are christmas lights in series or parallel
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ARE CHRISTMAS LIGHTS IN SERIES OR PARALLEL?
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.
Click to Open Overlay GalleryA
simple circuit with a battery and bulb. Rhett Allain
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.
Click to Open Overlay
GalleryA circuit with two lightbulbs connected in series. Rhett Allain
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.
Click to Open Overlay
GalleryA circuit with two lightbulbs in parallel. Rhett Allain
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.
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.
CHRISTMAS LIGHTS
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.
Click to Open
Overlay Gallery Rhett Allain
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).
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.
Click to Open Overlay Gallery
Rhett Allain
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.
Click to Open Overlay GalleryA broken
filament in a Christmas light. Rhett Allain
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.
Let’s take a closer look at one of the light bulbs in the strand.
Click
to Open Overlay GalleryA shunt wire (bypass wire) in a Christmas light. Rhett Allain
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.
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.
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).
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.
Click to
Open Overlay GalleryResistors in series Rhett Allain
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:
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.
Click to
Open Overlay GalleryResistors in series with a break in the circuit. Rhett Allain
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.
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.
Click to Open Overlay Gallery
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:
Click to Open
Overlay Gallery
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.