Series  vs. Parallel Circuits

What are series and parallel circuits?

Introduction

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Understanding the difference between parallel and series circuits is key to understanding electronics. In this exercise, we'll experience the difference by drawing both and seeing how they differ.

Let's start by thinking about the words "series" and "parallel."

A series is a sequence of things in order. (That's why they play the World Series games one after another.) In a circuit, a series is like a string of current that goes one by one through different components like lights, motors, buzzers, etc.

Parallel refers to lines that run alongside each other. In a parallel circuit, the current splits into two or more paths to do work and then comes back together to go back to the battery.

Step One

Basics: Draw a Series Circuit

As you probably know, a circuit is a closed loop of current that does some kind of work, like lighting a light bulb or turning a motor.

Let’s jump in and start drawing these two kinds of circuit, and then we can see what lessons we can take from what we observe. At the end you’ll find some ways to extend this exercise and explore in more depth.

Series Circuit

Let’s start by drawing the circuit pattern you see here. Use the stencil to set the distance between the pads where the feet of the modules will go.

Also gather up your materials - this circuit will use a battery module and 4 LEDs. (You can also make this circuit with any number of LEDs greater than one.)

Step Two

Connect Modules for a Series Circuit

Place a steel sheet underneath your paper, and then add the modules to your series circuit. The modules go over the spaces where there is no ink drawn.

Turn on your battery, and you should see the LEDs light up!

Step Three

Basics: Draw a Parallel Circuit

Next up: Take a trip to parallel-land. Draw the parallel circuit pads and lines you see here.

Next place the battery on the bottom or top, and add LEDs horizontally across the others.

Step Four

Examine the Differences

Now let's dig deeper and observe what's going on here. First, think about how the current is traveling through each of these circuits.

In the parallel circuit, it can go from the battery all the way around the outside loop. It can also go around each of the inside loops, all at the same time. Every loop is a parallel circuit that can act independently.

In the series circuit, the current goes through each light in a row, like runners running the bases in baseball.

Mess With It

What happens when you remove one light form this circuit? Do the rest go out? You should experience the same result as in the second photo. Does the parallel circuit stay partly on while the series circuit go out completely? Why do you think this is?

Step Five

What in the World is Happening Here?

Let's take a look at WHY these circuits are different.

In a series circuit, the battery has a certain voltage (let's say 9 volts) which is the potential to power your circuit. When it travels through the first LED, that LED does work (it lights up!) and there's a voltage drop on the other side. The reduced voltage keeps on truckin' until it reaches the next LED, which again drops the voltage when it does work.

Each time the current goes through an LED, there's less voltage on the other side. This is why they get progressively dimmer.

In the parallel circuit, the voltage is still 9V at the battery. Here, though, that 9V can travel to each new LED - that is, each branch of the parallel circuit - without traveling through any other LEDs first, so it appears at each LED with its full potential. The voltage drops as it goes across each LED, and meets back up with the conductive ink line that goes back to ground.

(Also note that in a parallel circuit the battery will drain faster since it's sending more current through each branch to every LED.)

This might be counterintuitive, but the more LEDs (or any kind of resistors) in a parallel circuit, the less resistance you encounter. There's a great analogy to make this clearer: the toll booth analogy.

Let's think of the LEDs as toll booths on a highway. If you have one toll booth and 100 cars, they'll be backed up quite far. Add four new tollbooths, and now you have 5 toll booths with just 20 cars each - the highway is flowing much faster with cars now. Every new toll booth reduces the resistance - or the waiting time - for cars on the road. Similarly, in a parallel circuit, the current has more paths to go down, and since the current can in our case go through two branches at the same time, the resistance across the LEDs is half as much as it would be across a single LED.

Step Six

Extend This Exercise

Finally, let's extend this exercise. Here are some great ways to explore the concepts of parallel and series circuits in more depth.

Measure Voltage with a Multimeter

Set a multimeter to measure DC volts. (Batteries are DC, while your wall outlets are AC). Measure the voltage across the battery's + terminal and the - side of each of the LEDs. (NOTE: You can touch a multimeter to the metal circles on the top of any module - these go down through the feet. See photos).

You'll notice the voltage drops about 9 volts across each of the LEDs in the parallel circuit, but only a fraction of that on each LED in a series circuit to add up to a total of 9 volts on the - side of the final LED.

Increase and Decrease the LEDs in Each Circuit

Play around with adding more or less LEDs to each circuit. How many LEDs can you add before they refuse to light up at all?

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