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Relay question for switches

Discussion in 'Lighting' started by PaidtoPlay08, Feb 10, 2020.

  1. Feb 10, 2020 at 9:28 AM
    #1
    PaidtoPlay08

    PaidtoPlay08 [OP] Well-Known Member

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    I have a 08 Tacoma and i am replacing the factory switches with ch4x4 ones. I am also adding another one for rigid ditch lights. A total of 3 switches. 2 replacing the factory ones and one specifically for ditch lights. They say the switches are 3 amps and MUST be used in conjunction with a relay. I know NOTHING about wiring.

    My question: Exactly what kind of relay do i need to buy. And do i need just one relay, or one for each switch. Also how do i hook the relay up. Any help would be appreciated.
     
  2. Feb 13, 2020 at 11:18 PM
    #2
    tacoma_ca

    tacoma_ca Well-Known Member

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    A relay is a switch which is operated by an electromagnet-- passing a small current through the electromagnet allows a larger current to be controlled once the electromagnetically actuated switch closes. You should probably read up on it if you are going to do it yourself. Just about everything low voltage in automotive (lights, switches, stuff the driver interacts with) is 12 volts. A quick rule of thumb is get a relay that is controlled by 12V at the coil and can carry perhaps twice the current (amps) that your accessory requires that will be on that circuit. You can over-spec the current capacity of your relays, the only downside is the electromagnetic actuators might draw a bit more current but neither the magnitude of nor the change in current is large enough to really matter. Your first step is to draw your circuit out of what you want to do.
     
    Last edited: Feb 13, 2020
  3. Feb 14, 2020 at 9:08 AM
    #3
    Pj40

    Pj40 Well-Known Member

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    Here is some wiring diagrams I drew up. I can draw up a specific one for you if you cant follow these. But yeah a relay is also a huge safety feature. A lot of people will just wire a switch with no relay so that means you have a hot wire in your cab passing a metal firewall and is a huge risk of electrical fires. A relay will make it so the switch inside is only sending low voltage signal just to turn the relay on so that it can send the power from the battery to your accessories

    20191222_165839.jpg
    20191231_201422.jpg
    20191222_163722.jpg
     
  4. Feb 14, 2020 at 9:09 AM
    #4
    Pj40

    Pj40 Well-Known Member

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    For every functioning set of auxiliary lights you want it's own relay. A relay cant be used for multiple things unless you want all the lights to come on with one switch.
     
  5. Feb 14, 2020 at 11:09 AM
    #5
    tacoma_ca

    tacoma_ca Well-Known Member

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    Good point about safety. One clarification here though is it is not the voltage that is different between the control circuit and the switched circuit, they are typically the same, and the peak voltage in the control-side circuit can exceed the peak voltage on the load-side circuit for typical automotive applications.* What is different is the current, whereas the switched circuit has a higher current capacity and the control circuit which penetrates the firewall passes less current and can thus have a fuse that blows earlier than the controlled circuit or the thinner control circuit wire through the firewall would fail sooner and thus generate less Joule heating if a short were to occur.

    *A caveat of actuating a relay coil with a square wave from a switch is the coil will try to ramp voltage to maintain constant current as the magnetic field changes based on its inductance just as we see in the ignition coil. But given the low impedance of the battery this is typically not a problem. As such, one could point out that the maximum voltage in the coil-side circuit is expected to breifly spike when switched. This is important to watch out for if you have anything sensitive connected to the switched side of the relay. This is why the use of ‘solid state relays’ are sometimes preferred as they are solid-state devices that are not electromagnetically actuated.
     
    Last edited: Feb 14, 2020
  6. Feb 14, 2020 at 12:13 PM
    #6
    Pj40

    Pj40 Well-Known Member

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    Your absolutely correct I was just trying to put it into simpler terms. The entire system has no difference in voltage but o didn't want to get into the scientific reasoning of the actually circuits and how they open and close. Just trying to keep it simple for the OP.
     
  7. Feb 14, 2020 at 1:38 PM
    #7
    tacoma_ca

    tacoma_ca Well-Known Member

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    I’d just add that current and voltage are very different so confusing the two might just make it more complex for somebody trying to learn what a relay does, especially since this is a record on the internet that may be referred to in the future. But I know what you are saying in terms of unnecessary complexity. I was just trying to capture that the situation is the opposite of what you described and that might not be obvious to some people.
     
  8. Feb 22, 2020 at 11:25 AM
    #8
    PaidtoPlay08

    PaidtoPlay08 [OP] Well-Known Member

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    Yeah I'm illiterate when it comes to electrical. Sounds like I need to buy a relay mount with 3 12V relays (One for each switch) and pay someone to do it. i don't want to colossally F something up.
     
  9. Feb 22, 2020 at 11:49 AM
    #9
    tacoma_ca

    tacoma_ca Well-Known Member

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    Or maybe look at how somebody else did the whole thing. It is very similar to plumbing, voltage is like pressure and current is like the mass flow rate in a pipe. Resistors are like a constriction and relays do the same sort of switching as solenoid sprinkler valves. A small current in the coil lets a large amount of water flow through the sprinkler sys. In your lighting system a small current in the relay coil pulls a switch that lets a large amount of electrical current flow through the lights. Just dont short anything out and make sure your wires will not rub off the insulation with repeated vibration.
     
  10. Feb 22, 2020 at 6:59 PM
    #10
    tacoma_ca

    tacoma_ca Well-Known Member

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