Relay Info and tips

[Jeff]

LEGAL BIT  This info is provided as is where is and without any warranty or promise of fitness for any particular purpose. It is provided by me free of charge and is used at your own discretion and risk. If you do not know how to work with 12v electrical systems you should take you car to a licensed mechanic or dealer for work. Always fuse any 12v source with a proper fuse and make covered and proper connections to prevent fire. Once again using this data is at your own risk and any result is solely your responsibility. Unless it works then it is all meYAY ME!

Ok boys and girls, it has come to my attention that it might be useful for there to be a guide to relays here on this site to help out folks who might be using them to run high current accessories in their automobiles.

Q- What is a relay?

A- A relay is an electro-mechanical device that employs a tiny 12v solenoid to make an electrical connection across a contact. The most basic is a 30/40amp relay that can constantly flow 30(normally closed)/40(normally open) amps.

Basic benefits to a relay
1. Run high current devices that would burn out most switches. (Amps are key here. AC amps and DC amps are different. Running AC rated switches is not the answer)
2. Remotely switch certain devices so high current 12v does not need to be run through the car or to a specific location.
3. Trigger 12v devices with a ground rather than 12v. (smaller wire and less fire risk)
4. Install multiple switches on the same device (using ground and 12v.)
5. Consolidate high current 12v sources to one place and switch it via a relay bank.
6. Run smaller (and cheaper!) switches and wire

Q- Jeff, why would I need to use a relay anyway? Why not just run 12V though a switch to my lights (fuel pump, heated seat, power window, vibrating dildo, etc)?

A- A relay allows you to do what we in the business call "low side driving." Basically switches do not like to handle large amounts of 12v AMPERAGE. So when you are running say lights...or a large vibrating dildo you might discover this causes lots of heat to build up in your switch and may eventually cause failure. The answer here is to run a relay to handle the high current load (high side) and drive it with a lower current (low side) driver circuit.

NOTE ÔÇô As stated above the situation you are fighting here is too many amps for the switch. You may see your switch is rated for 20 amps on an AC circuit and say ÔÇ£IÔÇÖll just run thatÔÇØ but the problem is DC current is harder on switches then AC. You should probably not be loading that switch with much more then 10-15 amps of DC load.

Q- Ok, so I want to run some relays. How should I do it and what do I need?

A- Well, this is a question with many answers and I will try to outline some of those here now. You will need a basic 30/40amp relay which should service MOST any need that does not come with its own specialty dedicated relay (like a winch or starter or something large like that). I will outline the ways to wire a relay after I give you some basic theory as I see it.

Basic Relay theory
A relay should be thought of as a valve. Current comes in one side and runs out of another. The lever on this valve is the solenoid. The lever can either be normally (at rest) open or normally closed; so with either current flowing all the time until you ask for different or not. The lever is moved by its own lower load 12v circuit. This circuit is a simple power and ground that can be provided by any rated switch. Further it can be provided by either switching its ground component or its 12v component or even both for a very versatile setup. The standard 5 contact relay can function as a normally open or normally closed relay, however there is such a thing as a 4 contact relay and most of these are of the normally open (provide switch for current) type. The contacts on a relay have a standard numbering system. The flows can be reversed with no affect on function.



Here is the basic layout of a 5 pin relay. On a 4 pin 87a the normally closed contact would not be present. 87(87a) and 30 are the high side. This is where the heavy current running your device flows. I use 30 as my output ALWAYS because it keeps me straight. This is not always the case though. 87 and 87a can also be outputs. If current is flowing in to 87 it will come out of 30 when the relay is energized, if it is flowing to 87a it will come out when the relay is not energized and stop flowing when it is. 85 and 86 are the contacts for the solenoid. These may be wired in either direction for either form of trigger. Ground one and send 12v to the other (positive trigger) or send constant 12v to one and ground the other (negative trigger) however you need. The following diagrams attempt to explain the two types of triggering systems for a relay in a graphical form. Notice I do not label any of the other terminals. It is easily explained that what comes in 87 (ground or 12v) come out of 30 when the trigger is on and what comes in 87a comes out of 30 when the trigger is not on.





This is a basic guide and explanation. If you need any further help or there are further questions I will make diagrams for each instance. They can also be found at <!-- w --><a class="postlink" href="http://www.the12volt.com">www.the12volt.com</a><!-- w --> If someone has more complex needs please let me know and I will do my best to explain.