Coloured flashes of light
Behind the seventh door you’ll find another
cable. Now build your circuit and insert
the red LED. Pay attention to the direction
of installation! The red LED appears to
be installed the wrong way around, i.e.
with the cathode towards the positive
pole of the battery. Using the button, you
can alternately charge (contact open) or
discharge (contact closed). When charging,
a green flash of light is generated, when
discharging, a red light appears. You
can repeat the alternating charging and
discharging as often as you like. Each time
you press the button, a red flash will appear;
once you release it, a green flash will appear.
Your battery delivers direct
current. This means that
the current always flows
in the same direction. In
your circuit, however, an
alternating current is created
when activating your push
button. In one direction
the green LED shines, in
the other direction the red
one will shine. Therefore, both LEDs in this
circuit had to be installed with different
directions.
7
The PTC fuse
All your experiments are
secured by a fuse to help
if a mistake occurs. If you
accidentally short-circuit a
cable, it could become glowing
hot, or the battery could heat
up, break, or even explode. But
the fuse will prevent that from
happening.
Many fuses just blow when you
cause a short circuit. You will
then need a new fuse. But your
special fuse is different. This is
a self-resetting fuse, also known
as a PTC fuse. If too much
current flows during a short-
circuit, the PTC fuse becomes
hot and lets only a little current
through because its resistance
rises sharply. That’s where
the name comes from. PTC
stands for “Positive Temperature
Coefficient” and means that
the resistance increases as
the temperature rises. If you
then switch off the power and
eliminate the fault, it cools down
and is as good as new again.
Please do not try it out, as the battery
will quickly become unusable in the
event of a short circuit. Also, the PTC
fuse can reach about 60 degrees
and can easily burn your fingers. But
that would still be better than glowing
wires and exploding batteries. So
always remember: The safety device
is only there for emergencies, similar
to the emergency brake on a train.
Attention!
