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View Full Version : What's a relay and how does it work


Vastier
10-23-2008, 11:19 AM
To help out with guys wiring up new lights and so on, here's the when, why and how to of using relays. Thought I would take a little of the "bad voodoo" out of these things and pass on some info for ya.

In the simplest of terms, a relay is a remote control switch. It can be used to redirect power (Either positive or ground) to a device, be used to isolate a device, trigger a factory device, or be utilized to control or deliver higher currents more efficiently to a device.

The key to know is the numbers, usually found on the bottom of the relay.

Pin 30- Generally used as the source input pin. ie a fused lead from the battery
Pin 87a- This is the normally closed position. With the relay "off" or not in use, this will be a direct line to Pin 30
Pin 87- This is the normally open position. With the relay "on" or energized, this will be a direct line to Pin 30
Pins 85 & 86 These are your relay control terminals. Suppling a positive charge to one and a ground to the other will "turn on" or energize the relay

Here's an example.
Instead of running a heavy wire from your battery, through your firewall to a switch, then back out the firewall to a set of lights on your front bumper, you could mount a relay under the hood by the battery. Then you take a good gauge wire (WITH A FUSE) from your battery to terminal 30 on a standard Bosch style relay. Next you take another good gauge wire and run it from terminal 87 out to your lights. Now you have just shortened your power wire by about 10 feet or so, allowing a much shorter and direct route to the lights, making them brighter. But how do I control my lights now? Easy! There are a couple of way to wire this now, all fairly simple. I will suggest the way I commonly wire them, which will give you the benefit of the factory headlight reminder. Find the parking light wire under the drivers side dash, it will be coming out of the headlight control switch (Make sure it is actually for the marker lights and not the dash lights or it will dim with them). Tap into that wire with a small gauge wire (18-20 ga will be fine) and run it to the switch you wish to use. Now run the other side of that switch wire out through the firewall to the relay and plug it into terminal 85 on your relay. Finally, take another small gauge wire and run it from terminal 86 on the relay to a good ground. Now whenever you have your marker lights on and the switch flipped, you will have foglights, and when you turn your lights off, they will all go off.

Now that was a pretty basic one that most guys may know, but did you know you can also use a relay to break a circuit such as an electric fan?

Now if you have an aftermarket electric fan with a thermostat controller, then you can use it's trigger wire, but for you guys running an existing factory electric fan, what you can do is find the power wires running to the fan. Find a section of wire that leaves you room to work on it and cut the power wire. Put female spade connectors on both cut pieces and plug the supply side into terminal 87a, and the fan side to terminal 30. Some of you may think that is backwards, but it will work either way, and by putting the feed on 87a in this case will prevent you from having a live terminal (87) exposed on the relay when this circuit is turned on. Connect a ground wire to terminal 86. Now you just have to run 1 small gauge wire from terminal 85 through your firewall to your switch and connect it to a feed, personally I would recommend an accessory feed. Now that this circuit is installed, your fan will work as it normally does, but when you activate the circuit, it will shut off power to the fan (recommended before water crossings).The nice thing about this is that the fans work with the circuit off, and it will allow them to run properly when the vehicle is turned off. I would recommend adding a small LED to the switched side of your switch, just to remind you that the fans are off. Also for those of you who always seem to find trouble, make up a short piece of heavy gauge wire (couple of inches) and put a male spade connector on each end. That way, if the relay ever fails, you can simply pull the fan wires and jump them.

Here's a great interactive link for those still confused.

http://www.bcae1.com/relays.htm

Have fun [cheers]

nbrowser
10-23-2008, 08:50 PM
Good write up mang, it'll be helpful to a lot of people that's fer sho !

Apathy
10-23-2008, 08:55 PM
Ya, that is a pretty good write up.. However, get rid of the numbers. they are never the same. what maybe 87 on one relay may not be on another.
for automotive this is not so much of a problem. But when your dealing with "relays" in general, this info could be false.

Vastier
10-23-2008, 10:05 PM
The relay I'm refering to is the common Bosch 12VDC 30 Amp (which is now Tyco Electronics, not Bosch). Pathy is correct, there are many relays but this is the most common for the uses I'm trying to touch on.

This link should spell it out nicely for how to use a Bosch Relay for their lighting projects or what ever else.
http://www.bcae1.com/images/swfs/relaycoilvoltagedemo.swf
[cheers] [cheers] [cheers]

epic3
10-23-2008, 11:25 PM
Ya, that is a pretty good write up.. However, get rid of the numbers. they are never the same. what maybe 87 on one relay may not be on another.
for automotive this is not so much of a problem. But when your dealing with "relays" in general, this info could be false.

Common five pin relays ALL use the same numbering and allocaton for the pins
Bosch Potter brumfields being the most common good quality 30/40ampones but even thecheapy chink ones are the same pin out.
It's only untill you get into vehicle manufacturer four and five pin molex style mini or not that become different pinout but they largley function the same.

Tunabomber
10-24-2008, 12:07 AM
Common five pin relays ALL use the same numbering and allocaton for the pins
Bosch Potter brumfields being the most common good quality 30/40ampones but even thecheapy chink ones are the same pin out.
It's only untill you get into vehicle manufacturer four and five pin molex style mini or not that become different pinout but they largley function the same.


with all this info i could finish the job phill started and never finished
and add power to my amp cooling fans :rolleyes:

epic3
10-24-2008, 12:10 AM
with all this info i could finish the job phill started and never finished
and add power to my amp cooling fans :rolleyes:

Yup

Or you could bring it by my shop with that light and we could go for pizza buffet and still not get it done.:D

Vastier
10-24-2008, 12:15 AM
with all this info i could finish the job phill started and never finished
and add power to my amp cooling fans :rolleyes:

Ya know, I had you in mind when I was writing this [f]
:D

85_GMC_4x4
10-24-2008, 10:38 AM
Good writeup. Definately leave the numbers.

Relays have other purposes as well, but you've done a great job of explaining the basics.