[Olpc-open] Foot power revisited...

[Steve Holton]

On Wed, Jan 7, 2009 at 8:57 AM, Mike Dawson <mikeofmanchester at gmail.com>wrote:

> Hi Steve,
>
> Thanks for the quick reply - sure enough  it comes back with 3V or so
> now...  Dynamo claims to be 12V.  Now it is though related to the rpms
> that we are doing - if I changed the gearing / speed then that should
> sort it out...


If it's similar to ones I've seen, the voltage out will be related to spin
speed. You'll need 12-14 volts to power an XO, and should avoid going above
18v.  For a prototype that shouldn't be an issue.

You'll also need about 15-20 watts to recharge an XO in about the same
timeframe you're expecting from an AC wall  source.
That means you'll need power recitfiers rated for at least 20 watts, or
about 2 amps at these voltages.

Good info, as always, at Wikipedia:
-  http://en.wikipedia.org/wiki/Bridge_rectifier


> I don't know why I thought the dynamo was DC...  Too long since doing
> physics properly I guess...
>
> Would you be able to let me know about what the suitable criteria
> might be for a bridge rectifier?  If you have any web link to any
> online catalogue that would be great so I can see if we can find that
> locally...  Or is it anyway possible to modify the rectifier from a
> normal AC brick adaptor to do the job?


If you can find a disfunctional AC brick which supplies 12 or so volts at
1-2 amps it will likely include either a monolithic bridge rectifier package
(a 4 pin block with a heat sink) or a bridge built from four discrete power
diodes.  Generally speaking, any device which runs on AC and has both
electronics and moving parts will need a bridge rectifier similar to what
you're looking for. If I were scavenging, I'd look at broken printers, FAX
machines, VCR's or audio equipment with high power output.

I'm sure some place like DigiKey would have a 'technically correct' part.
As a general rule look for something rated for roughly 12 or more volts, 1
to 2 amps, or around 20 watts.



> 2009/1/7 Steve Holton <sph0lt0n at gmail.com>:
> > On Wed, Jan 7, 2009 at 7:36 AM, Mike Dawson <mikeofmanchester at gmail.com>
> > wrote:
> >>
> >> Dear All,
> >>
> >> We tried to create a sewing machine based trial to power an XO - we
> >> geared it at about 90:1 so that the two dynamos should spin at about
> >> the same speeed as they would on a bike.
> >>
> >> When connecting the bike light it does light - a little dimmer than
> >> normal - but the digital voltmeter for some reason when just starting
> >> slowly to spin the wheel by hand registers 0.1V or so when it's going
> >> smoothly at full speed it registers absolutely nothing.  I tested
> >> measuring a few batteries and it seemed fine.
> >
> > Most "bicycle dynamos' will produce alternating current. The XO requires
> > direct current for charging. A light bulb can operate off either.
> >
> > I suggest you re-test the bike dynamo with the AC setting of your volt
> > meter.
> >
> > If this is the case, you can use a bridge rectifier to change the
> > alternating current into direct current, at the cost of about 1.4 volts.
> >
> >
> > --
> > Steve Holton
> > sph0lt0n at gmail.com
> >
>



-- 
Steve Holton
sph0lt0n at gmail.com
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