Skip to main content
All of the Lights
Email share

Christmas lights are the best, y’all. They twinkle and shine, making us all a little merrier this time of year. That is, until…. one of them burns out and the whole string of lights stops working. Why does this have to happen?!?


Usually, when a bulb on Christmas lights goes out the entire strand stops glowing…  at least, that’s how it worked with old school Christmas lights. Nowadays a lot of Christmas lights don’t do that (making dads across the country take a huge sigh of relief!).  What’s the difference? Well, it has to do with what type of circuit they use in the string of lights.

Series and parallel

A series circuit according to  is when “the current through each of the components is the same, and the voltage across the components is the sum of the voltages across each component”

How about in English, right? Basically in Christmas lights that are wired in series the electricity travels from the power source (battery or wall socket) through the wire to light one bulb and onto the next, and the next, and the next…, all the way back to the power source.

A parallel circuit is when “the voltage across each components is the same, and the total current is sum of the currents through each component” (

In this case, each bulb isn’t connected one after another - they are connected separately and each light has its own circuit to the power source.

Image - ChristmasLights-02_2.png

Shout out to for the awesome illustrations to understanding series and parallel in lights.

So, those old school lights that would all go dead if one light burned out (and made your dad say dirty words) were wired in series.

That would mean that newer strings of lights are wired in parallel, right?  Turns out, today’s Christmas lights are actually a combination of several series’ of lights put together in parallel. How’s that for high-tech holiday spirit?!


Image - ChristmasLights-03_0.png


Not only that, newer strings of lights have another handy feature that allows current to keep flowing through the series even if a light goes out! It’s like a little detour for the electricity to follow if the main highway through the lightbulb gets cut off. Genius.

This means you don’t have to struggle with going through an entire strand of lights to determine which one is burnt out. The lights will continue to shine bright even if one is out! You might have also noticed that Christmas lights these days are usually LEDs instead of incandescent (the OG of lightbulbs).

That’s because LED lights are durable, have better longevity, lower operating cost and are extremely energy efficient. In fact, LEDs use about 90% less energy than incandescent bulbs.  Incandescent lights function by creating heat which causes the bulb to be hot to the touch and can lead to a spark. That makes LEDs way safer, as well.

Let’s have a hand clap for these well engineered bits of Christmas cheer!

Image - giphy (1).gif