Why don't your Christmas lights work? Sadly, 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. However, this is a great time to look at the difference between series and parallel circuits.
A Simple Circuit
Just about the simplest circuit you can create uses just one battery and one light bulb. In one fun activity, you can give a person one wire, one battery and one light bulb. Ask the person to make the bulb shine. It's possible to get it to work, but it can still cause some trouble for people.
In order to get the bulb to glow, you need a complete circuit. There has to be a path for the current to come out of one end of the battery, go through the filament in the bulb and then return to the battery. Here's how that could work.
A simple circuit with a battery and bulb.
When current passes through the filament, it makes the filament get super hot. Super hot things glow. That's all the bulb does. But why the glass on the outside? That is there to keep air away from the super hot filament. With air in contact with the filament, it would burn out.
If you take away any one of the parts for the complete circuit, there would be no current and no glowing bulb.
Light Bulbs in Series
Now you want to connect two light bulbs to battery. Here is one way you could do that.
A circuit with two lightbulbs connected in series.
In this case, there is just one path for the current to follow. The current that goes through the first bulb also goest through the second bulb. If you remove any part of this circuit (like take out a bulb), everything goes out. If you wanted, you can make a similar circuit with 10 or 20 bulbs. Just connect them all in a line so that the same current goes through each one. This is a series connection.
Light Bulbs in Parallel
There is another way to connect two light bulbs. Check it out.
A circuit with two lightbulbs in parallel.
The difference in this circuit is that each lightbulb has its own "path" to the battery. If you break one of these bulbs, it doesn't really do anything to the other bulb.
这个电路的不同之处在于，每个灯泡都有自己的 “路径” 连接电池。如果你打碎了其中一个灯泡，它不会对另一个灯泡产生任何影响。
Here is a quick demo showing the difference between bulbs in series and bulbs in parallel.
Oh, and just like the lightbulbs in series you can do this with many, many bulbs.
Now we can get to the fun stuff. What about the lights you put on your Christmas tree? Are those lightbulbs in series or parallel? What do you think? Here are two hints.
Hint number 1: if you pull out one of the lightbulbs from a strand, the rest of the lights go out. Ok, this isn't always true. Many lights are actually two sets of strands. If you pull out a lightbulb, half the strand goes out.
Here you can see that this strand has one bulb removed and non of the other lights are on (even though the power is on).
在这里你可以看到，从这条线路线取出一只灯泡，其他的灯也熄灭了 ( 即使电源是开着的 ) 。
Hint number 2: In some cases, one of the lightbulbs in a strand may "burn out". When a lightbulb burns out, typically it will be due to a broken (or melted filament). In this case, the strand would look like this.
提示2：在某些情况下，串联电路中的灯泡可能会被 “ 烧坏 ”。当灯泡烧坏时，通常是由于灯丝断了(或熔化了)。在这种情况下，电路是这样的。
Notice the one bulb is indeed out but the other bulbs are on. But is the filament actually broken? Yes, if you look closely, you can see something like this.
A broken filament in a Christmas light.
Ok, now what's the answer? Are the bulbs in series or parallel? Based on hint 1, it seems the lights are in series. Based on hint 2, it seems they are in parallel. The answer is that the lights are in series. Yup.
But how can this be? How can you have one light with a broken filament and still have the other lights on? In the physics world, we call this "the magic of Christmas". Ok, not really. The answer is that the lights are connected in series but the bulbs have a trick.
但这是怎么回事呢？怎么可能一只灯泡的灯丝断了，而另一只灯还亮着？在物理学界，我们称之为 “ 圣诞魔法 ” 。好吧，这只是幻想。答案是这些灯是串联的，但是灯泡之所以发亮是因为有个小窍门。
Let's take a closer look at one of the light bulbs in the strand.
A shunt wire (bypass wire) in a Christmas light.
圣诞灯中的一根分路电线 ( 旁路电线 ) 。
This is a view of a broken bulb. At the end of the metal posts you can see the broken filament - so this is a burned out bulb (plus I broke the glass so you could see better). But what about that metal wire wrapped around the posts (indicated with the red arrow)? What is that for? If it was just a normal wire, this would short out the bulb. It would mean that most of the current going through this bulb would run through this wire and not through the filament. With very little current, the filament would not glow.
这是一个坏掉的灯泡。在金属杆的末端，你可以看到灯丝是烧断的——所以这是一个烧坏的灯泡 ( 加上我打碎了玻璃，所以你可以看得更清楚 ) 。但是绕在柱子上的金属丝(用红色箭头表示)呢？那是做什么用的？如果只是一根普通的电线，灯泡就会短路。这意味着大部分通过灯泡的电流会通过导线而不是灯丝。如果电流很小，灯丝就不会发光。
So, in order for this bulb to function properly in a strand (with a good filament), there shouldn't be a current going through this bypass wire. Ok, now let me make some guesses as to how this magic Christmas light works. Yes, I could ues the internet to find out more about this bulb, but I really like guessing.
