Lights out!

November 10, 2009 8:03 AM

I need burned out christmas lights (the miniature variety), or I need to burn them out myself.

Basically, I have 100-light outdoor miniature light strings that I only need the LAST 90 of. the first 10 or so (nearest the male plug) are not needed and when clumped together produce too much light in that area and ruin the effect we are trying to achieve.

These sets are the variety that when one burns out, the remainder remain lit, however, if you remove one, the whole set goes out.

Covering them is not an option because there is a fire hazard to having 100 or so of these lights on and in a box (10 from 10 strings)

Rewiring isnt an option as I dont have the talent and dont want to compromise the "outdoor-y-ness" of the strings.


So basically, I have to get burned out bulbs, or figure out a way to burn them out myself. I have a couple of strands to experiment with.

Any thoughts?

Thanks in advance!

posted by sandra_s to technology (16 comments total)

why not use exterior-grade extension cord?
posted by Think_Long at 8:08 AM on November 10

Wild guess - could you break the unneeded bulbs and then cover them with electrical tape?
posted by biggity at 8:09 AM on November 10

There's actually a safe way to cover them - "blackout light caps" - http://www.creativedecoratinginc.com/clips-attachments/black-out-caps.html
posted by Sweetie Darling at 8:10 AM on November 10 [2 favorites]

By doing this the remaining lights are going to burn out much quicker as they are going to get approx. 10% more voltage.
posted by raildr at 8:10 AM on November 10

Second raildr. Minilights are wired in series (this is why when one is unplugged the whole string goes out). Each bulb in a 100 light string is only rated for 1.2V and bypassing 10% of the bulbs will result in a significant over voltage to the rest of the string. They have a mechanical bypass on the filament to complete the circuit when the bulb burns out which is why the string keeps working as long as all the bulbs are in place. The bypass doesn't draw any current though.

What you need to do is buy two 50 bulb strings and a 15 or 25 (less is better) bulb string. Mark the plastic bases of the bulbs from the 15/25 and 50 bulb strings with a dab of nail polish so you can tell which is which. Obviously use a different colour for the 15/25 and 50 bulbs so you can differentiate.

Two at a time take 100 string bulbs out of their wiring harnesses and put them in the 15/25 bulb wiring harness. Plug the 15/25 bulb string in and each 100 string bulb will see 4-5 times the voltage it was designed for and burn out pretty quick. Do not replace all the bulbs in the 15/25 bulb string with 100 string bulbs. The bulbs will all burn out at once and because of the mechanical circuit continuity a dead short will occur. You need the 13/23 correctly rated bulbs to prevent the dead short.

Voila 100 burned out bulbs to put in your 100 string wiring harnesses.

But before you plug your 100 bulb string in replace ten working bulbs in each of the 100 bulb strings with bulbs from the 50 bulb string. This will give you the correct voltage drop in the 100 bulb string once you have ten burned out bulbs in the string as each 50 string bulb drops the same voltage as two 100 string bulbs. Now that you have the correct voltage drop your modified string won't burn out prematurely.

Caveat: the 50 string bulbs may not be the same brightness as the 100 string bulbs. If you spread the 50 string bulbs out in the 100 bulb string instead of bunching them together it won't be as noticeable instead being perceived as natural variation.
posted by Mitheral at 9:19 AM on November 10 [1 favorite]

I just came here to suggest maybe using an extension cord too as you can bluk that up how ever you'd like. though i guess if you knwo you only need 90 bulbs, then having 100 is too many.

Do what Mitheral says.
posted by bDiddy at 10:16 AM on November 10

They have a mechanical bypass on the filament to complete the circuit when the bulb burns out which is why the string keeps working as long as all the bulbs are in place. The bypass doesn't draw any current though.

I was under the impression they use a diode shunt - you know, a diode with a voltage drop of, say, 1.5 volts, so it doesn't conduct when there's 1.1v across a working bulb. As shown in, for example, this patent and this patent.

Do you have any pictures or anything showing how such a mechanical bypass would work?
posted by Mike1024 at 11:24 AM on November 10

Go door-to-door and ask your neighbors?
posted by filthy light thief at 11:50 AM on November 10

I can't find a picture. The ones I were thinking of had a loop either attached or formed into one of the wires holding the filament. The other wire holding the filament went through the loop and when the filament failed the two wires touched.

