# Tis’ the Season of Broken Christmas Lights – Understanding Series vs. Parallel Circuits

Oh no! Why aren’t your Christmas lights on? Oh, you thought it would be funny to pull one of the bulbs out, and now the whole thing has gone belly up! If you’re one of those unlucky souls that managed black out their entire light setup, don’t be sad, you’re not alone. Every year, millions of lights go dark around the world for one critical lesson – to teach you the difference between series and parallel circuits!

## First, the Basics

Before we dive into the difference between series and parallel circuits, let’s go over some basics terms that we’ll be throwing around.

• Current. Electricity has work to do, and when the electrons are flowing around a circuit, that’s current at work.
• Circuit. If it’s a closed, continuous path, then electricity will flow on it. Along this path, electricity can do a ton of amazing things, like power your smartphone, or send humans to space!
• Resistance. This is what electricity encounters when it flows along physical material, whether that’s a copper wire or a plain old’ resistor. Resistance restricts the flow of electric current.

Below you’ll find an image of a simple circuit, which includes a battery, a switch, and a light bulb.

The simplest of circuits, powering a light bulb with a battery.

## The Season of Series

Let’s go back to our Christmas lights to understand exactly how a circuit wired in series works. Say you have a strand of lights, connected one after the other. If you viewed this in a circuit, it would look something like this:

Your Christmas lights in series, notice the lights are all connected one after the other. (Image source)

When we plug our strand of lights into an outlet, what will the current do? Let’s follow the flow:

• Powering it up. When we plug our Christmas lights in, current starts flowing from our wall outlet.
• Flowing along. It then moves along the strand of copper wire and through our Christmas light, making them shine brightly.
• Coming home. When our current reaches the end of our strand of lights, it heads for a ground to get some rest, and so the cycle continues.

It doesn’t matter what kind of components you place in a series circuit, you could mix and match capacitors, resistors, LEDs, and a bunch of Christmas lights together and the current would still flow the same, from one part to another.

Now, this is where Christmas lights tend to have their downfall. What happens if you yank out one of those bulbs in your strand of lights? If your lights are anything like ours, then all of them turned off! Why is this? Think about it, if the current is flowing from light to light, and you disrupt that connection, then you’re cutting off the path on which the electricity is trying to flow. This is called an open circuit.

## Current and Resistance in Series

There is a fundamental law of the universe to remember for how current and resistance work in a series circuit:

The more work (resistance) that a series circuit does, the more its current will decrease.

Makes sense, right? As you add more resistance to a circuit, like some Christmas lights, or even a resistor, then the more work for your circuit has to do. Let’s say you take the circuit we introduced at the beginning of this blog that had one light bulb. Now, what would happen if you add another light to this circuit? Will both bulbs shine as bright? Nope. When you plug in that second bulb, both will get equally dim, because you have added more resistance to your circuit, which decreases the flow of current.

Adding another light bulb in series decreases the current because our battery now has more work to do!

But how do you go about figuring out how much resistance you have in a series circuit? You just add all of the different resistance values together. For example, in the circuit below we have two resistors, each being 10k Ohms. To get the total resistance in this circuit, just add all of the numbers together. That’s 10k + 10k, which comes to 20k Ohms of total resistance.

Adding our resistors together in a series circuit is easy, just add each one together.

And what would your current be in this circuit based on that amount of resistance? Here’s how you can figure it out.

• Using our trusty Ohm’s Law Triangle, we get the equation we need to use: I = V/R, or Current = Voltage divided by Resistance.
• Plugging in the numbers that we know, we get I = 10V/20k. 0.5 milliamps (mA) are flowing through our circuit!
• What if we took out one of the resistors? Now our equation is I = 10V/10k, and we’ve increased our current to 1 milliamps (mA) by reducing our resistance.

## Working in Parallel

Now, wouldn’t it be great if you pulled out one of the bulbs in your strand of Christmas lights but the rest of them stayed on? If your Christmas lights were all wired in parallel, then this is exactly how they would behave!

In a parallel circuit, imagine your strand of lights all connected together. But instead of each bulb being connected one after the other, they are all connected separately, in their circuits like in the image below. As you can see, each bulb has its own mini circuit that is separate from the other, but they all work together as part of a larger circuit.

Your Christmas lights now in parallel, notice how each light has its own circuit. (Image source)

But how does the current flow in this kind of circuit? It doesn’t just follow one path; it follows all of them, all at the same time! Here’s why this is awesome – Imagine that you yank out one of the bulbs in this type of circuit. Rather than stopping your whole Christmas light operation, the rest of the circuit will keep on flowing because each light is not dependent the light before or after it for its source of electricity.

## Current and Resistance in Parallel

When a circuit is wired in parallel, current and resistance start to do some strange stuff that you might not expect, here’s what you’ll want to remember:

In parallel circuits, as you increase the resistance, you’ll also increase the current, but your resistance gets cut in half as a result.

Wait, what? That sounds crazy! But think about it regarding your Christmas lights. As you add more colorful lights to your circuit, then you need to draw more current to power all of those lights, right? And so a magical thing begins to happen, the more lights that you add, the higher your current climbs, but that increased current has an opposite effect on your resistance.

This might be a bit tough to wrap your mind around, so let’s go through a simple example. Check out the circuit below:

Here we have a parallel circuit with two 10k resistors and a 10V battery.

Here we have a 10V battery source and two 10k resistors that are connected in parallel. Now, since each resistor has its own circuit, we need to figure out how much current each will use:

• Going back to our Ohm’s Law Triangle, we know the equation we need to use is I = V/R, or Current equals Voltage divided by Resistance.
• And plugging our numbers in, we get I = 10V/10k, which comes to 1mA. But that’s only one of the two resistor circuits; we now need to double the current to get our total for the entire circuit, which is 2mA.
• Now, what happens to our resistance at two amps? We can use Ohm’s Law to figure it out with R = V/I, which comes to R = 10V/2mA = 5k Ohms. Because we doubled our current, our original 10k resistors are now only doing half the resistance!

Yeah, this all gets pretty crazy, doesn’t it? It’s just one of those laws of the universe.

## How Your Christmas Lights Really Work

So how do those Christmas lights of yours really work? Here’s a hint – they are neither 100% series or 100% parallel, they’re both! Those smart engineering elves decided that the most efficient way to make your Christmas lights work is to connect several series of lights together in parallel. Check out the image below to see what we mean:

You’ll find many of today’s Christmas lights wired in a series/parallel combination. (Image source)

Here’s why this series/parallel hybrid is great – if you yank out one light, only one section of your lights will turn off, not all of them. This is because you have only affected one of the series circuits in your larger parallel circuit. But why didn’t the engineering elves just make all the lights in parallel? That would require a ton of wires, and Santa needs to watch his manufacturing costs just like us!

But wait, you might remember that one year when you had a light burn out, but the rest of your lights kept working, what happened there? You can thank this little magic trick on what’s called a shunt. This little device allows current to continue moving through a circuit even after the light burns out. How so? Let’s take a closer look at one of your Christmas lights below:

The shunt wire keeps electricity moving even after a light burns out. (Image source)

See that wire that’s wrapped around the bottom part of the light? That’s the shunt, and it has a coating on it that prevents any electricity from flowing through it while the light is working properly. But when the wire at the top burns out, the increase in temperature melts the coating off of the shunt wire, allowing electricity to keep passing from one terminal to the other of the light, and so your Christmas lights keep working!