Christmas Light Puzzler - HELP

The fact that are getting voltage at the end of the string does not mean alot to me. You didn't indicate what method you used to determine this? Did you use a field won maybe? if so, That most likely will yield incorrect results.. The Safety lamp might be in series with the return path.. First try a small 120 Volt incandescent lamp in place of the safety lamp. The whole idea is to have that lamp absorb the whole load in the case where the string may become shorted.

that is just an idea.. May not be fact but I wouldn't use anything other than another bulb to test the string.

I've never seen a set with a "safety bulb", but apparently it serves as a fuse? Personally I would bypass it and instead put an inline fuse in the set, but I don't recommend others to bypass safety features, use common sense with this.

For the other bulbs, I've seen a lot of problems with bad connections in the sockets or broken wire leads on the bulbs cause sets to be out. It can be frustrating, but a multimeter works well to track down the open.

quoted text -

Hi,

I used a Fluke multimeter, plugged into the female socket at the end of the string. Good voltage and it doesn't change when I move the string of lights around, so I think the string is solid.

I can't replace the "safety bulb" with anything without cutting into the string. These are the very small incandescent lamps with the tiny wire leads.

Here's an update.

In tracking down and replacing numerous burnt lamps, The back half - the side closest to the far end of the string - suddenly lit up.

What is strange is that I was working on the end closest to the plug at the time.

Also, I have replaced all burnt bulbs with new ones that were tested prior to installation. Now I have half a string that does not light for no obvious reason.

Keep the thoughts coming . . . I'll keep playing with this as long as my sanity holds out . . . ..

Thanks.

"Reactor" hath wroth:

It's not quite what you think. See explanation at:

The first bulb is a little different from the others. It does NOT contain an automatic shunting crowbar or shunt resistor inside. If it blows, the whole string goes off. That's because it's connected directly to the 117VAC power mains. If the bulb were removed, and some kid shoved a paper clip into the socket, they might get the full force of the 117VAC power. If they did that with any other lamp in the circuit, the safety lamp would limit current and possibly act as a fuse.

If all the shunts in the string decided to close after the bulb filament blew, then it's highly likely that the safety bulb would also blow. My guess(tm) is that the only way for that to happen is overheating or excessive inrush current. Do the other lights get unusually hot or seem unusually bright? Is there proper air flow? Do you have it near a fireplace or heater?

Poof. The 3rd wire goes to the connector at the end of the string, so that the next block of lights can be powered. In effect, it forms an extension cord for the 117VAC. If you short it, you short the 117VAC. Bad idea. Look at the wiring diagrams in the above URL's.

A better question would be why are your bulbs burning out? It might be that the safety features is actually doing its job and preventing a meltdown. 2500 bulbs belches quite a bit of heat. Before you disarm the safety features, do some calculating:

Each bulb burns about 1/2 watt. 2500 lights burn about 1250 watts, most of which goes up in heat, not light. That's quite a bit of heat that has to go somewhere. My guess(tm) is that your tree is overloaded with lamps and they are blowing because they're getting too hot.

Another possibility is that you're killing them with a light flasher. The inrush current is perhaps 10 times the normal operating current of the bulb. This inrush current happens every time you apply power to the bulbs. If they can't handle the high current, they'll blow.

Switch to Judaism. Our Hannukah menorah only requires 9 lights. If that doesn't help, there are LED lamps, which burn far less power and last almost forever. I bought a small string 2 years ago. They've been on continuously for about 2 years and none have blown.

Put one probe in the neutral slot of the outlet, then pull the bulbs out one at a time and check for mains voltage at one of the contacts. If you start in the center you can figure out which direction the open is, and quickly narrow it down. Keep in mind you might have multiple faults.

No way, incandescent lamps will work fine in environments hot enough to burn the insulation off the wires. Unless the tree is on fire, heat will absolutely not cause these lamps to fail. This is not rocket science, bulbs burn out, connections in sockets get bad, series wired lights are notorious for problems like this. Additionally, as each lamp fails and shunts, the voltage across the remaining lamps increases and if left unchecked they'll start to burn out too. More than once I've had this cascade in smaller strings to the point that every lamp burned out until the fuse in the plug opened.

When did this "safety bulb" thing come about? This thread is the first I've ever seen or heard of it. Is it a European thing or is this here in the US? Every string of miniature series lights I've ever seen has all identical shunted lamps and a fused plug.

I bought very long strings in Walmart a few years back, as house decoration lights are VERY expensive here in the UK compared to in the US. As each 'set' is made up of two strings, originally paralleled by the end-of-string connectors for use on US 110v line power, it was easy enough over here to just remove the plugs at one end and series two strings for 230v line power. You could then just plug two more 110v strings, one into each 'far-end' connector, to have two 110v paralleled strings in series with another two, across atotal supply of 230v for 2000 lights. Total cost about $16 as I recall. Over here, that amount of lamps would have cost $200 or more, equivalent.

Anyways, the point is that these are a three wire circuit, and each new three wire sub-string, has a replaceable safety fuse lamp at its beginning, so there are multiple safety bulbs per fully wired string. So no, it's not a European-only thing, as these are US purchased lights. Every year, these ones drive me up the wall as well. Every year, I think that I've got the wiring arrangement 'fixed' in my head, then every year, it all seems to go wrong. The fact that I have paralleled strings in series, *really* confuses the issue, because if one section of one string goes out on one series'd half, then the voltage no longer divides equally across each paralleled string, so one half lights up like flashlamp bulbs, whilst the other half go as dim as candles ...

