[Gllug] MP3/Ogg player hardware?

[David Damerell]

On Thursday, 13 Oct 2005, Andrew Farnsworth wrote:
>Could be, Dynohubs (Link Below) generate 6 volts at 2 watts (or 1/3
>AMP) so you could drive quite a bit with this. 
>http://www.sheldonbrown.com/dynohubs.html

That's very misleading. That page discusses the old Sturmey-Archer
dynamo hub, which is not made any more.

Modern dynamo setups essentially all are designed around the modern
German standard; 6V, 3W, 1/2 A, with this power output being
attainable at circa 6mph. This then powers either a 3W headlamp or a
2.4W headlamp and a 0.6W rear light in parallel.

However dynamos behave quite oddly in terms of their output;
essentially a dynamo likes to put half an amp through whatever is
attached to it. As the bicycle speeds up the voltage across the
terminals rises roughly proportionally to the speed until the current
reaches 1/2 A; then internal saturation means that the voltage and
current only rise quite slowly as speed increases.

[Obviously there's a bend in the curve, not a sharp transition.]

This is used to power twin-headlamp setups, with the two lights in
series and a switch which shorts out the second light. At low speeds
only one light is used; once you get up to about 12mph you switch the
secondary light into circuit and the dynamo blissfully puts out 12V.

[This only works with hub dynamos, not with bottle dynamos - they'd
put out that power if they could, but they can't grip the wheel
tightly enough.]

Obviously this makes attaching devices not expecting to have 1/2 A
pumped through them quite difficult. If you proceeded on the
assumption that the dynamo was just a 6V supply and attached a device
expected to draw 1W, the dynamo would drive it fine at 6mph; at 18mph
it would be shoving 18V through it.

There are some pages with circuitry for attaching other electrical
devices to dynamo setups - Google 'em up.

-- 
David Damerell <damerell at chiark.greenend.org.uk> Kill the tomato!
Today is First Mania, October.
-- 
Gllug mailing list  -  Gllug at gllug.org.uk
http://lists.gllug.org.uk/mailman/listinfo/gllug