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Trailer LightingI am building out an enclosed trailer for work that I am adding interior LED strips to for lighting as winter is arriving and it gets dark pretty early. I would like to be able to power the lights in 2 separate ways. One way will be to tap off the existing switch & incandescent light (light will be eliminated) that came with the trailer that operates while connected to my truck when its hooked up; that's the easy one. I am a builder so the trailer is dropped at a customer location 99% of the time. I am adding an AC plug on the exterior that will power receptacles in the trailer and an inverter that will power the LED strip lighting. My question is do I need to do anything special with relays, or as long as both power sources (truck or AC power) aren't connected at the same time will I be fine? Please excuse my offensively crude drawing. To summarize: LED strips powered by 12v truck power OR ac inverted power, never powered by both at the same time. Both ways are fuse protected. Can I just wire this up straight forward or will I need any kind of relay? Thanks so much!

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    \$\begingroup\$ Please use the integrated schematic editor to create a proper schematic... the diagram you've given is very difficult to comprehend. Short answer is that if you don't connect both at the same time you're probably fine. The easy way to ensure that is to only have one input plug that you physically connect to the source (be it the AC adapter or the truck's battery power). \$\endgroup\$ – Shamtam Nov 11 '17 at 15:24
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Best bet is always power from Truck battery and use 10A 14V trickle charger for battery from AC that is hardwired to battery with AC plug , with plug under front hood or mounted somewhere conveniently. LED strings vary in current , so measure what you need , maybe 5A is enough. This way battery drain is nill.

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  • \$\begingroup\$ Thanks Tony, the issue with that is that the trailer will very rarely be attached to or powered by the truck, and no battery in the trailer. Another way to put this is that the lights will be powered by AC & inverter 99% of time. I just want the same lights to work off factory switch that is powered by the truck when hooked up while I'm moving the trailer. I'm just making sure that while the lights will be connected to 2 different power sources, with only 1 of the sources providing power at any given time that I don't need to use any relays and wont have any shorts anywhere. Thanks again. \$\endgroup\$ – AndrewG185 Nov 11 '17 at 14:30
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    \$\begingroup\$ Stripleds have resistors in each segment so they operate from 9V (dim) to 14.4V. While 14V is brighter, 12.5V is OK. So I would still use a battery charger hardwired to trailer power. If truck is connected then it also charges battery and maybe drops the voltage a bit if the charger to battery current does not exceed fuse current. (10A?) so unlikely. \$\endgroup\$ – Sunnyskyguy EE75 Nov 11 '17 at 14:52
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You could use a relay, with the normally closed side to truck power, and the normally open and the coil to the AC inverted power side. If AC is present even while the truck is connected, it will use the AC side automatically.

You could use a simple diode or bridge. One diode on each side. Whichever voltage is higher will power the leds. You would need to connect the grounds together for this.

Or the simplest thing. A on-off-on dpdt switch. You manually select which power to use, or none at all.

If the current is too much for the switch you may need to use a relay anyway though.

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