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Thread: Need help wiring a relay please..

  1. #1
    Raw Wave tbird2340's Avatar
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    Need help wiring a relay please..

    I bought some LEDs that I want to install as daytime running lights on my car.. I want them to come on when the car is on but go off when the headlights are on..

    I would I wire this to accomplish that?

    Thanks!
    Current Vehicle: 2007 Dodge Nitro

    Second Vehicle: Sold it :( 2005 Dodge Ram 1500 Quad Cab

    First Vehicle: 2003 Ford Ranger

  2. #2
    Constant Bitrate
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    You could use two relays. Use the N.O. connection on one to turn on the DRL's when you turn on the key. Then use another relay in series with that, but use the N.C. connection and power that relay off the headlights.

  3. #3
    Raw Wave tbird2340's Avatar
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    Any chance you could be more specific? Pin numbers and the like?

    Thanks much.
    Current Vehicle: 2007 Dodge Nitro

    Second Vehicle: Sold it :( 2005 Dodge Ram 1500 Quad Cab

    First Vehicle: 2003 Ford Ranger

  4. #4
    Constant Bitrate
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    http://www.autoshop101.com/forms/hweb2.pdf

    On the top of Page 9 shows a 5 pin relay, you would need two of those. They are usually good for 30A, and you can get sockets for them so they are easy to wire.

    Relay 1 DRL on (Normally Open Relay) Closes when power is applied
    Wire 86 to Ignition
    Wire 85 to Ground
    Wire 30 +12Vdc Power (Fused)
    Wire 87 Power out to Relay 2

    Relay 2 DRL off with headlights on (Normally Closed Relay) Opens when power is applied
    Wire 86 to a wire that goes hot when headlights are turned on.
    Wire 85 To Ground
    Wire 30 Power in From Relay 1's Wire 87
    Wire 87A out to DRL's + Wire

    On page 12 of the document they show Diodes to suppress the voltage spikes when turning off the relay, not a bad idea. A Radio Shack general purpose diode is fine. 1N4001 I think.

    Ground DRL's as normal.

    Is that a little more clear?

  5. #5
    Maximum Bitrate
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    With standard automotive relays a Diode is not necessary since it is already included in the package.

    Also you CAN do this with one relay. The coil of the relay needs to see negative on one site and positive on the other side.

    Using a combination of resistors and possibly a Diode or two you can accomplish the same thing as using two relays. Also if you do use two relays you can use a standard 20/30 amp automotive relay to power the lights and a much smaller 12volt relay to control the coil when the lights are on or not since the second one really doesn't need to carry any current.

  6. #6
    Newbie daywalker03's Avatar
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    Quote Originally Posted by redheadedrod View Post
    With standard automotive relays a Diode is not necessary since it is already included in the package.

    Also you CAN do this with one relay. The coil of the relay needs to see negative on one site and positive on the other side.

    Using a combination of resistors and possibly a Diode or two you can accomplish the same thing as using two relays. Also if you do use two relays you can use a standard 20/30 amp automotive relay to power the lights and a much smaller 12volt relay to control the coil when the lights are on or not since the second one really doesn't need to carry any current.
    True; I also know that some newer vehicles use the existing lights (usually the high-beam at a lower intensity) to serve this function.

  7. #7
    Constant Bitrate
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    As an alternative, you can use one heavy duty relay, and one light duty.

    Relay 1 DRL on (Normally Open Relay) Closes when power is applied. Heavy Duty Relay
    Wire 86 to Ignition
    Wire 85 to N.C. Of Relay 2
    Wire 30 +12Vdc Power (Fused)
    Wire 87 Power out to Lights

    Relay 2 DRL off with headlights on (Normally Closed Relay) Opens when power is applied This can be like a 1 Amp Relay.
    Wire 86 to a wire that goes hot when headlights are turned on.
    Wire 85 To Ground
    Wire 30 Power in From Relay 1's Wire 85
    Wire 87A Ground

    How much current do your LED's Draw? Maybe you don't even need a really heavy duty relay. Some at Radio Shack are good for 10A.

    Lots of ways to skin this cat.

  8. #8
    Raw Wave
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    Why not just connect the LED+ to IGN +12V, and connect LED- to headlight +12V.
    That's assuming halogen or similar headlight bulbs - the LEDs will ground thru the bulbs when they are off.
    Use one diode each for hi and low beams - ie, LED- into 2 diodes; diode-band towards hibeam +12V & lobeam +12V. If LEDs are under 1A, any 1N400x diode will do.


    Quote Originally Posted by redheadedrod View Post
    With standard automotive relays a Diode is not necessary since it is already included in the package.
    No - very few relays come with inbuilt diodes. (My preference is plain relays with no inbuilt resistors or diodes. I then add diodes externally if required.)

  9. #9
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    Couldn't you just use one relay, and not connect one of the pins?




    Basically, connect 30 to the ignition, 87a to the LEDs, 85 to the headlights, 86 to ground, and then just not connect 87 to anything? When the coil is activated, it should flip to 87, but since the LEDs aren't connected, they should turn off. Am I missing something?

  10. #10
    Raw Wave
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    That should work noting that that if using the relay that redheadedrod described, you'd blow the fuse or the internal diode. (IE - hence the convention that 86 is more +ve than 85).

    But as I described, a relay probably isn't necessary.

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