Arfa

James,

Thank you for the information. I got this tree 5 years ago and this is the first time I have seen this ""safety bulb" I no longer have the directions to the tree, but will take a picture of this special lamp and sent it to you.

It does look exactly like the other bulbs, except that it's base is white and it is permanently installed into it's base, as if it were molded in. Each layer of branches has its own string of lights; they either plug into the previous string or into a main extension cord that traverses the trunk of the tree. Each layer of branches/string of lights has one of these bulbs.

Arfa,

If you need more lights shipped over there, just send me an email direct, tell me what you want, and I'd be happy to help you.

And thanks for your response to the post.

Why not replace entire strings? Factor in the replacement cost of your house, do the math, and then do the safe thing.

The shunt bulbs are designed to fail in a shorted condition. As each bulb dies, the voltage to the others is raised by a small amount causing them to be stressed more and shorten their life . . . so it makes a lot of sense to keep up the replacements and not let several lamps in a string die and then ignore them.

With 2,500 lamps you might expect a shunt or two to fail open in than number. It isn't like they are designed to rocket science standards.

The safety bulb is there to fail when the current is high enough to cause unacceptable heating in the dead shunted bulbs - keeping the lamps from melting the plastic sockets. It serves as a fuse, so just taking it out is not the thing to do - replacing it with a fuse might work for a short time and still be safe, but you still have to keep up the lamp replacements to keep the fuse from blowing.

Well, your still around to complain about it . . . must be doing what it is supposed to - fail open and turn off the power to the string.

Too late.

If the safety bulb is blown, then it is no good, toss it and get a new one.

Thanks, Bruce. Kind of you to offer. I do get over to the US fairly often ( I will be in Vegas in a few weeks time ) so I usually take the opportunity to stock up on stuff like this that I can't get back home - like Advil for instance, or Bengay patches for my dear old mum's arthritic neck. Last time I was there just before Christmas, I bought another whole set of lights. They were literally about $5 in Walmart, and I got them just for the 500 spare bulbs it gave me ... Good luck with fixing your problem. Believe me, I know how frustrating it can be, in my case, up a ladder, in the cold, with it getting dark, and a pocket full of spare bulbs that keep slipping from my frozen fingers !

Arfa

"James Sweet" hath wroth:

True. Heat does not cause the lamp to fail, but does aggrivate the problem. Although the author does not specifically mention the obvious, the hotter the bulb, the faster the evaporation, and therefore, the faster the failure.

How light bulbs burn out.

Due to the high temperature that a tungsten filament is operated at, some of the tungsten evaporates during use. Furthermore, since no light bulb is perfect, the filament does not evaporate evenly. Some spots will suffer greater evaporation and become thinner than the rest of the filament.

These thin spots cause problems. Their electrical resistance is greater than that of average parts of the filament. Since the current is equal in all parts of the filament, more heat is generated where the filament is thinner. The thin parts also have less surface area to radiate heat away with. This "double whammy" causes the thin spots to have a higher temperature. Now that the thin spots are hotter, they evaporate more quickly.

It becomes apparent that as soon as a part of the filament becomes significantly thinner than the rest of it, this situation compounds itself at increasing speed until a thin part of the filament either melts or becomes weak and breaks.

I haven't. There are usually 50 bulbs in series. When the Noma "Stay-Lit" bulbs blow up, a roughly 5 ohm shunt ends up across the bulb. With 50 bulbs, it would take quite a few bulbs to blow, before the current would creep up to the point where the others might blow.

However, if your lamp string use some other method of maintaining operation, such as shorting instead of introducing a 5 ohm shunt, then it might blow the other bulbs.

I have a similar problem with a 9ft. prelit tree we bought 4 years ago. Try flicking the safety bulb with your finger while the string is plugged in. This has worked for us over the years, but I have one string for which this technique no longer works. I'll figure out how to repair it, then I'll update you with the answer unless you've already come up with a solution. Let me know if you have.

Bruce, I solved the problem on my tree. The "safety bulbs" appear to be a "safety" for the manufacturer so the strings will all go out eventually and force you to buy another tree. They are simply bulbs without a shunt. I was able to pull the non replaceble white bulb bases out of the sockets with pliers and dental picks and I replaced the bulbs with ones that had shunts using the same white bases, and everything works fine. If you can't get the white bulb bases out, you could just cut the socket out of the circuit and splice the wires together. The only result will be slightly brighter lights since you've removed a little resistance form the series circuit.

Merry Christmas! Dish

And that, of course, is the whole purpose of the safety bulb. When one shunt bulb burns out, the shunt shorts it, and each remaining bulb gets a little bit more voltage. When another shunt bulb burns out, it happens again - and again and again. Pretty soon, there are only a few working bulbs left, and the string is drawing more current than it was designed to. The question then becomes which burns up next - the remaining bulbs, or the wire connecting the bulbs.

Bill

In my experience, one or more of the shunts does. They're a lot smaller than the wire in the string.