sms wrote:
On 4/2/2021 11:59 AM, Ralph Barone wrote:
sms wrote:
On 4/2/2021 9:00 AM, jbeattie wrote:

snip

I don't love it. I would love a dyno light with a solid 800 lumen
output, a little more upward spew and a stand light that was stronger
than the light on my give-away key chain from Wells Fargo -- and one
with a battery so I could use the light off the bike. We
transportational cyclists often need a light for use off the bike. A
flasher would be nice for dusk and dawn, but not required. And while
we're wishing, how about something lighter and more efficient than a
bunch of magnets whirling around. There must be some other way of harvesting electrons.

Such a light would be wonderful but it would be a stretch with a 6V/3W
dynamo, even at higher speeds where you can get more than 3 watts out of
it. Some LED makers are claiming 300 lumens per watt, at least in the
lab, but 200-250 lumens per watt are what is available commercially at
this time.

A 12V/6W hub dynamo (or even a 9V/4.5W hub dynamo) would make dynamo
lights with sufficient intensity more practical, including a beam
pattern where some upward spew would be possible. DRL flash capability
is trivial to add, as are internal batteries to be able to use it off
the bicycle. But there is just not much of a market for any of this.


You seem to have it stuck in your head that the internal impedance of a hub
generator is some immutable quantity and not a design parameter. Why not a
6V/12W hub dynamo? Hell, if you were willing to do frequency dependent
series capacitor switching, you can get a lot more than 3W out of a 3W
labelled hub.

Not stuck at all. The problem with that approach, and it's already been
done via lowering the impedance by putting two bulbs (or LEDs) in series
is that you don't reach sufficient power at lower speeds.

OK. First off, putting two LEDs in series raises the impedance, and it
works by allowing the voltage from the generator to go up. If you had a bit
of intelligent electronics (a frequency sensor with some hysteresis), you
could short out the second LED at low speeds where there isn’t enough
voltage to drive two in series.

One thing that some people have done (though it may be illegal in some
countries) is to use a dyno hub designed for a larger wheel on a smaller
wheel. A 20" wheel is around 1010 revolutions per mile, a 28" wheel is
about 720 revolutions per mile. This would get you more output at lower
speeds, but the current would still be 500mA, just at a higher voltage.
And of course it assumes that you want to ride a bicycle with small
wheels! And of course the drag would be greater than a dyno properly
sized to the wheel.

Practically speaking, the advantage of higher output dynamo would be
used with a bridge rectifier and a DC-DC buck converter to charge a
high-power battery powered headlight, or to operate it continuously at
lower than maximum power.

You’re almost there. The other advantage of a DC-DC converter (don’t get
stuck on buck - it could be that a boost or buck-boost or Cuk is the way to
go) is that it acts as a DC transformer allowing the system (through a
change in duty cycle) to change the effective impedance of the LED to a
value that better matches the source impedance of the dynamo hub at that
particular speed, thereby maximizing power output at all speeds.