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Relay question

Discussion in 'Audio' started by Muddytazz, Jul 1, 2003.

  1. Muddytazz

    Muddytazz 1 ton status

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    This isn't audio specific, its more electronic in general. What is the purpose of a 12v relay and what does it do (for any circuit).
     
  2. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    The purpose is to provide a "safe" way to switch high current draw items using a smaller circuit.

    The relay has a trigger side that can use any kind of small wire (high numeric gauge numbers...18GA, 20GA, etc)....the high current side can run the BIG wire (smaller numeric gauge numbers...12GA, 10GA, etc).



    A perfect example would be for running offroad lights. They take a LOT of current (MANY AMPS!) to power, and you wouldn't want to feed all that through a typical dash mounted toggle switch....it would probably just burn up (or melt) as soon as it was turned on. What you do, is use that switch to trigger the relay, and let the relay SAFELY handle connecting the HIGH CURRENT part of the circuit for you. (in this case, the actual wires that go to the lights)


    Clear.....? or does that need mo-better explainin'?? /forums/images/graemlins/grin.gif
     
  3. Muddytazz

    Muddytazz 1 ton status

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    [ QUOTE ]
    Clear.....? or does that need mo-better explainin'?? /forums/images/graemlins/grin.gif

    [/ QUOTE ]
    Crystal

    Now for the next question /forums/images/graemlins/rotfl.gif are all relays rated the same, or are there different rated relays for different useages?
     
  4. Dale fan

    Dale fan 1/2 ton status

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    You can get different relays for different amperages and wiring configurations.
     
  5. HarryH3

    HarryH3 1 ton status Author

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    Here's my reply from a similar post over in the 2nd Gen Forum:

    A relay allows a small current to control a large current. The small current is all that's required to energize the coil inside the relay. The coil is wrapped around an iron core, which becomes an electromagnet when voltage is applied to the coil. This magnetic field then pulls the contacts for the high-current portion of the relay into the closed position (and actually opens the normally-closed side of the relay, but that's for another time... ) which now completes the high current side of the circuit, sending voltage to your lights (in this case).

    In electrical wiring, resistance is the enemy. In a 12 volt system, just one or two ohms of resistance can create a noticable voltage drop in the circuit (which means dimmer lights in this instance). If you route power from the battery, across the engine bay, through the firewall, to the switch, then all the way back to the lights, you have a lot of wire between the power source and the lights (and you have to use a really beefy switch to handle the high current). Longer wire = higher resistance. Not to mention you now have some fat wires stuffed up under the dash. This becomes a real problem if you're running mulitple sets of lights.

    The relay is mounted under the hood and now power goes from the battery, to the relay, and then directly to your lights. You have a much shorter current path and less resistance in the path, due to the shorter wire between the battery and the lights. You can run very small wires to the switch on the dash, as that part of the circuit is VERY low current. (On newer vehicles, where the computer controls things, a single transistor is all that completes the circuit for things like fan relays, fuel pump relays, MAF relays, etc. When the computer commands one of them to turn on, a transistor switches on and completes the circuit to ground for the coil in the particular relay.)

    Oh yeah, the biggest relay on your truck is the starter solenoid. GM solenoids use a beefy coil that draws 30-40 amps, since in addition to being a switch, it also has to have a strong enough magnetic field to throw the starter gear out to engage with the ring gear on the flexplate or flywheel. The relatively small amount of current that goes through the actual ignition switch is small compared to the 200+ amps that the starter draws. Imagine if they ran the fat battery cable all the way into the cab, then through a huge switch that could handle the current required by the starter, then back to the starter... A relay is a much better solution.

    The whole thread can be found at:

    http://coloradok5.com/forums/showflat.php?Cat=&Board=blazer4x4&Number=775052&Forum=All_Forums&Words=small%20current%20control%20large&Match=And&Searchpage=0&Limit=25&Old=6months&Main=774527&Search=true#Post775052
     
  6. Muddytazz

    Muddytazz 1 ton status

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    Very good info /forums/images/graemlins/thumb.gif /forums/images/graemlins/thumb.gif

    Relay rating now. (Example) Battery to relay with ??awg wire and 20-amp fuse, relay to dash switch with then to fuse box with ??awg wire, relay to york compressor with ??awg wire. Can i use just 14awg wire, and what rated relay would be good?
     
  7. HarryH3

    HarryH3 1 ton status Author

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    First you need to know how much current is required by the load (in this case, the clutch on your York compressor). I don't think they draw a lot of amps, but I don't know the current draw offhand. Then do a Google search for American Wire Gauge and Current Rating. That should find a chart that lists the proper wire size to use for a given load.

    Relays are typically rated for the current they contacts can handle. So once you know the current, then you can determine the relay required. Bosch 30 amp relays are pretty cheap and are used in a lot of automotive applications these days. They're also pretty small, so they don't take up much space under the hood.
     

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