[Peripherals] Charging the XO off a bicycle dynamo

[Richard A. Smith]

Arjun Sarwal wrote:
> Also, we were told that dynamos don't generate any output, if they are
> run below the rated rpm. Alternators however do generate an output
> below rated rpm and the output(number of amps) is proportional to the
> rpm...

Strictly speaking I don't think this is correct.  Unless the device in 
question has specific circuitry designed to prevent output @ < rated 
rpm.  Like some sort of DC/DC converter or linear voltage regulator perhpas?

If you pass a coil of wire through a magnetic field you get a current. 
It might be small but its not optional.

I think what they were trying to describe is that dynamos or permanent 
magnet generators are designed around a much smaller target rpm range. 
When you operate them outside that range they are not very efficient.
This has to do with the strength of the field.  In a permanent magnet 
setup you can't adjust this strength.

In an automobile setup where you have lots of rpms they are designed to 
operate at a high rpm.  The reasons alternators can achieve so much 
higher efficiency is because they have control of the field strength. 
This also allows them to operate across a wide rpm range.  But even then 
there's still a design range. If you look at the output of most 
automobile alternators you will see that they don't produce rated output 
until higher rpms.

-- 
Richard Smith  <richard at laptop.org>
One Laptop Per Child