From: John Popelish
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Subject: Re: Drive white leds from bicycle generator
Date: Wed, 20 Nov 2002 21:30:30 GMT
NNTP-Posting-Date: Wed, 20 Nov 2002 16:30:30 EST
> How can I drive 20 white leds from a 6V 3W (AC) bicycle generator
> (dynamo)? Output voltage of the dynamo varies with cycling speed. How
> can the light output be at maximum at a low cycling speed, and remain
> constant with increasing speed (=3D increasing voltage)?
> Putting as much leds in series as possible is a bit of a problem, due
> to the output voltage of the dynamo.
> The circuit has to be simple, since there is not much space in a
> bicycle headlight.
> Any ideas?
It would be good to include a small rechargeable battery in the
circuit, since, below some speed, the light cannot remain usable
without some storage. I think I might g with a 7 cell nicad battery
that looks like a 9 volt rectangular battery (about 8.4 volts
nominal). I would run the LEDs in pairs, anti parallel, 10 pairs in
series, so that the string could be driven with about 30 volts AC, 30
to 40 ma, so that each diode got about 3 volts forward 20 ma, on
average. A small push pull current mode switching regulator and a
small inductive filter would allow high efficiency conversion of 8.4
volts DC into +- 30 volts AC, while regulating the current. I would
have to do a bit of research to pick the best control chip for this
job, but I think there are some that would supply the power regulation
with no external switches.
Then you need a battery charge regulator to keep the battery from over
charging and to minimize the current load on the generator to minimize
drag. I would go with a two schottky diode voltage doubler rectifier
to produce about 15 volts max., and buck regulate that to hold the
battery voltage constant. Again, at this voltage and current level, I
think there are control chips that would need no external parts,
except a schottky diode, and maybe even that would not be needed.
This sort of sophistication would be a good introduction to switching
power supplies, without the danger of being electrocuted.