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  1. #1
    Elite Member
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    Default Why relays?

    I have a kind of dumb question. I'm no stranger to simple electronics, but I'm relatively inexperienced with automotive electronics, and I'm trying to figure out the proper usage of a relay. My understanding is that a relay is used when the current draw of the load is more than the switch that controls the load would be rated to handle. But what's confusing me is, I see switches all over the Internet that are rated to handle 15-20 amps at 12 volts, which is plenty for many applications. So why not just run the hot wire directly through the switch and be done with? And then I look under the hood of my tractors, and there's relays all over the place. So there must be something I'm missing. There must be some reason to use a relay other than that the switch isn't rated to handle the current draw of the load.

    The example I'm pondering right now is, I'm thinking about putting a light set on my ROPS. I was thinking, if I wired a relay into the hot wire running to the headlights, then I could control the ROPS-mounted lights with the headlight switch. But wait. Why not just splice into the headlight wire and run my lights directly off that? Assuming, that is, that the headlight wire could handle the current draw.

  2. #2
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    Default Re: Why relays?

    relays can allow a shorter path from the battery to the load. for example a set of lights on the front, the battery is located at the front and the lightswitch is at the back. with the switch and no relay, the power has to travel from the battery 5 ft to the switch and then 5ft to the lights. with a relay, the current path is much shorter, say a foot or 2. the lights will be brighter with the shorter path. relays can also be useful in creating a sequence that items are powered or not. say you have multiple lights and want them to come on in a sequence, with a relay that can be done. or another is one set turns OFF when another set turns on.
    2010 JD 2320, 200cx, Frontier 1060R RFM, 2048 BB, LR1072 rake, IMatch, 5 suitcase weights.
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  3. #3
    Veteran Member sparc's Avatar
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    Default Re: Why relays?

    Lighter gauge wire in the dash area takes less space too, as well as the reduced cost of the smaller wire.

  4. #4
    Bronze Member
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    Default Re: Why relays?

    The main reason Relays are used is to take the load of resistance off of the switch.

  5. #5
    Elite Member Baby Grand's Avatar
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    Default Re: Why relays?

    I'd rather have any switch that I have to touch in an outdoor environment be low current.
    That's the problem with trouble.
    It always starts out as such fun."
    - Randall Brown

  6. #6
    Silver Member
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    Default Re: Why relays?

    These days there is so much computer control going on, and a micro chip can only handle current in the milliamp range. Therefore, the relay.

  7. #7
    Platinum Member
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    Default Re: Why relays?

    You need to be sure that switch you see is rated at 12 volt DC. DC arcs much more than AC and the switch must be heavier duty. Relays are a good thing.

    Kim

  8. #8
    Elite Member
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    Default Re: Why relays?

    Quote Originally Posted by Baby Grand View Post
    I'd rather have any switch that I have to touch in an outdoor environment be low current.
    That's a good point. Just because the switch is rated for so many amps doesn't mean you want those amps hitting you in a failure mode.

    Quote Originally Posted by KWentling View Post
    You need to be sure that switch you see is rated at 12 volt DC. DC arcs much more than AC and the switch must be heavier duty. Relays are a good thing.
    Yeah, they definitely are 12 volts. I can't hardly find a 12-volt switch rated for less than 20 amps.

  9. #9
    Gold Member gtrippleb's Avatar
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    Default Re: Why relays?

    Quote Originally Posted by sparc View Post
    Lighter gauge wire in the dash area takes less space too, as well as the reduced cost of the smaller wire.
    This is one of my understanding of using relays also in that you can use smaller wire to and from the switch and then the larger waire to the relay and the load.

    Here's some information on Relays from the following website Relays This website seems to do a pretty good job of explaining relays.

    Two Reasons to Use a Relay:
    A relay will allow you to use a small current to control a larger current. Generally, a relay coil has a relatively high resistance and will require only a small electrical current to engage the contacts. Typically, the contacts are rated to carry much more current than it takes to engage the relay. This means that a relay can be used if you need to make/break the circuit path where there is a relatively high current flow and the control circuit can only supply a small amount of current. In your vehicle, the ignition switch cannot pass a significant amount of current without being damaged. You probably also know that the engine starter motor needs significant current to be able to start the engine. Since it would almost instantly destroy the ignition switch if you were to try to power the starter motor with the ignition switch itself, manufacturers use a relay (also known as a solenoid) as a buffer between the ignition switch and the starter motor. In old Ford vehicles, the solenoid was mounted on the fender. On other vehicles, the solenoid was mounted onto the starter. If you ever have to work on the starter, you will see that there is at least one small wire and at least one very large wire. The small wire drives the coil of the solenoid. The larger wire supplies power to the actual starter motor. In this application, the relay/solenoid is used to allow a small current to control a larger current. In car audio, the most common use for the relay is a buffer for the remote output of the head unit. Since the head unit's remote output is limited, the relay allows you to power many more devices than you could with the remote output otherwise.

    The second reason to use a relay is to isolate two circuits. If you needed to control a very high voltage circuit with a 12 volt controller, you could use a relay. Since a relay coil is 'generally' isolated from the contacts, you typically have complete isolation between the 'input' and 'output' section of the relay. Of course, the input of the relay is the relay coil and the output would be the contacts.


    Here's a couple more websites I came across with some decent information:
    Single Pole Double Throw (SPDT), Single Pole Single Throw (SPST) Automotive Relays
    Special Applications with SPDT Relays, Diagrams
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  10. #10
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    Default Re: Why relays?

    Quote Originally Posted by gtrippleb View Post
    The second reason to use a relay is to isolate two circuits. If you needed to control a very high voltage circuit with a 12 volt controller, you could use a relay. Since a relay coil is 'generally' isolated from the contacts, you typically have complete isolation between the 'input' and 'output' section of the relay. Of course, the input of the relay is the relay coil and the output would be the contacts.
    That's really interesting. I hadn't considered that.

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