Unique wiring challenge on travel trailer

Discussion in 'Automotive Electronics' started by Farmaller48, Jul 1, 2016.

  1. Farmaller48

    Thread Starter New Member

    Jun 30, 2016
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    Background: I am working on adding a third brake light to the back of my new 5th wheel travel trailer (doesn't have one from factory). I will be using a 15" LED light bar mounted between the two stock tail lights. Diagram of how I wired it in is shown below.

    Challenge: Since tapping into only one of the brake light circuits (right side or left side only) for power to the new 3rd brake light would result in the 3rd brake light becoming a turn indicator when the turn signal is activated to the stock brake light from which the new 3rd brake light is getting its power, I have tapped into both right and left side stock brake light circuits in order to provide constant 12v power to the new 3rd brake light when the brakes are applied AND one the turn signals is activated. This works, with one small glitch. When the brakes are applied AND one of the turn signals is activated, the new 3rd brake light properly illuminates as a brake light, but it has a very slight pulsing action that corresponds with the blinker cycle from the turn indicator.

    Question: Is there any sort of resist or or something similar that I could wire into the circuit feeding power from the two stock brake lights to the new 3rd brake light that would eliminate this pulsing from the slight variation in the voltage due to the on/off blinker cycle on one side or the other?
    5th wheel 3rd brake wiring.JPG
     
  2. roadey_carl

    Active Member

    Jun 5, 2009
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    5
    This is a little confusing.
    We need to work out how the system works...
    The brake lights are the signal lights also?

    How many wire are comnected in the trailer socket?
     
  3. Farmaller48

    Thread Starter New Member

    Jun 30, 2016
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    Think I found a solution. Tapping into the 12v power supply for the trailer brakes at the axle, going directly from there to the new 3rd brake light. This should supply power to the new third brake light each time the brakes are applied without any interference (voltage variance) from the turn indictors.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    Are you sure that power only comes on when the brakes are applied and is steady DC?
    Many trailer brakes use PWM to control the amount of braking as determined by how hard the brake pedal is pressed.

    Edit: If it is PWM than you could use a diode and capacitor to capture the peak PWM voltage (12V) and use that to turn on a relay for the brake light.
     
    Last edited: Jul 2, 2016
  5. Farmaller48

    Thread Starter New Member

    Jun 30, 2016
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    Crutschow, I really like your thought process. I was pondering the same thing... using a relay to open a circuit to power the new 3rd brake light. In that scenario, what would trigger the closing of that circuit so that the 3rd brake light turns off when the brake is released?
     
  6. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Assuming you don't already have this in place,

    The simplest thing I can think of is to have both brake/turn signal leads that feed the third light go through diodes so that each can not feed back to the other side and pull the supply voltage down.

    The other issue with the dimming could be from insufficient grounding between the trailer and the vehicle power supply points or simply from the lengths of wire and the associated lighting loads producing a large enough voltage drop to be able to make the LED light dim a bit.

    Or a combination of all of the above.
     
  7. Farmaller48

    Thread Starter New Member

    Jun 30, 2016
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    Actually, tcmtech, the LED light is not dimming but intensifying slightly (spiking) when the right or left side stock tail light is in blinker mode. I assume this is the result of voltage bleeding over from that side into the LED light. Is there something that can be put in the lines leading from each side to the LED light in my diagram that would suppress the voltage bleed over?
     
  8. crutschow

    Expert

    Mar 14, 2008
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    If you want to use the PWM brake signal then it could be something like this:
    upload_2016-7-2_16-27-48.png

    x in the diode designation is any number between 1 and 7.
    The relay contacts aren't shown. You could connect the the wiper to +12V and the normally-open contact to the brake light.
    The relay will close the normally open contacts and operate the brake light when it receives the PWM signal.
    The capacitor keeps the relay pulled in during the short off periods of the PWM signal.

    The contacts will open about a quarter to half second after the PWM signal stops.
     
