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Thread: Whats a relay and how does it work?

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    LugNutz's Avatar
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    Whats a relay and how does it work?

    Well it seems that over the last couple of days I've had to give a few Electronics 101 courses, particularly when it comes to the when, why and how to of using relays. I know a lot of you guys are scared by wiring, so thought I would take a little of the "bad voodoo" out of these things.

    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.

    There's a lot more you can do with a relay, and I will write up a few other possibilities as time allows, in the meantime, here's a great interactive link for those still confused.

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

    Enjoy
    Last edited by LugNutz; 08-14-2008 at 01:10 PM. Reason: typo
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    Nice write up.

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    As simple as they are, they drive guys crazy. Thanks for the write-up, will be referencing when ready to install my e-fan.
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    Very nice - thanks.

    I'm one of those guys who can sort of figure things out, but I have no idea - usually - how it might be working wrong, or how it could be made to be a better.

    Now if I can just figure out how to wire in a fuse so it doesn't keep blowing up.

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    saw this great write up, reminded me of a site I have bookmarked.

    now these are for SPDT (single pole double throw) relays, but they may come in handy for some.
    Relay Applications
    the site itself has some interesting reads, found near the bottom under 'Relays'
    insert something witty here

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    Just thought I'd send out complements on a great informative write up.
    Great job LugNutz..
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    LugNutz's Avatar
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    Had a couple of guys asking me again so lets bump this up.

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    Lugs you do good wrk what about wiring stereo's now. and any tips besides dont let me touch anyones truck?
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    Thanks for this write up!
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    Very good intro to relays, which are an easy and powerful for adding some cool toys and options! Only thing that I would add is the control circuit (terminals 86 and 85) is an electromagnet and therefore generates a voltage spike when is turns off which can damage circuit boards. Most quality relays have an internal flyback diode to safely prevent this voltage spike but the relay must be wired correctly for it to work; voltage in to 86 and ground to 85 (I remember by thinking that 85 being the lower number is closer to the ground).

    Paul

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    Thanks Paul. I'm comfortable with relays and have had to wire a couple in the Jeep before, but I never knew about the diode.
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    sorry to dig up an old post but , what about the new relays with the little pins . On the cover I believe it shows a capasitor and a diode I think.
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    ^lmao I'm not surprised that you bumped is thread at all Mike.

    As for relays.... They all work on the same principal.
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    I don't know about a capacitor...I think you may be looking at the electrical symbol for a coil, which is present in every relay...I have never heard of a relay with a capacitor inside.


    As for the diode, yes there are relays with diodes inside...they are called "suppression circuits" and it has nothing to do with the "new relays with little pins", it is common on the standard Bosch style 20/30 or 30/40 relays as well. It prevents a reverse voltage spike when the coil is de-energized by removing +12v or ground from the coil terminals (85 and 86). When a suppresion circuit is present inside a relay then you MUST use terminal 85 as ground and terminal 86 as +12v.


    These diodes can also be installed outside of the relay on the backside (bottomside) of the relay harness or socket. There is also the option of the person constructing the circuit to install there own diode while putting the harness (socket) together.


    Something to keep in mind with a diode when they are external to the relay or installed by the individual constructing the circuit. The cathode side (side with the stripe) is the side you would want to apply ground to...so you would want to verify which side of the coil the cathode is on...with an external diode it might not be terminal 85 as mentioned above for an internal diode.


    In practice, all of my techs have always been trained to use terminal 85 as ground, regardless of the presence of a diode or not, or if the diode is internal or external. It is a good habit to have because there is never a question of which terminal is ground. They are also trained to recognize when the cathode is not on terminal 85 (in an external diode setup) and to correct it before continuing with the job. By doing this, everybody is on the same page and if one tech has to go back in and inspect or service anything that another tech originally worked on then it is known that terminal 85 is ground without question.
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