On 2017-03-13 11:50, sms wrote:
On 3/13/2017 11:06 AM, Joerg wrote:

snip

That's how some cheap Chinese chargers work.

And a lot of non-Chinese chargers as well.

Not good as that is no
lomnger single-fault safe. It is as if you'd control a pump via tripping
a GFCI when a certain fill level has been reached.

Technically there are two safety circuits. First, with an 8.4V output
charger you're limiting the charging voltage to 4.2V per cell.


That doesn't really work because then the charge current would all but
vanish towards the end, a charge would take forever. You've got to
monitor cell voltage twice, once from the charger side and then in the
cell protector circuitry.


... Second,
the protection circuit on each battery, or on the battery pack, limits
the voltage to 4.2V per cell just in case someone plugs in a higher
voltage charger.


The usual bicycle packs have only one such supervisor ciruit and then
the bare cells.


I think the simplest dynamo to Li-Ion 2S pack circuit would consist of
only three parts. A 2A Schottky bridge. A filter cap. An 8.2V, 5W zener
diode. I don't think you even need a current limiting resistor because
the dynamo isn't going to put out much more than 500mA.


Zeners have huge tolerances. I wouldn't do that. If you must use a shunt
limiter at least employ a TL431 plus a big transistor or FET. Those are
way more accurate and just as cheap.


I don't think you gain much with a buck switcher unless you're often
putting out dynamo voltage well in excess of 6VAC, or unless you want to
do 5.25V for USB charging. You're going to lose some power with the
switcher which is likely to be only 85%-90% efficient.


You'd gain a lot. An old trick for well-trained commuter cyclists was to
hang two lights in series for 12V which resulted in a lot more power and
thus light. For slower pace one lamp could be shorted out.


You could stick in a Murata LXDC55FAAA-203
http://power.murata.com/data/power/LXDC55FAAA-203_data_sheet_E.pdf
instead of the zener and do a USB version also with only three parts.
But I know that you love designing your own switchers.


$3.50? Mon Dieu! :-)

My main gripe would be the 14V limit. I'd like this to be able to go a
tad higher for long downhill stretches.

I don't enjoy designing switchers all that much anymore after having
done 50-100 professionally. Gets boring.


I'm in shock when I see the prices of some of these dynamo to USB
adapters, since the parts cost is only a few bucks.

http://www.sinewavecycles.com/products/sinewave-revolution


I often have the impression that cyclists get taken advantage of.
However, in this case the produced quantities will likely be so low that
they have to charge a lot. It's not going to be a roaring market success.


Here's someone that did it DIY but using a linear regulator.
http://www.14degrees.org/diy-bicycle-dynamo-usb-charger-for-smartphones-and-battery-packs/


One of my bicycle rear lights (homemade) contains two MC34063. Not
terribly efficient but it can safely be used to a little over 30V and
costs 50 cents even if you only buy one. Can be pilfered from scrapped
electronics. Diodes Inc makes one on steroids that goes ... hold on to
your hat ... to 180kHz :-)


At lower output levels, a dynamo could even run a 2 cell battery powered
light if the batteries were removed.


Then you'd be back to this inferior dynamo lighting which goes out at
the traffic light, or uses dimmed or short-lived light if there is a
supercap installed.

No, I'm more thinking of a way to have a good light but with dynamo
back-up should the batteries go flat. As well as charging the batteries
during daytime riding when all you're using is the DRL flash mode.


That would be my goal. I have good lighting on both bikes but on the
road bike it would be nice to have a dynamo in the game. Like a hybrid
car which combines gas and electric. Over the course of a long ride 50%
of the juice would come from the dynamo and 50% from the battery,
allowing to use half the number of cells. Should I ever ride more than
5-6h I could dim the front light and milk the dynamo for 80-90% of the
budget. Or use more MUP where I can turn the lights off and then the
dynamo would deliver a net average of 100%, only the road sections where
I need full bore and have to wait at signals would need to be covered by
the battery.


Unfortunately, since dynamo lights are so rare in the U.S. we end up
with sub-standard lighting with poor beam patterns, low intensity, no
flash mode, and inferior brightness when stopped or riding slowly.

I thought it was interesting that Barry Beams mentioned that his light
could work, at less than full intensity, off of dynamo output. He may
have a compelling product for dynamo users that want a brighter, safer,
light but that still want the assurance of not being caught out with
flat batteries. Even Frank realized the need for better lights and
bought an Oculus. And while the Oculus may not be a bargain battery
powered light (even though the price is still pretty reasonable), it's a
lot less than dynamo lights that are not as good.


I'd keep my lights as-is, MagicShine clones with diffusor lenses. The
dynamo would just be added into the mix, doing the same as the
alternator on a car except at standstill.

--
Regards, Joerg

http://www.analogconsultants.com/