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DiscoverCircuits.com -- Hobby Corner
"Hobby Circuits for
your Hobby Projects"
Last Updated on:
06/19/2009 02:18:38 PM
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Shake to Charge
Flashlight -- Page 3
designed by David A. Johnson, P.E. |
| Most white LEDs draw about 20ma
of current with a voltage of about 3.6 volts. As the voltage drops from 3.6v, the
current will also be lower. Without any regulation, the above circuit will not have
a consistent light output. I measured the LED current in this circuit at only a few
milliamps, even after many minutes of vigorous shaking. This suggests that they
decided to sacrifice light intensity for light duration. |
| The human mechanical power to electrical
power conversion efficiency for a shaking device, such as this flashlight, is poor.
To measure how much power I could get from the shaking magnet generator, I first
completed the flashlight dissection process by disconnecting the coil from the
flashlight circuit. I then connected the coil to a Schottky diode bridge, made
from four 1N5817 diodes. These diodes have a much lower 0.35v drop instead of
the 1.0v for the 1N4001 diode. I then placed a high quality 10 Farad super capacitor
from Maxwell (www.maxwell.com) across the output of the bridge. I connected a
digital voltmeter across the cap, to measure the capacitor voltage. |
Maxwell 10F Super Capacitor
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| Before the test, I made sure
the capacitor was completely discharged. I started a stopwatch, and then started
shaking the flashlight’s magnet. After 120 seconds, the capacitor was charged up to
1.0 volts. This corresponds to an energy increase of 5 joules using the equation
0.5CVV, where C is 10 Farads and V is the 1.0 volts across the capacitor. So, I got
5 joules (watt-seconds) of energy in 120 seconds. That means that the magnet shaking
was only able to produce about 0.042 watts of power. This is a dismal amount.
To put this into prospective, a single 1.5” x 1.5 “ solar cell, placed in bright sunlight,
would generate more power than the shaking magnet generator. I bet many of the hand
crank generator flashlights I have seen for sale would do much better. A pull string
type generator would work even better. |
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