Christmas lights are a big part of the holiday season. As November and December roll around, you 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?

Now, we'll look at Christmas lights so you can understand everything about them!

Christmas Past
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.

 Standard mini-light bulb

 Mini-lights in a typical strand as you buy them in the store

These bulbs are not much different from any incandescent flashlight bulb (see below for details).

Given that you are plugging these 2.5-volt mini-lights into a 120-volt outlet, the obvious question is, "How can that work?"

The key to using these small, low-voltage bulbs with normal house current is to connect them in series. If you multiply 2.5 volts by 48, you get 120 volts, and originally, that's how many bulbs the strands had. A typical strand today adds two more bulbs so that there are 50 lights in the strand -- a nice round number. Adding the two extras dims the set imperceptibly, so it doesn't matter. The lights in a 50-bulb strand are wired like this:

You can now see why mini-light strands are so sensitive to the removal of one bulb. It breaks the circuit, so none of the bulbs can light! When mini-lights were first introduced, any bulb burning out would darken the entire strand. Today, the bulbs can burn out and the strand will stay lit, but if you pop one of the bulbs out of its socket, the whole strand will go dark. This difference in behavior occurs because the new bulbs contain an internal shunt, as shown here:

 Today's standard mini-light bulbs contain a shunt wire below the filament. If the filament burns out, the shunt activates and keeps current running through the bulb so that the rest of the strand stays lit.

 Stay-lit Strands It is not uncommon to find strands of lights today that claim they will not fail even if bulbs are removed. These strands simply place a second shunt inside the socket. If you pop a bulb out of one of these strands, the strand will stay lit and you will feel the socket heat up because of the current flowing through the shunt.

If you look closely at a bulb, you can see the shunt wire wrapped around the two posts inside the bulb. The shunt wire contains a coating that gives it fairly high resistance until the filament fails. At that point, heat caused by current flowing through the shunt burns off the coating and reduces the shunt's resistance. (A typical bulb has a resistance of 7 to 8 ohms through the filament and 2 to 3 ohms through the shunt once the coating burns off.)

Although you can buy simple 50-bulb strands like the one shown above, it is more common to see 100- or 150-bulb strands. These strands are simply two or three 50-bulb stands in parallel, like this:

If you remove one of the bulbs, its 50-bulb strand will go out, but the remaining strands will be unaffected. If you look at a strand wired like this, you will see that there is a third wire running along the strand, either from the plug or from the first bulb. This wire provides the parallel connection down the line.

The big advantages of mini-bulb strands are the low wattage (about 25 watts per 50-bulb strand) and the low cost (the bulbs, sockets and wire are all much less expensive than a 120-volt parallel system). The big disadvantage is the problem of loose bulbs. Unless there is a shunt in the socket, a loose bulb will cause the whole 50-bulb strand to fail. It's not hard to have a loose bulb because the sockets are pretty flimsy. There are testers on the market now that can help find loose bulbs faster, and they only cost \$3 to \$4. You point the tester at each bulb and it tells you which bulb is loose (see How do proximity-type Christmas-light testers work? for details). But it is still a pain to test each bulb. C'est la vie...

Mini-light Flashlights
If you hook a mini-light bulb up to a normal AA battery, the bulb will light just like a flashlight bulb. It will be dim, however, because the bulb expects 2.5 volts rather than the 1.5 volts the battery is generating. You can put two batteries together to create 3 volts, or you can hook the bulb up to a 9-volt battery as shown below:

Because you are driving the bulb at a significantly higher voltage than it expects, it will burn extremely brightly and will not last very long (perhaps 30 minutes or an hour).

The crude method involves the installation of a special blinker bulb at any position in the strand. A typical blinker bulb is shown here:

The extra piece of metal at the top is a bi-metallic strip (see How Thermometers Work for details on bi-metallic strips). The current runs from the strip to the post to light the filament. When the filament gets hot, it causes the strip to bend, breaking the current and extinguishing the bulb. As the strip cools, it bends back, reconnects the post and re-lights the filament so the cycle repeats. Whenever this blinker bulb is not lit, the rest of the strand is not getting power, so the entire strand blinks in unison. Obviously, these bulbs don't have a shunt (if they did, the rest of the strand would not blink), so when the blinker bulb burns out, the rest of the strand will not light until the blinker bulb is replaced.

The more sophisticated light sets now come with 16-function controllers that can run the lights in all sorts of interesting patterns. In these systems, you typically find a controller box that is driving four separate strands of mini-bulbs. The four strands are interleaved rather than being one-after-the-other. If you ever take one of the controller boxes apart, you will find it is very simple. It contains an integrated circuit and four transistors or triacs -- one to drive each strand. The integrated circuit simply turns on a triac to light one of the four strands. By sequencing the triacs appropriately, you can create all kinds of effects! Patent 4,215,277 is a good one to read if you want to learn more about sequencers.

For more information on Christmas lights and all sorts of related topics, check out the links on the next page!

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