    Last edited: Jul 2, 2016
  9. Farmaller48

    Thread Starter New Member

    Jun 30, 2016
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    Thanks so much for the diagram, crutschow. It is very helpful. I have looked on the net for diodes, capacitors and relays as shown in your diagram, but I don't know enough about them to confidently say I know what I am looking for. Is a 2700uF capacitor the same as a 2.7 uF? It looks like maybe so, but then the voltage ratings differ greatly. I assume I just need one rated at 12v or higher, correct? Then there are non-polar, bi-polar, etc. It's a bit confusing for a novice like me. Any suggestions as to where to look and what to look for are greatly appreciated. Are these items I could possibly get at Radio Shack? (I'm really not stupid... just electronically ignorant when it comes to diodes and capacitors and such... :-/)
     
  10. crutschow

    Expert

    Mar 14, 2008
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    No. 2700μF (2.7mF) is 1000 times larger than a 2.7μF capacitor.
    You want its voltage rating to be at least 15V such as one of these 16V units.
    You would get a polarized cap since a non-polarized capacitor of that size would be humongous in physical size.
    The relay can be a common automobile type relay.
    You may be able to get all that at Radio Shack if there's still one near you.
    Otherwise you can likely order it all at Amazon, Digikey, Mouser, Jameco, etc.
     
  11. Farmaller48

    Thread Starter New Member

    Jun 30, 2016
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    Again, great help and much appreciated! So, when I hear polarized, I think of designated POS and NEG. Are the polarized caps marked (+) and (-), or does it matter?
     
  12. Farmaller48

    Thread Starter New Member

    Jun 30, 2016
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    Wow... that last question was probably useless. The cap isn't grounded, is it? It just becomes a part of the (+) circuit leading to the relay doesn't it? (Up at 4AM several days in a row is getting to me...)
     
  13. crutschow

    Expert

    Mar 14, 2008
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    The negative marked side of the cap goes to ground, as my schematic shows.
    The positive side goes to the junction of the diode and relay coil. also as shown.

    A capacitor is a two terminal device and both ends have to be connected otherwise it's useless.
    A single terminal device cannot do anything since an electric circuit always has to have a return path connection
    (rule no. 1 in electronics).
     
  14. Farmaller48

    Thread Starter New Member

    Jun 30, 2016
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    crutschow, you asked yesterday about what the current rating is for the LED light I am working with. When I set my ammeter to 10A, I get no reading at all (light won't even illuminate). When I set ammeter to 200m, light illuminates normally and I get a 124.9 reading (with LED attached to a standard fully charged 12V auto battery). Am I correct in converting that to a .125 amp current draw?
     
  15. Lestraveled

    Well-Known Member

    May 19, 2014
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    Why not just connect the middle light into the running lights? (Edit: Sorry, I forgot what the question was.)

    trailer-plug-wiring-diagram-7-way-7-way-rv.jpg
     
  16. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    @Farmaller48
    This will do it with only one relay. When both turn signals come on the middle light will come on.

    Brake Light.jpg
     
  17. crutschow

    Expert

    Mar 14, 2008
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    But you need to keep the middle light from blinking with the turn signal.
     
  18. crutschow

    Expert

    Mar 14, 2008
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    If the light wouldn't illuminate on the 10A scale then you didn't have the meter correctly connected.
    Yes, the LED apparently draws 125mA.
     
  19. Farmaller48

    Thread Starter New Member

    Jun 30, 2016
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    On the ammeter, I had the black lead in the COM port and the red lead in the 10A port, with the ammeter set to the 10A setting. I think something may be wrong with my multimeter on that setting (although I can't see why... it is practically brand new, and I haven't ever used that setting). I grounded the light to the batt, then touched the red lead from the ammeter to the POS post of the batt and the black lead from the ammeter to the hot (red) wire on the LED light. Same procedure that I used when I tested it on 200m (and it worked on 200m).
     
  20. crutschow

    Expert

    Mar 14, 2008
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    Well, it does sound like something may be wrong with the meter.
    If the LED didn't light then you had no continuity through the meter.
    Perhaps it's fused and the fuse blew.
     
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