|
|
|
DiscoverCircuits.com
-- Hobby Corner
Last Updated on:
07/10/2008 08:39:45 PM
|
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
|
|
| 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. |
|
<< Previous Page
Next Page >> |
| |
|
|
