Re: Rechargeable v. throwaway batteries

On Oct 22, 3:33 pm, thirty-six <thirty-...@xxxxxxxxxx> wrote:
On 22 Oct, 18:13, Frank Krygowski <frkry...@xxxxxxxxx> wrote:

Now it's interesting that LEDs need constant current, the exact thing
that generators deliver.  And they put much less resistive load on the
generator, so voltage and power output are less.  I've recently put an
LED headlight on a bike with a cheap Union bottle generator, and it
seems to have significantly less drag, with more light.  And this is
not a high end lamp at all.

That's the sort of input I like to see.   Is there any risk of
overdriving the LEDs?

Well, one of my new acquaintances is vice-president of research for a
company that specializes in LED products. He's done tons of
experiments with high powered LEDs, so I asked him. He assures me
everything should be fine.

As an example of what I'm talking about: I previously tested the
output current of the Soubitez generators I use. Here are the results
at a road speed of 26 mph, for various resistive loads:
6 Ohms, 0.63 Amp ; 12 Ohm, 0.58A; 18 Ohms, 0.54A; 24 Ohms, 0.50 Amp.
And at just 6 mph, this one put out over half an amp into the 6 ohm
load.

So you see, load resistance has an effect, but not nearly the
approximately inverse-proportionality effect it has with a battery.
With zero resistance, even this relatively high output dynamo won't
put out much more than 0.63 Amp.

Diodes usually require something less than a volt to conduct in their
forward direction, but then they display very little resistance. A
battery with no resistance in series can dump very excessive current
into them and pop them, so batteries need current limiters of one type
or another to drive LEDs. (The exception is a coin cell, which
apparently comes with enough internal resistance to let the LED
live.) Bike generators give LEDs exactly what they want.

If the LED handles that 0.63 Amp (which most high powered ones do)
there should be no problem. I hope! We'll see how it does long term.

The headlamp I installed is this one:
http://www.velo-orange.com/spluledhewst.html
and although I took it apart far enough to re-design the mount for my
purposes, I resisted the temptation to dig deep enough to see its
circuitry, but the internal wiring made it clear that it was
originally designed for either generators or batteries.

- Frank Krygowski
.

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