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Thread: Relays

  1. #1

    Relays



    While relays are nothing new, very few people understand them and how they work. With a little luck, this page will explain the relay and show some examples of what can be done with them. The unfortunate thing is that so many of the people that use relays don't understand how they work. I've met more than my fair share of (stereo) installers that only know what to do because of instruction books and pre-made wiring harnesses. I was once one of these people, but the amazing simplicity and potential of the standard 12 volt relay was something far too good to go unknown and unused. If it sounds like I'm overdoing it a bit, I'm really not. If you think about how often you use and need relays, you'll soon agree.



    The standard 12 volt relay is single pole, double throw. There are five terminals; two to "activate" the relay and the other three are the "switching" terminals. As you can see in the picture above, there are three terminals that are facing up and down, and two that face side to side. For reference, we will consider the uppermost terminal (#30 facing up and down) to be the "top" terminal. Now that we have our orientation, it's time to understand which terminal does what. The two side terminals (#85 and #86) are the "activation" terminals and have no polarity. They simply require +12 volts on one and ground on the other to activate the relay. The terminal on the top will be the common "switching" terminal. This means that it will be the one which is always connected to one of the other two switching terminals. When the relay is not "active," the top terminal is internally connected to the middle terminal (#87a). As soon as the relay becomes activated, the top terminal will break it's internal connection to the middle terminal and become connected to the bottom terminal (#87). Essentially it's just like flipping a normal switch, only its flipping action is controlled from a remote source.

    Now that you see how a relay works, what can be done with it and why not just use a switch. There are so many advantages to using relays, I will only be able to mention a few, but I'm sure it will be more than enough to make other ideas and applications come to mind.

    One of the most important uses of relays is to control a high power source from a lower power source. As an example, consider that you want to be able to turn on and off something like a set of driving lights that will pull about 30 amps. Obviously, you would need a powerful switch to safely take care of that. I'm sure many can relate to the supplied rocker switch that came with a set of lights, burning out and melting after only a few weeks. Using a relay will allow the use of a small, good looking switch mounted in the dash to activate a relay mounted under the hood to supply power to those lights. Most standard relays (the good ones) can safely handle loads of 30 amps each. There is also the option of tapping off of another switch, such as the headlight power wire, without pulling enough current to affect its circuit. Keeping the main power wires located under the hood and near both the power supply (battery) and the end user (the lights) is also helpful and convenient.

    A huge benefit, in cases like this where high current is being used, is the life span of the switch or relay. Even though it may not seem like it, when you flip a rocker switch, it actually takes a long time for it to make contact inside. A long time in electrical terms is still very short, but either way, a switch takes much longer to make its contact than a relay. This long switching time causes internal damage to the switch as an arc is passed across the terminals as they come in contact. The longer it takes to make contact, the longer that arc will burn up the switch. Because a relay is activated much faster, the arc doesn't have the time to cause damage. Sure some will occur, but at such a small amount that a relay will still have a very long life span.

    Another use for relays is the ability to control multiple switches at the same time. Say you have a competition stereo system and you want to be able to disconnect all but two speakers from a two channel amp that is running eight speakers. (long story) Obviously, you can't just connect all these speaker leads to a simple switch. There are four wires that would need to be disconnected, two positive (left and right) and two negatives, and none of them can touch each other. Now the advantage of the relay is clear. A small switch can be installed in the dash to activate four relays in the back of the car, disconnecting all the speakers at the same time and without any hassle.

    For another use, consider that you want to be able to supply one device from one of two sources. Perhaps you have a temperature gauge and you want to be able to monitor the difference between the coolant temperature in the motor -vs- the temperature in the radiator. Obviously you could use two gauges or a switch, but neither of these would be the best of ideas. Using a relay, you could easily have a small (good looking) switch control a relay that had the gauge going to the top terminal (#30) and each temperature sender going to each of the other switching terminals (#87a and #87). This is a great example of the flexibility of the relay.

    While most of the advantages of the relay (I hope) are easy to see, sometimes it's not so obvious. Using switches can have unseen problems. As a switch deteriorates, the internal connections start to build up resistance. This resistance can offer intermittent connections and voltage drops, both of which can be disastrous. The perfect example is when controlling nitrous solenoids. This is the last place you want problems with your wiring! One other thing to add to this is the quality of the relay. As with anything else, quality is everything. For this reason, I highly recommend using only top quality relays such as those offered by either Potter & Brumfield or Bosch (in that order). All too often, products are supplied with low grade relays that wear out quick and cause problems. Any time you buy a product (such as air horns), replace the supplied relay with a better one. While some people will try to screw you on the price, a good relay should never cost more than five dollars. At this price (or hopefully lower) it's just good insurance against problems.

