How Christmas Lights Work

By: Marshall Brain

This homeowner shows off his Christmas decorations. Rob Lewine/Getty Images

Christmas lights are a big part of the holiday season. As November and December roll around, you might see strands of lights everywhere -- on Christmas trees, houses, shrubs, bushes and even the occasional car! Have you ever wondered how these lights work? Why is it that if you pull out or break one of the bulbs, the whole strand of lights goes out? And how do they create the lights that sequence in different color patterns?

­If you were to go back in time 30 or 40 years and look at how people decorated their houses and trees with lights, you would find that most people used small 120-volt incandescent bulbs. Each bulb was a 5- or 10-watt bulb like the bulb you find in a night light. You can still find strands of these bulbs today, but they aren't very common anymore for three reasons:


  • They consume a lot of power. If you have a strand of 50 5-watt bulbs, the strand consumes 250 watts! Consider that most people need two or three strands to do a tree and five or 10 strands to do a house and you are talking about a lot of power!
  • Because the bulbs consume so much power, they generate a lot of heat. When used indoors, three strands at 250 watts per strand are generating as much heat as a 750-watt space heater! The heat from the individual bulbs can also melt things.
  • They are expensive. You can buy a 10-pack of miniature bulbs for about a dollar this year. The large bulbs might cost five to 10 times more.

The one advantage of this arrangement is that a bulb failure has absolutely no impact on the rest of the bulbs. That's because a 120-volt bulb system places the bulbs in parallel, like this:

You can have two, 20 or 200 bulbs in a strand that is wired in parallel. The only limit is the amount of current that the two wires can carry.

In this article, we'll look at Christmas lights so you can understand everything about them, starting with energy-saving Christmas mini-lights.