Are Christmas Lights in Series or Parallel? | Hacker News Reader

Are Christmas Lights in Series or Parallel?

wired.com

96 pointsdnetesn50 comments

Unfortunately, the bypass wire doesn't actually always burn out to make the rest of the lights work. There is a device called a LightKeeper pro that sends a larger "zap" through the string to encourage the needed bypass wire to burn out. Its saved me a bunch of time figuring out which bulb was bad in a string of lights.

Or you can also DIY with a lighter : https://www.youtube.com/watch?v=7foDiXX-CcE

tl;dr Christmas light bulbs each have a bypass wire which is initially insulated to avoid shorting the filament. If the filament burns out, the new high electric potential burns off the insulation, thus re-completing the circuit for the rest of the bulbs.

Actually this is what I hate about such articles, they extrapolate two sentence explanation into huge corpus.

I understand the article is about series vs parallel but I'm sort of... Taken aback by the premise setup in the beginning that:

A.) Its hard to find the burnt out bulb(hint its almost always the one with the broken filiment)

B.) You should just throw the strand away

Maybe I'm just too old at 30(?) or I grew up underprivileged because I recall untangling lights and replacing burnt out bulbs well, as far back as I can recall. 7 or so? And throwing the strand away is just wasteful.

I understand there is an article to write but on one hand we have some guy writing about creating a sou-vide oven and on the other someone suggesting burnt out light bulbs are too hard to find. :|

I agree with you, and I also replaced burnt bulbs as a kid, but nowadays I'm seeing more and more Christmas lights using molded plastic instead of sockets (also, usually with LEDs instead of bulbs), so replacing them is much harder than it used to be.

True, but you shouldn't need to replace an LED bulb like you did the incandescent's. More likely to be a problem with the transformer or cheap wire and connections.

It's not always easy to see the broken filament, or at least didn't used to be (I've not really examined many in detail recently).

As a kid in the 80s, I used to work in my dad's hardware shop and people regularly used to bring in lights that didn't work. We had a little test area set up where we would put each bulb in turn to find out which one(s) wouldn't light.

I used to use an ohm meter and a binary search algorithm to find the broken bulb in the series strands (waaaaay back in the day). My dad gave me the task as a challenge. I was quite young and into electronics, but it was before I had access to computers (or even calculators for that matter).

I completely agree.

"in the event of a string of lights going out on your tree, replacing the strand is usually the best option. It’s pretty difficult to find that one bulb that’s causing the problem"

That one sentence sums up most of what is wrong with Christmas, and society, today. I find the "just throw it away if it's broken" attitude really difficult to accept. I would hope everyone reading HN has the self-awareness to consider where their possessions come from and what happens to them when they're thrown away.

A truly appropriate article for a site whose name is "wired".

LED sets are usually wired in series too, since they're current-driven devices.

Although most LED matrices have parallel columns of series LEDs, since they are usually multiplexed at high speed to create the persistence of vision effect.

The answer to this, at least for longer strings, is usually 'both'. You'll usually find several sets of series wired runs connected in parallel and sometimes mid run branches.

The price ratio between comparable LED and incandescent holiday light strings is now between 2 and 2.5 to 1. Incandescent holiday lights probably have about two to three years of life left as a product.

The white LED strings look very nice and I definitely agree that white incandescent strings have a very short market life left. But the colored LED strings which are sold at Home Depot and similar merchants in the USA have very different colors than the incandescent strings.

I personally find the colored LED strings to have very unpleasing colors compared to incandescents. I've tried a few different "brands" (in quotes as the brand is mostly meaningless afaict) of LED strings and all were not to my liking.

Oh interesting, I'd noticed that at some point between my childhood and now, burned out bulbs stopped taking out the entire rest of the light string. I wonder if the bypass wire is a new(ish) development, or if we just had cheap Christmas lights when I was younger.

You had cheap Christmas lights. We had both. We had the actual bubble lights. Those were more expensive at the time. A bulb could burn out in one of the bubblers and the remaining bubblers would work.

So why aren't the bulbs hooked up in parallel?

Probably to save wire, if you hooked em up in parallel you'd need to send a wire to every bulb from the outlet.

With 10 bulbs at 1 inch spacing, you're wiring 1 + 2 + 3 + ... + 9 + 10 = 55 inches of wire instead of 10

Follows a roughly N^2 law

Huh? Why not just use two wires and connect one wire to the other with each bulb.

Like this: http://www.planetchristmas.com/Images/MinisA.gif

I think the real answer is that if you did it this way (in traditional pre-transformer light sets), each bulb would have to be rated for 120v, or whatever your local line voltage is, which would make them both more expensive, and more dangerous.

