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?
this article, we'll look at Christmas lights so you can understand
everything about them!
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
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!
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.
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.
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:
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.
The 1970s saw a revolution in decorative lighting: Mini-lights
were introduced. They now dominate the market when
it comes to strands of lights. A mini-light is a small, 2.5-volt
incandescent bulb that looks like this:
in a typical strand as you buy them in the store
bulbs are not much different from any incandescent flashlight
bulb (see below for details).
that you are plugging these 2.5-volt mini-lights into a 120-volt
outlet, the obvious question is, "How can that work?"
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:
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:
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.
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.)
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:
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.
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.
There are two different techniques that are used to create
blinking lights. One is crude and the other is sophisticated.
crude method involves the installation of a special blinker
bulb at any position in the strand. A typical blinker
bulb is shown here:
extra piece of metal at the top is a bi-metallic strip
. 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
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!
How Stuff Works, Inc http://www.howstuffworks.com/