    For some good examples of what can be done with relays, check out the pictures below. I will add more over time. If you need help with any other uses, please feel free to let me know.



    How to wire a Line Lock

    How to wire a Nitrous Kit

    How to wire a Trans Brake

    How to wire a Nitrous Kit with a Trans Break
    (This will prevent the Nitrous from flowing while the Trans Brake is on.)

    How to wire Power Windows
    2006 F150 Stx
    1989 Ls1 S10
    Future King of the Slow car Class
    http://img.photobucket.com/albums/v217/JGoldinger/b.jpg

  2. #2
    Join Date
    Jun 2005
    Location
    Way up in North Dakota
    Posts
    351
    Thanks for the information, even old dogs like me can learn new tricks.


    [ just ask my wife ]
    2002 GMC 1/2 ton Sierra,4WD, Xct cab, camed, mid-length headers, Nelson tune, electric rad. fan, trany cooler, Heavy 5750 lbs with topper.

  3. #3
    ff_jeff Guest
    good write up, and pics. I smell a sticky.

  4. #4
    Yeah this stickyed will keep me from trying to remember the right way to wire a relay. I need to wire the relay into my air compressor for my slam air helper bags. Thanks Dr. WOOWOOWOO
    99RCSB Broke because I wasted thousands and thousands of dollars on my truck.

  5. #5
    Join Date
    Nov 2003
    Location
    Denton Texas
    Posts
    20,689
    We can't sticky everything though, if we did new topics would be on the second page.
    See my truck data in the "My Garage" section here... http://www.ls1truck.com/forums/my-ga...tml#post191709

  6. #6
    Quote Originally Posted by RedHeartbeat View Post
    We can't sticky everything though, if we did new topics would be on the second page.
    How about a sticky section.
    99RCSB Broke because I wasted thousands and thousands of dollars on my truck.

  7. #7
    Join Date
    Nov 2003
    Location
    Denton Texas
    Posts
    20,689
    There is one, its at the top.
    See my truck data in the "My Garage" section here... http://www.ls1truck.com/forums/my-ga...tml#post191709

  8. #8
    Join Date
    Dec 2009
    Posts
    169
    While I was installing my E-fans the other day I noticed all of my small relays were installed backwards (just like in this pic) (I didn't take this pic so pay no attention to the red circle around the big fuse).

    Pic of underhood fuse block ~~>

    I checked out the diagram in the shop manual and the numbers for the relay terminals also indicate the relays (marked "8567" in the pic) are all upside down. I used a test light and it indicated that the relays are installed upside down (relay terminal "30" was hot on the fuse block in the location the shop manual showed it would be). The other relays (not 8567 relays) show to be installed the correct way. I pulled all the 8567 relays out and installed them the way the shop manual shows they should be. Then I ran across the pic in the link above and all the 8567 relays in the pic are also upside down. What is up with that?
    Last edited by Fasglas; 04-26-2010 at 02:50 PM.

  9. #9
    Join Date
    Sep 2007
    Location
    Broken Bow, OK
    Posts
    1,441
    Quote Originally Posted by Fasglas View Post
    While I was installing my E-fans the other day I noticed all of my small relays were installed backwards (just like in this pic) (I didn't take this pic so pay no attention to the red circle around the big fuse).

    Pic of underhood fuse block ~~>

    I checked out the diagram in the shop manual and the numbers for the relay terminals also indicate the relays (marked "8567" in the pic) are all upside down. I used a test light and it indicated that the relays are installed upside down (relay terminal "30" was hot on the fuse block in the location the shop manual showed it would be). The other relays (not 8567 relays) show to be installed the correct way. I pulled all the 8567 relays out and installed them the way the shop manual shows they should be. Then I ran across the pic in the link above and all the 8567 relays in the pic are also upside down. What is up with that?
    On the 8567 it doesn't matter which way they are installed. The 87-30 and 85-86 are diagonal from each other so that if you flip it upside down they still work correctly.

  10. #10
    Join Date
    Dec 2009
    Posts
    169
    Quote Originally Posted by PitchBlackC3 View Post
    On the 8567 it doesn't matter which way they are installed. The 87-30 and 85-86 are diagonal from each other so that if you flip it upside down they still work correctly.
    OK, thanks. I didn't notice that. Makes sense.

    They seem to be working fine. I guess I'll leave them the way they are since I turned them around.

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