Re: That Lowe's flashlight, again

SMS wrote:
pm wrote:


Has anyone tried running the head of the Lowe's light off a dynamo?

I know I would need a bridge rectifier at least, but I'm wondering
whether it reaches full brightness, or would run too hot at speed.

A bridge rectifier and a switching regulator to lower the voltage to 3 volts or so, and keep it constant. If the output of the dynamo is 6 volts, the rectified voltage is 6 * 1.414.

I think that's the reason there are so few LED dynamo headlights. The cost and inefficiency of the rectification and the regulation makes it unattractive. Also, the LEDs use a lot of current and generate a lot of heat and require steel or aluminum housings to act as a heat sink. Also, 3 watts isn't enough for the high-power Cree LEDs. A filament bulb is perfect for dynamos as it runs off AC just fine with no waste in doing rectification.

LEDs are actually a nearly perfect match for generators. Both incandescents and LEDs are non-linear, but incandescent non-linearity goes the wrong way -- "delta" resistance increases with power -- (which is why they have to be voltage clamped to prevent bulb burn out). Incandescents have a very narrow power range, becoming extremely inefficient at less than full power, and very short lived at over power -- plus, they don't age well and don't like vibration, especially as they age.

LEDs are actually more efficient at less than full power. They age, but more gracefully, and are insensitive to vibration. Being more or less constant voltage devices, LEDs need to be current limited, but with a generator that is accomplished with the winding impedance -- the inductive component limits the current, as does the rising frequency (with increased shaft speed). All this makes them the perfect generator driven bike light.

As has been discussed before, a high-power LED with a low-drop (Schottky) bridge would have an electrical efficiency of ~80%. A 2 LED stack, ~90%. LED efficiency is much better than even best-case incandescent efficiency to begin with , so there's no comparison really at less than best-case, even with some rectification losses. Unlike an incandescent, which requires voltage clamping, LEDs, if properly matched to the generator, require no regulation. They do require heat sinking, but that is generally integrated with the housing, and not a significant cost factor.

The reason that LEDs haven't completely displaced incandescents in bike lights yet is that they're still (relative to bulbs) expensive -- the bike headlamp market does have rather obscene markups to support. They are displacing incandescents in the battery-powered market -- an application which is also cost and efficiency sensitive, and has a more challenging regulation (electrical) problem. The bike industry is slow to change, and volumes are low, which is why you're touting using a $30 flashlight as a bike light. That same product would cost $60 in a bike shop -- $90 if it came from Germany. LED bike lights are on the Moore's law curve, so it's just a matter of time before they're cheaper than incandescents, but they will have eliminated them long before that.



http://www.ebbc.org/?q=node/1286

"LED vs halogen headlamps
Submitted by Robert Raburn on January 3, 2008 - 2:20am.

Dynamo lighting has reached a new milestone. A pair of Lumotec Fly IQ Senso
Plus* headlamps arrived today and I mounted one on Pat's bike equipped with
a Schmidt hub (6V 3W). We took a short ride this evening to compare her new
LED with the halogen on my bike (a Lumotec Oval also powered by a Schmidt).

Holy smokes! The first thing we noticed was the LED fired up as she walked
to the front gate. On the road the LED was considerably brighter than the
halogen--perhaps the 300% claimed by Lumotec. Plus the pattern was useful.
After stopping, the standlight was also impressive by maintaining full
brightness for a few minutes--sufficient to light our front door.

Downsides include high cost (about $100) and lack of availability in the US
(Peter White is the only distributor with stock so far)."

.

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