Shortening battery powdered light string

oldalgebraJanuary 5, 2011

I'd like to cut the last six lights off an LED battery powdered string of lights (which contains 20 lights).

From beginning to end, there are only two wires going in and coming out of each individual light.

1. Do I have to keep the original LAST bulb on the string as my last (no. 14) bulb after shortening the string?

2. Or can I simply cut the last six off? If that's so, do I tape off the two wires that come out of the last bulb so that they DO NOT touch, or do I wire them together?

Thank you

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Ron Natalie

You'd not want to do that with 120V as the're wired in series and rely on the voltage drops, but for the battery ones they are typically in parallel (two wires going in and two coming out for four total, right?). Yes, you can lop off a few without problems.

    Bookmark   January 5, 2011 at 12:15PM
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brickeyee

Depending in how thew string is wired (series/parallel/sections & the battery voltage) you may have a problem with to much voltage on the remaining LEDs.

    Bookmark   January 5, 2011 at 2:15PM
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DavidR

If this string is battery powered, I'd be surprised if the LEDs are in series. Typical forward voltage for an LED is in the 2-4v range, so if they're powered by 2 or 3 flashlight batteries, they're almost certainly wired in parallel.

If that's the case, most likely you can remove any number of LEDs with no damage. There's a slight risk that the battery box has a series resistance to limit the current, which wouldn't do its job right with fewer LEDs in the circuit. But most of these cheap LED strings are set up to use as little material as possible, so that's not too likely.

It's kind of a final test, but if you cut off the last 6 LEDs and the rest continue to operate as before, at about the same brightness, then all you have to do is solidly insulate the cut ends.

If they go out, that means they're in series. In that case you'll need a series resistance to keep the remainder of the LEDs from drawing too much forward current and destroying themselves.

What resistance you need is determined by the forward voltage of the LEDs and their current. In most cases it's easiest to determine the resistance experimentally, working downward until you find one that makes the rest of the string glow at the right brightness.

But as I say, it's unlikely that they're wired in series.

If all this sounds somewhat iffy, it's because it is. Any time you modify a product, you need a little (sometimes a lot of) engineering expertise. In this case, the quick and dirty no-tech solution is to just put black electrical tape (or black heatshrink tubing) over the LEDs you don't want shining.

    Bookmark   January 5, 2011 at 7:55PM
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