Ignoramus16112 said:For my bike (nothing special, a nice simple road bike bought at a
garage sale) I would like to add a bicycle generator and LED lights.
I assume that output voltage of these generators varies with
speed. There are some on sale on ebay for not too much, 6v or 12v
models, most are 6 watts.
On the contrary, almost all bike generators are 6 volts, 3 watts.
(That's half an amp.)
Voltage does vary with speed, but these days most generators are
well-designed to self-regulate to a max of about 6.5 volts. However,
that's assuming that a standard generator headlight bulb (or headlight
& taillight) are fitted.
Generators are pretty much constant current devices. If a generator
sees more resistance, it still tries its best to pump out half an amp.
It does that by producing more voltage. It's not hard to get over 30
volts out of many generators, if the load has high resistance - like,
for example, an open circuit.
Quoted message said:
I want to do it in some nice way, such that
1) the LEDs would work across the range of speeds and be protected
from overvoltages and such
2) Most power would not be lost on some current limiting resistors,
but instead would go into LEDs. I think that 6W spent on LEDs would
give me a lot of light.
If there are products like that on the market already, I do not want
to "reinvent the wheel" and I would rather buy one, but I am not sure
if such a thing is available.
I would prefer LEDs to regular incandescent bulbs for many very
obvious reasons (more light and more reliability).
I am moderately handy and built some electronic things before, like a
control and inverter for a DC welder.
Any ideas?
First, recall that bike generators are AC devices. LEDs are DC.
Red LEDs are much more efficient than incandescent bulbs trying to
shine through red plastic lenses, so you could easily gain efficiency
in an LED generator taillight. But it's so easy to clip on a battery
powered LED blinky that the generator version is probably not worth the
trouble.
I think we're just now at the point in time where white LEDs are
significantly more efficient than halogen bulbs. (And that's not all
white LEDs, but the much more expensive Luxeon creations.) But it
requires some serious work to take advantage of that.
The biggest obstacle, I think, is optics. Generator lamps often have
excellent, excellent optics, so every lumen goes to a useful place.
LEDs put out a fuzzy cone of light, more or less. You want the former.
I once found an article in German where someone had taken a 3 watt
Luxeon and found a way to graft it into a commercial bike headlamp
unit. IIRC, they needed a big aluminum plate for a heat sink, and they
ended up with the LED facing _up_ at the bulb's focus point, slicing
the headlamp nearly in half.
Unfortunately, I don't read German; and I'm away from my bookmarks.
But that's what I recall.
If you can contact Andreas Oehler (sp?) he can give you all the info
you could possibly use. He drops in here occasionally when the talk
turns to generators.
- Frank Krygowski