondering about this since I have been messing with my lights lately and was hoping someone could tell me why this would or wouldnt work.

When hooking things up through a switch most people are saying use a relay. Like in this crude picture.



My question is, is the relay relay necessary? From the picture, you are still having a 12V current go through the switch so why not just use a switch with no relay? Am I missing something? I just want to better understand what I am doing. Thanks guys!

 

[ezdaar]

Thats exactly how it works.

The relay when wired properly allows high amperage to be drawn directly from the battery to the lights...
While a very low amp draw is pulled through the switch required to operate the relay..

 

[puckhead]

Thats exactly how it works.

The relay when wired properly allows high amperage to be drawn directly from the battery to the lights...
While a very low amp draw is pulled through the switch required to operate the relay..

So its an amperage issue and not a Voltage issue? How is there a lower amperage going to the switch then? Wouldnt it be the same as the amperage going to the relay? Thats were I am getting confused. I understand how to wire it. I am just wondering why.

Say you use the batter as the 12V source for both the relay and the switch. Why is there a different amperage? Does the end result lights, horn, wipers, etc. only draw a certain amount of amperage?

So the relay is a limiter for the switch but still always the lights to draw full amps from the batter? Is that correct?

 

[greenhornet]

As in the previous reply you need a relay .The switch and relay draw very little current when energized while the relay supply's a much higher load caring supply to the device.

 

[ezdaar]

Ok, bud heres the scoop.

the amps required to hold the relay in the open position (automotive light relays are a naturally closed type) is less than 2 amps give or take alil. A switch can handle that all day long.. Its a small electro magnet that closes the large brass contacts inside the Relay.

But Lights require anything up to 30 amps or more.
Reguardless if that switch can handle 30amps or not, the Immidiate amp draw when that switch is thrown is Way over 30, this immidiatly heats up the contacts in the switch, combine that with the excessive amp load of the switch. It will continuosly heat up untill melt down.

The lights will pull Direcly through that large brass contact (bridge) to the battery. This does not heat up unless the Relay is undersized, Most automotive lights dont draw even 30. Electric fans however can draw up to 45, this is where you use a 60amp relay. I like to use starter solinoids (Extra heavy duty 200 amp relay) from mid 80's motorcycles. They are full metal, rebuildable and waterproof.

Ehow, How a Relay works
This image is pressed into or printed on ALL quality Relays store bought.

C= contact 30, Comes direct from battery dont forget the Fuse! No more than 18" from battery!

D= contact 87, Goes to Lights or Electric fan. sometimes theres a 87 and 87A, they both do the same thing, they are doubled up so you can run heavy guage wite to a single female spade connector.

A= contact 86, Ground no explanation needed.

B= contact 85, 12volt SWITCHED source, This can be anything from your ignition to a lighted or OEM switch or even your brake light.


That is Standard issue Relay wiring.

Me I prefer to do it abit different for Safty reasons..

I put my Switch on 86 (ground) and simply jump my power from 30 over to 85.
This keeps me from running "hot" wires all over the place which is less chance of shorting andmelting down wires which turns into a vehical fire, IF the fuse didnt catch the short or was to high a amp rating.

To determin required amp rating (with out a mp guage) is simple. Get your lights all wires up.
Now place in a 5 amp fuse, if it blows, go up to a 7.5 amp, if it blows go up to a 10amp. Keep going up 5 amps at a time untill the fuse does not Blow under constant use for 15 minutes. If you hit 25amps and blow a fuse running lights (40 for a 16" electric fan) Check your wiring for potential shorts immidiatly.

Relays from 30 to 75amp and the Socket used to wire them up.

More Techincal information on Wiring Relays and their types.
Dunno if you needed all that info, but there it is.

 

[OmarR]

Here is the illustrated version:



Puckhead, you are correct in that 12 volts is 12 volts, but it's a matter of amperage draw as the other guys stated. When you put the switch on the relay, the relay is only asking (.1) 1/10 of an amp. If you put the switch on the lights/whatever, the draw can be anywhere from several amps up to (typically) 15-30 amps, depending on what you are running. The switch will burn up if you do that.

As ezdaar stated, I also like to wire the switch to ground like this:



Wiring the ground side to the switch makes it easier, even safer if you have go thru the firewall. I personally would only run 12v to the switch if I used one of those lighted switches, but that is another thread for you.

Also, if you put the switch on the ground of the relay, most of the time this allows you to just run 30 and 86 together from the battery. Sometimes, you will have to run 30 straight from the battery and 86 from a switched ignition source, it will just depend on the application.

Good luck!

 

[puckhead]

Man, Doug and Omar, that is more than I could have ever asked for! Thanks a ton guys!

I didn't doubt that there was a good reason to using the relay, I just like understanding why. And now I do and will be able to use this info in many other projects. (The list never ends! lol)

I was sorta on the right track just needed a good educated shove lol.

Thanks a ton!