Flashlight Has Dismal Light Output

Those portable shake-to-charge devices seem like a good idea, especially for the battery challenged, but how well does this sort of thing really work? Engineer Dave Johnson took this flashlight to the test and found the performance decidedly lacking: “The maker used some very cheap 1N4001 diodes in the bridge rectifier circuit instead of more efficient Schottky diodes. They also used a small 0.5 Farad cap with a 5.5V rating. I noticed that that this kind of super capacitor was originally designed for maintaining data in memory chips and has a rather high internal equivalent series resistance. This reduces the overall efficiency, since the device can’t be charged or discharge very quickly. Some of the power that should go to the LED will end up being dissipated inside the capacitor. Better super capacitors do exist.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 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. View circuit 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. To measure the capacitor voltage, I connected a digital voltmeter across the cap. 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 — and I am no weakling! To put this into perspective, 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. But let’s imagine a different kind of flashlight altogether.

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. 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. 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.



  1. It seems likely that this type of light may be worse for the environment than a standard torch using rechargeable cells. If you work out the impact of the food energy required to charge it (air miles, driving to Tesco etc etc.) compared to burning coal to charge it. A solar charger would be better. I bought my mum a solar / crank handle radio / torch a few years ago. 5 minutes of cranking, although quite hard work will make the radio last about 4 hours, or the torch for about 30 minutes (it has a conventional bulb). Much more efficient, and left in a bright place it hardly needs cranking at all.

  2. I bought a “NightStar” branded torch some time ago when they first appeared in a ‘gadget’ article in the house magazine of the then IEE (Institute of Electrical Engineers), now the IET.
    I mainly just for use it when I walk the dog in the dark and need to see where I am cleaning up. It may not have the light output of my led modified Mini-Maglite but the joy of this torch is that I do not have to worry about battery life… I have just checked it now having used it for a few months now, given that it is ‘Summer’ (tell that to the weather) and it is still charged.
    It is, of course possible, that later copy-cat models do have inferior performance but I remain impressed and, at the time, bought a few of the smaller models as presents and they are still used.. mostly kept in car glove boxes just in case.

  3. Whilst I agree with all that was said in the article, I suspect the New Torch as described would be vastly more expensive than the original one. I bought mine for £1 each. One is in my car, replacing the torch that is always flat when I need it. The other is in the process of being converted into a “how fast can you shake ” game.

  4. I was able to select a similar flashlight from a pile at a retail store, and I picked one that seemed to have pretty good output. I’m happy with mine.
    Part of the idea behind this device is it will work 5+ years from now, after being left in a car or drawer. Most battery lites would likely be dead.
    My unit provides plenty of light vs. the total darkness of a power outage – enough to go find my other flashlights and batteries.
    One nice thing about shaking, it requires little skill – you can had the flashlight to a kid and they can do something useful.
    I think the shaking may be more useful than the pull string, since it is easier to do with one hand. Useful if your arm is injured, or you are doing something else with it, like working a bilge pump, riding a bike, or holding your pajama bottoms up…

  5. I may not be the target audience for this article, but I think I might have understood it better if a _before_ measurement had been made using the original components.
    Pictures of the mods would be welcome, also.
    Also, the idea of a flashlight like this is that you can drop it, hammer nails with it, submerge it, and it will continue to offer a modicum of light. Cranks, ratchets, and pull-strings all fail in time.

  6. Andrew R. Morris

    I have bought (and later given away) a flashlight just like this, with a couple lithium batteries added. When the lithium cells run out, the flashlight’s dead. Maybe the charging circuit was supposed to top off the cells. I also bought an identical flashlight with the lithium cells, and no electronics. It had one layer of wire wound around the coil to make it look functional, and no components on the PC board. The PC board was needed to hold the battery contacts. The packaging, in broken English, clearly told you that a few seconds of shaking will recharge the light.

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