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Shake to Charge Flashlight -- Page 4
designed by David A. Johnson, P.E.

Improved Flashlight Design
Letís imagine a different kind of flashlight.  I like the idea of using a pull string human motion over crank.  I think this technique would be a much more efficient way to convert human power to electrical power. This device would be a miniature version of the technique often used to start a lawn mower.  The imaginary device would be held in one hand, while the other arm would pull the string.  Each pull of the string would spin a flywheel up to a high RPM, which would be connected to a brushless motor, acting as alternator.  I imagine that such a device could generate several watts of power, perhaps as much as 20 watts.  Such a machine would produce 50 or 100 times more power than the shaking magnet technique.
Letís then connect the output of the three phase alternator to a three phase bridge rectifier.  The output of the bridge would then be connected to a bank of quality super capacitors rated at 2.5 volts.  Perhaps the total capacitance would be about 50 Farads.  I would then use an efficient DC-to-DC converter (Maybe using Texas Instrumentís TPS61070 device), designed to maintain a constant current flow to one or more LEDs.  One LED might be adequate but an array of 7 LEDs would be even better.

Letís say the pull string approach could generate 5 watts of power.  Then, a 50 Farad cap would be charged up to 2.5 volts in less than a minute.  Once charged, a DC to DC converter with an 80% efficiency would keep a single 20ma white LED going for 30 minutes.

Such a flashlight would be much brighter than the above device.  A larger 350 farad capacitor from www.maxwell.com might be used to power a 7 LED flashlight for 30 minutes after 7 minutes of charging.  If a 20 watt device could be developed, it might be powerful enough to charge up a dead automotive lead acid battery after a few minutes of string pulling to start a car.  Such a device might be small enough to fit into a carís glove box and could make a nice Christmas gift.
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