tapping into a 12 volt dc lamp

[bobhenry]

I have eight 12 volt led lamps in the caboose. These are 120 volt ac with a transformer ( wall wart ). I would like to tap 12 volt from the on board battery pack into the cord/s running from the transformer to the light. This rather stupid question is directed to the electrical engineers and electrical minded folks. I realize the current will flow to both the light and the transformer. Should I place a diode to eliminate the feed back to the transformer while allowing the flow to the light from the transformer when ac is utilized. Will this ac created DC want to flow back to the battery banks. Is this bad? Is the back flow diode needed to isolate the transformer to eliminate the drain dc drain when utilizing dc current only. Is it even possible or should I just use the inverter to convert the dc to ac to run the lamps. Perhaps a simpler answer would be a double pole double throw and Feed from each sorce and then to the lamp keeping ac created dc and the battery dc isolated. I am starting the dc wiring and this problem has me scratching my head a bit.

[Greg M]

If it's a single transformer with eight lights off it then I'd say a double pole, double throw switch to select the power is your best choice. Otherwise, you need to put diodes in both feed lines.

[kludge]

bobhenry, please post a pic or the ratings on the ratings sticker on the wall wart.

Typical diodes will drop the voltage going to the LED lamps by 0.5V to 1V. Don't forget to fuse the 12V supply line.

If your electrical plans include a full 12VDC system and a converter/charger for shore power, then I would just permanently wire the lights to the 12VDC system, and let your converter/charger worry about making the 12VDC to charge the battery and therefore run the lights.

If the thought is that you have a fully wired 120VAC system and, oh by the way it would be nice to just have these 12V lights run off a battery from time to time, then the inverter would be an easy way to get there.

If this is truly dual use, then I think I would wire in some on-off-on rocker switches, so that I could choose whether to run the light either off the battery or the wall wart or put the switch in the middle to turn off the light. I personally wouldn't do the diodes... I mean you have to put a switch in there somewhere anyway, right?

[woodywrkng]

Chances are the wall wart already has a diode on it output, since it's putting out DC, more or less. I say "more or less" since a single diode on the output of a transformer doesn't really give you DC, but rather a pulsating DC. If there's a capacitor inside the wall wart then it's pretty close to DC. Anyway, without a DPDT switch like you suggest, the wall wart may be crudely charging your battery when plugged in. If the DC voltage is in the range of 13.5 - 14.2 this will do no real harm, but if it's higher or lower I wouldn't want it connected to my battery long term. Anyway, I would just go with the DPDT switch and then you'll have absolutely no worries.