# Light Bulb Circuits Lab Summary by liwenting

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Light Bulb Circuits Lab Summary

Battery orientation
does NOT matter!
Step 2: Since the light
bulb filament is
connected to the bottom
and the side of the bulb,
the 2 screws of the bulb
receptacle must make
contact with the METAL
SIDE (shell) and
BOTTOM (button) of the
bulb.
Step 6: Orientation of Batteries

Lights
Go
Out!!

Analysis Question # 3: What’s the advantage
of placing batteries in parallel? Longer Battery Life
Why are batteries placed into the back
of a device such as a calculator like…..

A typical battery holder

Are these batteries connected in
series or in parallel? SERIES!!
Step 7: The Different Wattage Bulb…
In Series: The different wattage bulb
did NOT light up, but the others did.

In Parallel: All bulbs lit brightly, but the
different wattage bulb was a bit
BRIGHTER.
Why?? More on that later…
Analysis Questions:
1. Parallel Circuits: Each bulb has a
DIRECT connection to the battery and is
therefore INDEPENDENT of the other
change the brightness and removing one
bulb does NOT affect the others.
Series Circuits: There is only ONE path
for the current, so each bulb is
DEPENDENT on the other bulbs. Adding
additional bulbs causes ALL the bulbs to
dim because they must share the battery’s
power. Also, removing one bulb causes
ALL the bulbs to go out.
Christmas Tree Lights:
A little History...
The original lights (over 40
parallel
________. Each bulb was a 5
to 10 Watt bulb (similar to what
you find in a “night light”) that
required 120 V. They aren’t
used anymore because they
consume LOTS of power and
generate LOTS of heat
(dangerous and expensive!)
Mini-Lights: Introduced in the 1970’s
Only 2.5 Volts were required to light
the bulbs! How can that work? They
were wired in ______, so the 120 V
series
was “shared” among the bulbs, BUT
when one bulb burns
out, they all go out!! Grrr…
Today’s standard mini-lights are still
wired in series, but when a bulb burns
out, the OTHERS STAY LIT!! How??
shunt
Demo with
Colored
Strand of
filament   MiniLights

Each bulb contains a shunt wire wired in
_______ with the filament. If the filament
parallel
burns out, the current will still run through
the shunt so the rest of the strand can stay
lit. What happens if a bulb is removed?
The shunt wire contains a coating that
gives it fairly high resistance until the
filament fails. This keeps most of the
current from going through the shunt
wire initially. When the filament fails,
heat caused by all the current that is
now flowing through the shunt burns off
the coating and reduces the shunt's
resistance. If the resistance didn’t drop,
the other lights would dim.
A closer look at the wires of a strand…

What is the purpose of the 3rd wire?
There’s typically a plug at the end of a
strand, so it’s used when plugging in another
strand. Then the 2nd strand will have its own
circuit, and the lights won’t dim.
WARNING: Don’t plug in too many strands
“back-to-back”.
Some strands have multiple circuits already
“built-in”. Every other (or in some cases,
every 3rd or 4th) bulb is on the same circuit…
An example of 2 circuits (every other bulb
on same circuit):

The # of wires is always one
more than the number of circuits.
Demo: Four circuit light strand
“There are two different techniques that are used to
create blinking lights. One is crude and the other is
sophisticated. The crude method involves the installation
of a special blinker bulb at any position in the strand.
A typical blinker bulb is shown at right….

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