所以，为了让这个灯泡在同一线路中正常工作 ( 用一个好的灯丝 ) ，电流是不允许通过该旁路导线的。好了，现在让我来猜测一下这个神奇的圣诞灯是如何工作的。是的，尽管我可以上网找到更多相关信息，但我真的很喜欢猜测。
- Like I said, in a functioning bulb this wire does not conduct electrical current. There must be some type of insulator between the bypass wire and the filament post.
- When the filament burns out, all the current going through all the bulbs in series stops.
- Without current going through the bulbs, there is a high potential difference between the bypass wire and one of the filament posts (I will show why below). This high voltage (120 volts) across a very short distance produces a very high electric field.
- If the electric field is high enough, it could cause and electric breakdown in the material and turn it from an insulator into a conductor.
- Boom. The rest of the lights work now (but it probably doesn't make a boom sound).
- 在没有电流通过灯泡的情况下，旁路导线和灯丝灯柱之间存在很大的电位差 (我将在下面说明原因)。这种高压 ( 120伏特 ) 在很短的距离内将产生很高的电场。
- “ 砰 ”。其余的灯现在可以工作了 ( 但可能不会发出轰隆声 ) 。
But how does a broken filament make a high voltage right at the bulb that is broken? Let's pretend like we have a different circuit. In this one, I have 5 lightbulbs in series connected to a battery. Oh, and all the lightbulbs work.
Resistors in series
Suppose each lightbulb has a resistance R (it doesn't matter than lightbulbs are non-ohmic). If I have a 10 volt battery connected to them, then the total change in electric potential around the loop must be zero volts. Also, since the bulbs are in series, they have to have the same electric current (I). This means that I can write:
假设每个灯泡都有一个电阻R ( 这与灯泡的非欧姆无关 ) 。如果我有一个10伏的电池，那么回路周围的总电势变化一定是0伏。此外，由于灯泡是串联的，它们必须有相同的电流 (I)，这意味着我可以做这样的运算：
So each bulb has a 2 volt electric potential across it. But now let's say one of those bulbs break. With an open circuit, there is no longer a current. Here is the new diagram.
Resistors in series with a break in the circuit.
Even though there is a break, the change in electric potential around this loop still has to be zero volts. But since there is no electric current, the change in potential across each resistor is zero. This means that the full 10 volts must be across the gap. This 10 volt gap potential creates a large electric field that breaks down the insulator. That's the magic of the Christmas lights.
My Lights Still Don't Work.
These lightbulbs have this awesome bypass wire inside the bulbs. However, if the light bulb is not even plugged in even the bypass wire won't work. So, if you have a strand that doesn't work here are some things you can try.
1. Is it plugged in? Yes, you should check this. A strand of lights that isn't plugged in won't work. If you have lights plugged in outside and they get wet, they will likely trip your Ground Fault Interrupter (GFI). You have these on the outlets in your bathroom (with the test and reset button). When the circuit gets wet it can trip this.
1. 插上电源了吗？是的，你应该检查一下这一点。一串灯泡不插电的话确实不会亮。如果你的电灯插在外面，它们被弄湿了，它们很可能会阻断接地故障断续器(GFI)的正常工作。你的浴室里的插座应该会带有该装置 ( 带有测试和重置按钮 ) 。当电路遇水时，断续器也就不工作了。
2. Check the fuse. I'll assume that you checked your household circuit breaker. If that was tripped, you could probably tell because of the other things that weren't working. But there are also fuses in the little plug on the end of the strand. There is a sliding door and it looks like this.
Rhett Allain Fuses in a strand of Christmas lights.
If you connect too many strands of lights together then the current will be too large. The fuse will go out to prevent these things from getting too hot. However, sometimes these little fuses just get loose after storing the lights in the off season. It doesn't hurt to check them.
3. Look for a loose bulb. If a strand doesn't work and you checked the other stuff, there's a good chance the problem is a loose bulb. It can be quite a challenge to locate a loose bulb. Normally, I just push on each bulb to see if I can get the strand of lights to come on. There is also a special tool you can use. Mine looks like this:
A Christmas light detector.
I'm not completely certain, but I think this thing detects small electric currents. If you have a loose bulb, there will be a slight alternating current going to and from the outlet to the break. On the other side of the break there won't be the same current. You can detect a current by detecting the magnetic fields that go along with the current. But now I'm not so sure this is the way these detectors work. I'll have to think about this is a little bit.
4. Check for squirrels. Yes. Look at these lights on a tree in my front yard.
A broken strand of Christmas lights.
I guess squirrels hate Christmas.
Oh, and just for fun I made a video version of this same thing.