converting 24v truck to 12v trailer

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silverstate55

Big Dummy
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I was wondering about if a 4 pin relay would work. But I thought I read on another thread that a 5 pin relay was required. But I haven't done the install yet, so I have no experience to verify. But I guess you answered that question. Oh well, the 5 pin relay was the same price, guess I'll just have an extra pin.

Humm... just had a thought. I wonder if you used a 5 pin relay, that you could use 1 relay for both brakes and tail lights? Connect the taillights to 87, and then connect the brakes through 87a...?? Sound reasonable?

As you can tell, I'm no electrician. Know the basics,and can usually repair stuff, but never had any reason to use relays for anything more than just replacing dead ones.


Thanks for the post!
I'm subscribed since I'll be doing this very soon.

Somebody please correct me if I'm wrong, as I frequenly seem to be... but with a 4-pin relay, doesn't that act as a SPST switch: 24V In, Ground, 12V In, 12V Out...single 24V In switches 12V In/Out on & off?

So a 5-pin relay would act as a DPST: 24V In, 24V In, Ground, 12V In, 12V Out...BOTH 24V Inputs switch 12V on/off? So this could act as a brake switch relay if BOTH brake lights come on to activate trailer brakes (so that the trailer brakes don't momentarily activate with just one side turn signal coming on, it would require BOTH sides of turn signals/brake lights coming on to activate)?

Or am I totally confused?

Thanks in advance!!
 

jamawieb

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I went with Mikew's set-up that he makes. Here is the link Civi Trailer Lights. I just got it in the mail yesterday and it looks very well made. It really looks like it was factory made, it's compact and you don't have to splice any wires.It also has a license plate holder and light.
 

John Mc

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Digging up a rather old thread, but wouldn't it be possible to put a 24v to 12v buck converter on whatever circuit you want to drop down to 12V? A stop/turn signal draws about 2 amps. Is there any reason not to just put one of these $10 24V to 12V 5 Amp buck converter in each of the lighting circuits before making the connection to the trailer connector on the back of the truck? For a couple $ more, you can get a 10 Amp version

A single, higher amperage converter could be used with 24V relays triggered by the truck's light circuits. It would probably be a bit less expensive, but using a separate converter for each circuit eliminates the moving parts of the relays.
 
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mikew

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Hey John,

We looked into doing this, even have a prototype on the shelf, but encountered an issue.

It's pretty standard design to put a turn on time delay in buck converter circuits, to give the system enough time for input power to become stable. It's a pretty short delay half second or so. But; the result is, running lights work great but turn signals may not even flash at all, or it they do it's a way too quick little blink.

The thing we never evaluated, because we never found an off-the-shelf buck converter with a short enough turn on delay, is; does the repetitive turn on surge kill the buck converter in a short time.

Add a couple of turn signal relays in your design, old school but time-tested, rugged and waterproof (if you get the correct ones).

Regards,

Mike W
 

John Mc

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Thanks for the heads up on that, Mike.

I found someone making adapter cable assemblies from a link earlier on this thread. From the schematic, it appears he's using a single larger buck converter, and using that to feed several relays, one for each of the needed circuits. I'll probably buy one of his cables. But I do wonder how he solved the problem if the blinker is the only load in use on the buck converter. I suppose you could fool it with a resistor that kept a constant load on it. It would have to be a separate circuit from the relays, rather than jumping around the relay contacts, since the ladder would leave a faint glow on the lights all the time
 
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John Mc

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It turns out that the "someone" I found that is making 24v to 12v trailer lighting adaptors is none other than @mikew. You're not exacty a "hard sell" type of guy are you Mike? Instead of sending me a link to his products, he's here giving me tips n how to make my own. After looking at the parts involved and my time to figure it out and put something together, I decided that his Ruggedized Frame Mounted Trailer Light Converter was exactly what I needed, was probably nicer than what I would have come up with myself, and is reasonably priced... so I ordered one. He does make other kinds of adapter cables, including one that plugs into an existing 24v trailer connector and provides a standard 12v 7-way RV/4-flat connector for your 12v trailer. The adapters Mike sells do not interfere with a vehicle's existing 24V trailer outlet, if it has one.

Between that and a brake controller that will run off of either a 12 or 24V vehicle electric system while providing output suitable for 12V trailer brakes, it looks as though I'll be all set. (Mrs. Santa Claus is giving me one of those for Christmas.) If anyone else is looking, the controller is a Redarc Tow-Pro Elite Trailer Brake Controller. It will run in proportional mode for highway use, or "User Control Mode" (a.k.a. Time delay control) which is generally better for off-road/rough terrain. (you can also manually operate the trailer brakes). The main body of the controller is mounted remotely. Just one control knob to mount on the dash.

Between these two items, I should be ready to pull any of my several 12v trailers, or trailers owned by friends.
 
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