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Unique wiring challenge for travel trailer

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Farmaller48

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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 resistor 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
 

Diver300

Well-Known Member
Most Helpful Member
The way that you have shown the wiring, all three trailer lights are in parallel and will light at the same time.

If you have a diode in each of the two positive wires, then the trailer lights won't be affected by the additional light, but the central LED light will flash when the brakes are not on and the car is indicating.

The tail light converter should allow the central LED light to work as a brake light only, but the dimming might still happen. That is due to the voltage drop in the wires, caused by the resistance of the wires and the current taken. To stop that, either increase the wire size, or get an LED light that has better current control, so that its light output doesn't vary if the voltage varies.

You could also use a connector that is separate from the standard towing connector to join the LED light directly to the vehicle's central brake light.
 

Les Jones

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One simple solution would be to use a relay with a set of NO contacts driven from one side stop lamp and the centre stop light driven via the relay contact from the other side stop lamp. That way the centre one would only come one when both side stop lamps were powered.

Les.
 

Diver300

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One simple solution would be to use a relay with a set of NO contacts driven from one side stop lamp and the centre stop light driven via the relay contact from the other side stop lamp. That way the centre one would only come one when both side stop lamps were powered.

Les.
The problem with that scheme is that when braking and indicating, one side is on all the time and the other is flashing, so the centre light would flash.
 

MikeMl

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I can think of seven cases:

1. Left and Right both off (rolling along or parked)
2. Left and Right both on steady (Braking)
3. Left on steady and Right flashing (Braking while turning)
4. Left off and Right flashing (Turning)
5. Left flashing and Right on steady (Braking while turning)
6. Left flashing and Right off (Turning)
7. Left flashing and Right flashing (Four-way flasher)


Does the RV have electric brakes? (The one I owned did). However, PWM is used to modulate the braking action. Light the middle bar only when the electric brakes are being pulsed by the braking controller in the towing vehicle. A simple RC circuit would prevent flickering....

Otherwise, I could envision a state machine that could work from only the Left and Right tail lamps.
 
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Les Jones

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Hi Driver300,
I overlooked that situation. Well spotted. When I was driving in Canada I found this method of using the same light as a brake light and an indicator confusing.
 

MikeMl

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MikeMI, pardon the ignorance, but what is an RC circuit, and how does it work?
Before I answer this, what is the current rating of the added light bar? Can you post a link to it?
 

Farmaller48

New Member
MikeMI, I am trying to find out the current rating. I bought them from an Ebay seller, and they don't list the amp rating in the description. Looked on the light units and it is not listed on them either. I contacted the seller to ask if they know. In the meantime, I have learned that your RC reference is resistor/capacitor. I will post the current rating as soon as I find out. Thanks for your help
 

MikeMl

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Do you have a multimeter? You can put the meter in either 10Adc range, or 1Adc range, and measure the current that the light bar draws when connected to 12Vdc. I'm guessing that since it is LED, it will measure 1A or less.
 

Farmaller48

New Member
I do have a multimeter, and will give that a try. I think your guess is going to be correct since the wiring coming out of the back of the LED light is either 18 or 20AWG.
 

MikeMl

Well-Known Member
Most Helpful Member
MikeMI... current draw on the LED light is less than 1 amp.
How much less? I was hoping for a measured number, like 304mA.

Another question: Most 6 and 7 pin trailer connectors have +12V power from the towing vehicle in the plug. Can you extend that wire to the LED bar to power a circuit?

Finally, are you up to building a small circuit that contains at least one diode, a transistor, a resistor or two, and a transistor. Inputs would be +12V, the braking line, gnd. Output would be the wire to the LED bar?
 
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shokjok

Member
I used two relays in this situation when I built my bike trailer in 1987. My circuit description may help you in your situation, found in another thread about tail lights as UTTI.GIF . I recently repaired that circuit, replacing two transistors with an SPST relay.
In my circuit, the NC contacts are fed from the brake switch, the NO contacts are fed from the flasher and the center arms feed the tail lights. Turn switches feed the relay coils. To feed a third brake lamp on my trailer ( optional in Alberta), I would be running a separate wire straight from the brake switch signal feed.
 
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