Manual
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46. Current Limiting Fuse
Build the circuit, hold the press switch (61), you will hear the alarm
(78) sound, while the bi-directional LED (71) turns on red. Release
the press switch (61), then install the bi-directional LED (71) in
the reverse direction. Hold the press switch (61), you will see the
bi-directional LED (71) will turn on blue and the alarm will sound
very faintly. If you release the press switch (61), the alarm (78) and
the bi-directional LED (71) will turn off. In this circuit the LED (71)
is acting like a current limiting fuse by limiting the current to the
alarm (78). Replace the LED (71) with a 4-wire (4) and the alarm
(78) will now be much louder. Current limiting fuses are used to
provide over-current protection in electronic distribution systems.
47. The Resettable Fuse
Build the circuit, move the magnet (7) towards the reed switch
(83), the alarm (78) will sound while the bi-directional LED (71)
turns on red. Move the magnet (7), then install the bi-directional
LED (71) in the reverse direction. Now if you move the magnet (7)
towards the reed switch (83), the bi-directional LED (71) will turn
on blue and the alarm will sound very faintly. If you move the magnet
(7) away, you will see the alarm (78) and bi-directional LED (71) turn
off. Positive-Temperature-Coefcient, or PTC, thermistors – also
known as resettable fuses are devices that have very low resistance
until a current is reached, then they get warm and the resistance
changes limit the current. Pretend the magnet (7) is a piece of ice
and the reed switch acts like a PTC fuse. Putting the magnet (7)
near the reed switch cools down the PTC fuse allowing the LED (71)
to light and alarm (78) to sound. Taking away the magnet (7) makes
the PTC fuse (the reed switch) heat up, which shuts off the circuit.
Red Light
Blue Light
Red Light
Blue Light