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Shake to Charge Flashlight --
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
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 received 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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