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Tuesday, June 01, 2021 03:06 PM
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Shake to Charge Flashlight --
Page 4
designed
by David A. Johnson, P.E. |
Improved Flashlight Design
Let’s 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. |
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Let’s 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 Instrument’s 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.
Let’s
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.
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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 car’s
glove box and could make a nice Christmas gift. |
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