Automotive trailer wiring question

My van, like many newer vehicles, don't like extra brake and turn signal lights for a trailer simply tapped into the existing wires.
I had a special unit hooked up which made things work, but it burnt up last week. I could wire in a new brake/turn light unit, but with that unit, between it's internal resistance and the overloading the little wires made to power only 1 or 2 bulbs, there was quite a voltage drop and the trailer lights were often dimmer than I'd like. (The voltage drop was over 2 volts in certain situations.)
I thought this would be easy with some relays, but I ran into problems.
First to power the trailer, I ran a fused hot wire from the battery junction box to the rear of the tow vehicle (van) and I have a good ground off the van.
Keep in mind that the van has separate lights for brakes and turn signals and the trailer uses the same lights for the brakes and the turn signals.
I first wired in the turn signal lights tapping a relay into the vans turn signals, they work great! Nice and bright! Then for the brake lights I wired another relay with 1 diode going to each brake (and turn signal) lights. I used the diodes to isolate the two separate turn signals.
Now the brake lights work great and bright!
But I forgot about 1 thing. When the brake lights are on, I need to be able to make the trailers turn signal/brake light flash, which they don't since the brake relay in on.
I believe I can do this using the normally closed terminals somewhere, but my thinker isn't thinking as well as it used to.
Any help appreciated! And remember the tow vehicle has separate turn and brake lights, and the trailer uses the same lights for the brakes and towing. I think when it's done properly, I won't need the diodes either, but hey, I'm asking for help so I'll shut up!
Thanks, Tony
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Tony Miklos wrote:

<snipped>
I'm replying to myself.
I *think* I've decided to get a generic adapter similar to the one that fried, but I'll have the adapter run relays so I can have my lights nice and bright. I think I'll still need the diodes after the the brake relay, but that's not a big deal.
I doubt I'll get this done in the next few days if anyone has a better/different approach.
Tony
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Tony,
You're on the right track thinking about using normally closed relay contacts. What makes these converters work is that the stop and turn signal inputs from the tow vehicle each open the circuit from the other input to the combined output, instead of closing their own circuit from input to output. My parents owned an RV business during the 1970's, and I first saw these modules when I was in high school. After some thinking, I worked out the circuit using two PNP transistors but never built it.
One effect of this circuit is that when the tow vehicle has both stop and turn signals on, the trailer turn signal flashes opposite of the tow vehicle. That's not the case in my 2004 Chevy Silverado pickup with factory tow package, so there must be some extra circuit tied to the turn signal switch.
Mike
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You need two relays for the turn / stop function. Make sure the relay you select does NOT have a suppression diode across the coil since it will have both polarities applied to the coil.
For the left turn /stop line on the trailer, connect the relay coil between the left turn line on the van and the stop light circuit on the van. If the turn signal is flashing alone, the relay will cycle with the coil ground provided by the stop light bulb filaments. The relay coil current isn't enough to cause the filaments to warm enough to light up. If the stop light is on without a left turn signal, the relay will be on solid with the coil ground being provided by the turn signal filaments. If the brake is on and the turn signal is flashing, the relay will cycle. The source will be the stop light circuit and the ground will be the turn signal bulbs. When stop light is on and the turn signal is in the off part of the flash cycle, the relay will be on with the ground through the turn signal bulbs. When the turn signal is in the on part of the flash cycle, both sides of the relay will have 12V and the relay will be off.
The right turn/stop relay is wired the same way, with the coil between the right turn wire and the stop bulb wire.
If the trailer has a bunch of marker lights (full clearance light package), you may want to consider a relay for the marker lights too. If it only has the tail lights you probably are OK.
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Matthew Beasley wrote:

I had time late this afternoon and wired it up. It works great! And yes I kept the relay for the clearance and running lights, they had the highest voltage drop of all, now they are most likely brighter than when new.
The only thing that looks funny is that when the brakes and turn signal are on, the van and the trailer turn signals flash opposite of each other. Not a problem as far as I can see.
While working on it days ago I had noticed that all four lights in the trailer had the dual filament bulbs in, but only the outer two would light for the brakes and turn signals. I pulled the light fixtures out of the rear doors and found that someone had CUT the wires to the STOP/TURN filaments of one lamp circuit on each side. They were without a doubt hooked up when new. Evidently the original owner had a problem with too much current draw for two bright lights on each side and cut the wires.
Thanks Again!!!!!!!!!
Tony
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Tony Miklos wrote:

Replying to myself again... I forgot the relays are all rated for intermittent use. The only one that presents a problem is the clearance lights which will stay energized for hours at a time.
I could have ordered a different relay, but instead I used an old trick that was used in some juke box circuitry. I put a #47 bulb in series with the coil in the relay. So when the filament is cool, it gets a good... surge? to turn it on. A split second later, the bulb lights and the voltage to the relay coil drops 3.75VDC
I could play with different bulbs to give it more of a voltage drop, but I have a couple hundred of the #47's around. Also the bulb only lights dimly, it's rated for 6.3vdc and it's only getting 3.75vdc so it should last indefinitely.
I left the coil on for hours (with the clearance lights disconnected) and it hardly got warm.
Curious if anyone has seen lamps used as current limiters and/or if that is really old school?
Tony
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Pretty clever idea with the light bulbs. Have you checked to see that it cools fast enough that you wont have a problem cycling the power? Try rurning the lights off and right back on to make sure the relay closure is reliable. If not maybe a capacitor in parrellel with a current limiting resistor (or bulb) will work better.
Tony Miklos wrote:

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**THE-RFI-EMI-GUY** wrote:

Yes the light bulb current limiter is a neat trick!
There is no problem cycling since the only relay that will stay on long and use the light bulb is the relay for the "running lights" and/or "parking lights", and/or "clearance lights". What ever you want to call them! And even so, I've turned them on and off many times to make sure the relay was working.
Tony
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