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How to Upgrade Your LED Dynamo Flashlight

There is this big buzz going on about this thing called ECO, and a lot of manufacturers got into this wagon only to profit and deceive. Some say this is all that big corporations do! But let's not go there for now…

Where we are

If You want to be eco, You cannot miss LEDs. They are (roughly) 25 times better than incandescent lights, and 5 times better than CFLs1). Also, they save You money (power, batteries, etc) and last longer. The pinnacle of eco-friendly LED flashlights is the Dynamo Flashlight - the one that gives You light if You give her muscle power. There are many types and models, see figure One, and I am going to talk about the last model, the green translucent one.

Before talking about it, please take a look at a lantern's box (figure Two): figure_two_led_flashlight_funny_box.jpg

If You're not laughing, You have no sense of humor… Although I didn't laugh when I bought it and read it at home!2) So, You've bought a LED Dynamo Flashlight, and You wonder if You can improve it.

What's wrong

There are many wrong aspects and we cannot improve all of them. Lets list a few for the start:

  • Huge number of pushes for charging the battery
  • Short lived (and also weak) accumulator.
  • Cheap - i.e. not for long life - execution and mechanisms

Let me prove my words: The battery is 3×1.2V NiMh and 40mAh in capacity, as buttton models are. This battery, fully loaded, could light for less than two hours, and by simply changing it with a better one (LiPo CR model), 3.6V and 210mAh, we get over 7 hours, a time that might help us pass the night! Also, we get almost three times the electrical power into LEDs, and that translates in more light. See the data below: Original Battery NiMh 3.6V/40mAh versus new one LiPo 3.6V/210mAh

Original NiMh New LiPo
Battery voltage U0 [V]4.343.83
LED working voltage ULED [V]2.812.94
Light relative intensity1.221.84
LED working current ILED [mA]12.0028.00
Usable Power Pu [mW]33.7282.32
Efficiency (yield)0.650.77
Battery capacity C [mAh]40.00210.00
Discharge Current Id (%C)0.300.13
Full discharge time t [h]2.507.14

Another factor is generator efficiency: one coil four poles permanent magnet generator (see figure Three). In fact, here I might agree it was the best affordable solution, but at least, they could rectify both halfs of the alternative current! Yes You read it right, they use only one diode, half-wave rectifier!

What we need

(as tools and parts) If You want to be able to do this work by Yourself, You need some electronic parts (diodes, electrolytic capacitors), Weller (soldering iron), wires, a multimeter and some more room inside the flashlight for the extra wires. We can’t get over the modifying (the tuning) of the original LED flashlight without having this conversation about rectifiers. I will try to keep it short, but it is a “good” topic3), and for a complete understanding of the phenomena, please go to wikipedia or, better than that, professional courses.

Single-wave rectifier:

This is the initial rectifier, which uses only half of the energy available in the generator (dynamo), not to mention asymmetries, bias and other “benefits”. If we would use this model in real life, we'd half of our fridge, half of the gasoline we pour in reservoirs, and so on. There are a few useful applications (of this rectifier), like the first radios, but not in our power source.

Double-wave rectifier:

This seems to be the ideal choice as it uses both waves and 100% of the available energy (minus the collateral losses – wires, diodes…) Unfortunately for us, the bridge rectifier, under practical circumstances, causes a loss of 2×0.7V = 1.4V, which is a bit too much. In real life applications, this is the main rectifier design (computer PSUs, wall adapters, etc.), and at 110 or 220 volts, the one and a half is acceptable.

For the center-tapped design, we didn’t plan for such a major intervention (riveted coil, unwinding, rewinding with center-tap, …) We probably had room for the necessary extra amount of copper and YES, we will give it a try sometime later… This is the usual configuration of a computer PSU, in the low tension (high current) side.

Voltage Doubler rectifier:

They were used in the vacuum tubes era, for high voltages, because of the lower cost (lower than copper/coil) and the small currents involved. Some of my Electronics colleagues were staring at those configurations (a sign for me that I'm an old-timer…)

We chose this option (it uses both waves, with one diode on branch and doubles the tension) for several reasons:

  • Lower voltage drop, outweighed by doubling;
  • Better charging at lower revolutions;
  • A relatively simple and fast intervention.

Our thanks to HyperPhysics website of Georgia State University for the rectifier images used above.

Before starting the work, we did another test (and my 12 years old son helped me): we measured the energy “pumped” by hand - by his and by mine. Mentioning only a few of the formulas involved, E=(C*U*U)/2, E=P*t, E=U*I*t and so on, we arrived at those results:

  • Child hand: 11k hand pumps for NiMh, and over 58k for LiPo
  • Adult hand: 8400 hand pumps for NiMh, and over 44k for LiPo

Those numbers show that our modifications, along with a future one (power connector for recharging) should give the best usage for the flashlight: Charged externally (wall socket, solar, bike, …) and using hand pump only in emergency. The actual modification should unfold smoothly: soldering and gluing components, assembling and – because we could no longer trust screws – loads and loads of transparent adhesive tape (scotch-tape).

The end result

As this was an already tested change – that we were already familiar with – the results were the ones expected: lower revs charging (You don't need to “hand-pump” like crazy), greater mechanical load (increased field) and lower spin-wheel inertia (unobtrusive), faster charging with a lower pace. Our work gave us a completely functional flashlight and a bonus: the adhesive tape behaves better at the impact, making it less destructive; maybe a thicker layer would do even better… This topic is far from being covered, there is more to experiment and build, at least recomputing and rewinding the generator, better batteries (Li-Fe-Poli or Li-Ion), LED driver circuit, so on. It’s quite a large area and we’re so much into it…

How To | Electronics | Eco | LED Lights

1) Compact Fluorescent Lamp: standard eco-friendly lights, i.e. Neon Lights
2) In the articles to come we will get into more details regarding the fake flashlights, with a dynamo but also with a non-rechargeable battery.
3) We remind our advanced readers that we need to keep our explanations to a certain level. For a proper explanation, we recommend (our) bibliography: Moseanu, Rudenko, Rice, Wells and so on.

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