Modifying LED christmas lights to run off of 120 volt DC

christmas lights on the front of a truck.

Normal Christmas lights will run off of a 120 volt DC source just as well as an AC source because they are a resistive load.

LED Christmas lights will also run off of a 120 volt DC power supply (such as the traction pack on my electric pickup), but because they are diodes the polarity of the supply voltage must be correct. With the string I bought, half of the string was oriented one way, and the other half of the string was oriented the opposite way. This results in each LED only being lit 1/2 of the time when powered from an AC line, but if you plug the string of LED’s into a DC source only 1/2 of the string lights up. By reversing the connections at the middle of the string I was able to get the whole string to light up.

Of course, it could be that the LED Christmas tree lights are designed to only operate at a 50% duty cycle, so I may be over-heating them, but I figure that since they are mounted on the front of my truck they should get plenty of air cooling. So far they haven’t died!

2 thoughts on “Modifying LED christmas lights to run off of 120 volt DC

  1. Is it safe to by pass the plugs with embedded fuses and connect 8 or 10 x-mass lights in one continuous run for outdoor tree ?

    Thanks

    David

    • As long as your “first” string still has it’s plug with fuse, you should be fine. The first fuse (between the tree and the power supply) will protect the entire line of x-mass lights. (Or rather, protect your tree from being burnt to the ground if you get a short anywhere in the string, by blowing and removing power before the short overheats and starts a fire.)

      Note that each string of xmass lights has a maximum number of other strings that can be attached to it based upon the number of amps that the wires can pass. Typically you’ll blow the fuse if you exceed this number by too much.

      Note that for my truck I kept the original plug with fuse (rated for AC, not DC, but in a pinch it will be better than nothing) on my lights, plus I installed a DC fuse on the cord powering them.

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