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Four 3-wire strings of Christmas mini lights. Incandescent. Likely identical and purchased same year. Bulbs physically interchangeable.

Each string has three circuits. The first and last bulbs in each circuit have larger bases that fit in to larger sockets.

Its bveen several years since they were last used as none of the 12 circuits light. All sockets are full, no broken bulbs. The male/female ends pass electricity, and the female distal ends passes electricity. So fuses are all good and cords are basically good.

I removed all bulbs and chose one proximal circuit as the test circuit. Starting at the proximal end of the test circuit, I added bulbs one by one, checking for continuity in the series wire between the male/female plug and the next (empty) socket using a free multimeter from Harbor Freight. If a bulb did not provide continuity, I set it aside. If it did, I left it in and proceeded to the next empty socket. Eventually the entire socket was full and it lit.

But some of the installed bulbs did not light. I then replaced those bulbs with ones that did light.

I then removed one of lit bulbs, which caused the circuit to un-light. One by one I went through all the bulbs, 100+++, using the empty socket as a testing rig. A few bulbs did not cause the circuit to re-light. These few were set aside as duds. Quite a few caused re-light and themselves lite up. These were set aside as good'uns. However, the vast majority caused the circuit to re-light, but did not themselves light. These were set aside as anomalies.

I filled the other two ciruits with the good'uns. The entire string lit. Jiggling the string to test for bad connections did not reveal any.

What's the deal with the anomalies? shorted out? higher voltage requirements? How can they and the other three strings be salvaged?

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  • Aren't most Christmas lights disposable? Prices are so low it's not worth it to try repairing, just buy a new string (and recycle the old)
    – mmathis
    Dec 12, 2017 at 20:13
  • really, how many hours did you just spend trying to salvage a $2 light string?
    – agentp
    Dec 12, 2017 at 21:40
  • @agentp, I spent a lot of time working on strings woven into a pre-lit tree, so it was worth the hassle. After going through that, I wouldn't mess with it at all on a loose string. Time consuming for sure.
    – JPhi1618
    Dec 12, 2017 at 21:48
  • yes I cringe every time I see one of those pre lit trees that are so $$$ you will be messing with the bulbs when they go.
    – agentp
    Dec 12, 2017 at 22:02

1 Answer 1

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Christmas light manufactures have started to make mini-bulbs that include a "shunt" device that allows the light to pass current even if the bulb has been burned out.

enter image description here

Image and more detailed information about holiday lights at Energy.gov.

Your "anomalies" are most likely burned out bulbs that have the shunt wire working. The other bulbs that don't work and also don't pass current have burned out and do not have a shunt, or it is not working. Neither of these bulbs should be used.

As for the strings that do not work, you can try a device that is made to try and get all the shunts working. There are products such as the LightKeeper that have a "quick fix" mode. These send an electrical shock down the wire to try and activate the shunts. I have used one with some success, but it doesn't always work.

Another procedure I have tried with some success is using a non-contact voltage detector to see where the voltage "ends" on a plugged in set of lights. I was able to untwist the wire every once in a while on the broken set and probe for voltage. Using a divide and conquer type search, I was able to find a bad bulb pretty quickly. That LightKeeper Pro has a built in voltage detector for this, but I think I used another detector I had in the tool box.

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  • Good answer. I’ll add that it’s important to replace non-glowing bulbs, not just rely on the shunt. The string is wired in series. If there are 50 mini lights the bulbs are 2.5 volt, for 35 the bulbs are 3.5 volt. For each light not glowing the voltage increases, causing the remaining bulbs to glow just a bit brighter--after several are out the accelerate burning out others... The more burned out the faster the rest begin to burn out. If we buy 20 new sets for something I get 21 or 22 and pull the bulbs out of the extras for spares.
    – Tyson
    Dec 12, 2017 at 21:44
  • The linked article mentions replacing the burned out ones, but it's good to call it out. I had a set that was in pretty bad shape and I watched the last 20 or so lights go out in a accelerating chain reaction as each remaining bulb got brighter and brighter.
    – JPhi1618
    Dec 12, 2017 at 21:46
  • That's exactly how I learned... Then sat down and drew the series circut, then I realized why... Eventually...
    – Tyson
    Dec 12, 2017 at 21:49
  • @Tyson I did wonder how that circuit keeps from cascade failing in a very, very fiery way once enough bulbs have burnt out.... and there we have it.
    – rackandboneman
    Dec 12, 2017 at 23:53

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