[Olpc-open] Foot power revisited...

[Mike Dawson]

Hi Steve,

Thanks again -sounds good.  I'm not sure we're expecting to recharge
the XO in quite the same timeframe as one would do from the wall - I'm
hoping that it could be more of a continuous supply to it...

I'm not sure where our nearest DigiKey, Maplin, or Radio Shack is here
in Afghanistan... ;) - and the dysfunctional bricks here are often so
because they got blown up by a power spike - but I can easily buy such
an AC adaptor at the electrical market.

Anyhow I'm optimistic about this - in theory we're good and you just
plugged the gaps between the theory we are practising and reality.

I think if we plug in two of these dynamos in series it should do the
trick at the right speed...

Regards,

-Mike

2009/1/7 Steve Holton <sph0lt0n at gmail.com>:
>
>
> 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
>