So I just looked at one of the minilight bulbs I've got in my electronics stuff (minilights are a cheap source of small low voltage bulbs) and it doesn't seem to work that way. There is a loop of wire above a dot of something below the filament. So not mechanical like I thought but I don't think that's a diode shunt either. The blob of something doesn't seem to contain any thing. Could just be a bit of glass to hold everything together until the envelop is installed. I don't have a burned out bulb to compare to though I might hook a rheostat and isolation transformer tonight and find out what appens when the bulb burns out.
posted by Mitheral at 12:36 PM on November 10

This page says: "If you look closely at a bulb, you can see the shunt wire wrapped around the two posts inside the bulb. The shunt wire contains a coating that gives it fairly high resistance until the filament fails. At that point, heat caused by current flowing through the shunt burns off the coating and reduces the shunt's resistance. (A typical bulb has a resistance of 7 to 8 ohms through the filament and 2 to 3 ohms through the shunt once the coating burns off.)" If true, and it makes sense, there will be essentially no voltage drop over the burnt out bulb and my instructions should still work.

sandra_s I was reminded on that page that not all 100 string sets use 1.2V bulbs. If you plan to follow my instructions make sure the 100 bulb string goes completely out if one bulb is removed. If only half or third goes out some of the math changes. Post here with the details in that case.
posted by Mitheral at 12:45 PM on November 10

The shunt is simply a wire that is insulated with a high resistance coating. When the filament burns out, the increased heat is enough to burn the insulator off, allowing power to flow through the shunt wire. The filaments typically have 7-8 ohms of resistance, whereas the shunt has 2-3 ohms. Running with 10 burned out bulbs will therefore increase the voltage to the others, but not as much as e.g. cutting the sockets off and splicing the wires.

There is another twist (and one that likely breaks Mitheral's idea) -- 100 (and longer) bulb strings are usually N strings of 50 bulbs simply joined together. This keeps voltages even for different lengths, to keep the bulbs universal.

You can verify if your string is like this by taking one bulb out and verifying that only half the lights go out.

It also means that the 40 good bulbs will actually be overpowered by more than 10%, but not a full 20% since the shunts offer some resistance.

The caps that Sweetie Darling linked may be your best bet. Then again, since a full new string costs about as much as those, you could just go for it with the burned out ones -- the good bulbs may well have enough tolerance that you can get through the season without much trouble.

Lots of info here.
posted by SpookyFish at 1:01 PM on November 10

D'oh, should have previewed.
posted by SpookyFish at 1:02 PM on November 10

More info and a follow up:

If one light is pulled out, all 99 of the others go out.


Ultimately, I'd like to hide the unwanted length of lights (the first 10%). My thought was to coil them up and put them in a weatherproof box, but I am concerned about the heat build up.

THe blackout caps would work, but then I would have kind of a cat-o-nine tails of unlit lights at the bottom of the tree.

Basically, I'm lighting a tree that has a 2 foot trunk, then has tall (say 20'-25') mini-trunks that that branch off it. but instead of hanging the lights from the tree limbs, I am stringing them straight up the trunks.

So that I didnt have 2'-4' of lights hanging off the ends of the limbs, I had to start at the ends and work down to the trunk. Thats where I have the about 10% of each strand extra.

My thought was that if i could have burned out bulbs in that 10%, i could coil them up, stick them in the box, and since no filament would be burning there would be negligible heat build up.

Wow, waht a project this has become! :-)
posted by sandra_s at 1:15 PM on November 10

THe blackout caps would work, but then I would have kind of a cat-o-nine tails of unlit lights at the bottom of the tree.

But you would have this same issue if you left burnt out bulbs in the sockets.
posted by SuperSquirrel at 5:56 PM on November 10

@SuperSquirrel - "My thought was that if i could have burned out bulbs in that 10%, i could coil them up, stick them in the box, and since no filament would be burning there would be negligible heat build up."
posted by sandra_s at 7:04 PM on November 10

Ah, I missed that part, sorry.

Maybe you could approach this as a feature, not a bug. Could you put a different color bulb in the last 10% of each strand, and then do something creative with those ends? Maybe swirl the ends around your tree skirt or something?

What about very darkly colored bulbs? I've seen dark green and dark blue bulbs (and even black at Halloween*) that seem to produce very little actual light. You could then snake them back up the trunk, but since they're a different color, it wouldn't be such a blob of brightly-colored light in one area?

*A house in my neighborhood was actually all decked out in lights for Halloween (like one would do for Christmas) using orange and black lights.
posted by SuperSquirrel at 8:19 PM on November 10