I have been pondering for many months the best way to wire in my Gumpydog trailer light relay module, and the one issue that I am questioning is the 24 volt to 12 volt conversion. I have a Vanner but about 30 amps of it is already in intermittant use for other on-road things like power seats, radios, fans, and such. It's a 60 amp unit so I do have the option of pulling more current out of it if I want to - but I am leaning away from this since I don't have spare wires of sufficient gauge in the bus wiring harness to schlep 15 or 20 amps front to back several times. the spare wires I can free up are 14 gauge mostly, or smaller.
So I had the idea to convert 24 volt to 12 volt nominal locally, where it's needed. I need a 12vdc 20 amp source for the brake controller at the front of the bus, and about a 10 amp source at the back of the bus to power the trailer lights and turn signals (which will be mostly LED so 10 amps is actually serious overkill. I found a lot of switching power supply based converters, but I also found an autotransformer based converter (which is actually bi-directional and can be wired to act as an equalizer) that has really got me intrigued.
The autotransformer unit is here: http://store.solar-electric.com/12to24or24to.html (http://store.solar-electric.com/12to24or24to.html) A typical switching converter is here: http://store.solar-electric.com/sa24voto12vo.html (http://store.solar-electric.com/sa24voto12vo.html)
I know that the autotransformer unit will be putting out unregulated voltage, basically half the input, which is just fine, there is no need for regulated power in this instance. I suspect that it can or should be more reliable than the switching unit as well, being fundamentally a bit simpler. Is there any advice on which one would be the best choice?
As an aside, my headlights use a kit from Universal Coach Parts that includes a 24 volt to 12 volt converter module of some sort. Does anyone know what these are or where to get them now? A search for Universal Coach Parts doesn't come up with much. As an old race car guy I like to have a good grasp on spare parts for stuff that might strand me...
Edit - it occurs to me that since my current load will be so low at the rear of the coach I can easily run that off the Vanner equalized supply, and send it to the rear over an unused wire from the AC junction box to the rear junction box, 14 ga will be ample to run a bunch of LED lights. But I still want an independant high current 12 volt supply at the dash to power the brake controller - I'm liking the little autotransformer thing more and more for this!
Thanks, Brian
Brian,
I had the same dilemma and so I decided to put the relay box above the muffler in my MCI 7. That kept it closer to my start batteries. I pull 12V by center tapping the start batteries that are in the space of the old AC compressor. I figure that my alternator will more than cover for the intermittent use of a center tap for a trailer only.
I copied Gumpy's design and it has been FLAWLESS!! You will love it!
As for the brake controller, I have not put one in yet, but I plan to put one on.
About your vanner, it sounds like it is mounted in the front of the bus. I don't think you would have a problem using it for your brake controller unless you had everything else running at the same time.
You might consider running proper gauge wire for the brake controller under the bus through a conduit strapped to the flooring..
Good luck!
Glenn
I used the same Samlex converter for some 12 volt needs except I think I have the 12 amp version, not the 15 amp version. I bought mine brand new from an Ebay seller for about $20 shipped. Mine works good so far. I have 12 volt and 24 volt at the dash already, but no switched 12 volt. I convert switched 24 volt to 12 volt with the converter.
Universal Coach Parts is now part of MCI. There also used to be another company that sold MCI coaches that is now part of MCI. I'm not sure if they were all related to MCI, or if MCI buoght out these companies.
Brian,
QuoteI suspect that it can or should be more reliable than the switching unit as well, being fundamentally a bit simpler
Actually, both devices will have electronics to "chop" or otherwise modify the DC since pure direct current (DC) will not induce a voltage in a transformer's primary to secondary winding. Transformers are alternating current (AC) devices, but as mentioned are used in "DC conversion" projects when the direct current is suitably pulsed to induce a voltage across the transformer. And electronic components are required to process the DC in all cases.
As noted above, their are many different ways to approach this. On my bus I built a box to house a few 24 vdc relays, the same type as the 12 vdc P&B or Tyco black cube relays used in automotive applications. The current draw of the relay coil is only 1/10th amp (100 ma) and is wired directly into the OEM light circuits at the rear junction box in the engine compartment. No lines from the front are necessary. The relay contacts switch 12 vdc from a Vanner converter mounted in the engine compartment. 24 vdc feeds the converter from an OEM power post in the compartment.
I mounted new lights on the towed van to convert it from a 3 way circuit to a 4 way circuit that matches the bus. Mainly because I prefer dedicated amber turn signals to combined red stop/turn lamps. Hence no towing lamp converter is required. But I did make my own 4 way-to-3 way converter for towing other 3 way light vehicles. Two receptacles on the back, one for each mode.
Chuck