Interesting: That would suggest that if you let enough bulbs on a strand go out, you reach a cascading-failure threshold at some point, where the voltage across the remaining filaments is higher than their rating. And then I guess the breaker trips.

Yes, if you shunted enough of the bulbs in the strand you would get a cascading overvoltage down the line. It wouldn't trip a conventional breaker, though. Those bulbs can't pull enough power, even in over-voltage conditions. Might trip an Arc Fault Circuit Interrupter, though.

Higher voltage on the remaining filaments really just shortens their life. So yes it's a type of cascading failure, but it's not catastrophic (each set has a fuse in the plug anyway, otherwise that 28AWG wire would function as the filament of last resort, if you get my drift).

This means you want to replace burnt out bulbs when you notice them, rather than waiting for the whole segment to go dark.

That's not how parallel circuits work. The voltage is independent of how many devices are connected. That's how your wall plugs always provide 110V.

Edit: Sorry, I misread, I thought this subthread was about parallel circuits.

It's not a parallel circuit. It's a series circuit. If you have 100 bulbs in series, each is getting 1.1V across it.

If half of them burn out (and, as the article explains, become a short-circuit), then you have 50 bulbs, and each is getting 2.2V across it.

Why would you need a separate wire for each bulb? Seems to me like you could have a ladder topology: two bus wires running from the outlet, and then the bulbs going between them.

You're right, I don't know why I thought the way I did back there

It requires 20 inches of wire in both cases. When connected in parallel, each socket acts as a pass-through to connect the hot and grounded wires to the next socket in the set, in addition to providing power to the bulb. Thus, the sockets are in series but the bulbs are in parallel. This is how electrical wall outlets are wired.

The problem is that a parallel connection requires bulbs that can take 120VAC directly. This is not a problem with C7 and C9 light sets (the small Edison base bulbs). The minis, though, can only take a few volts. Thus, they need to be series-connected to get the voltage drops across the individual bulbs to the level a single bulb can handle.

These days you'd think they could throw a regulator in the wall plug for a few cents to get around the voltage issue.

That doesn't work as well when you want to string together multiple strands end to end.

Or you could use a transformer.

Sure, and some sets did. It doubles the cost of a basic light set, though, so they were usually only found in the higher-end sets.

Draws too much current. Older strands used to be parallel and were a frequent cause of fires.

LEDs use less power but don't work well in parallel.

Interesting. And your theory seems to hold water: V=IR, so with constant voltage, current is inversely proportional to resistance, and the resistance of 100 bulbs in series is far far greater than 100 bulbs in parallel.

Your intuition is correct, but a minor nit: bulbs aren't linear devices and don't have a fixed resistance. At low power, a bulb will look pretty close to a short circuit (it's just a piece of tungsten wire, after all). As power increases, the filament temperature rises and so does its resistance. This is why bulbs generally burn out when turning on - 170V across a mostly short circuit, with most of the resistance being in whatever section of the filament is most worn through, so most energy is dissipated at a single spot. poof!.

170?

The nominal voltage on an AC power line refers to the "RMS" average of the waveform. The peak voltage of a sine wave is sqrt(2)*RMS. Presumably bulbs tend to blow when they're turned on while the voltage is near a peak.

Couldn't higher-resistance bulbs be used?

Apart from other reasons mentioned. LEDs don’t work well in parallel. Even if you have just 2 of them, one will be bit brighter (drawing more current) than the other.

Curious to know what would happen if 2 or more bulbs broke at the same time?

In Engineering, there is no such thing as "at the same time" unless it's a feature of the design.

I thought those christmas lights had bee replaced by led strips.

Even LEDs need electricity, and electricity can be delivered to them serially or in parallel too.

But LEDs have much lower failure rates than incandescent bulbs, and when they do fail, they're likely to fail closed rather than open. So this kind of trick shouldn't be necessary.

Piercing blue, icy white, and unfocusing violet aren't so inviting on a cold winter's night.

Black body 4 life.

What's the problem with coloring the lens?

LEDs emit a limited number of wavelengths of light.

Incandescent lamps emit a broad spectrum of light.

Filters (colored lens) can knock out certain colors, allowing others to pass. Filters cannot add colors that aren't there to begin with.

You can't just filter a monochromatic light into another color. Filtering LEDs doesn't work as straightforwardly as your incandescent-based intuition would lead you to believe.

Width of the emitted spectrum.

If you’re going to do it properly you’d actually attach candles to your tree, although this is slightly riskier (I’ve been woken and run out of the house by my mum in the past while my dad attempted to put out a flaming branch).

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