[Olpc-open] Foot power revisited...

[Mike Dawson]

Hi All,

Unfortuantely I am just stuck in Jalalabad at the moment as bad
weather cancelled our flight back to Kabul but I met the folks who are
doing the Fab Lab here (if anyone has heard of that).

So we did a quick back of the envelope calculation along these lines
that refreshed my high school physics as soon as they reminded me that
W = FD

1. Assuming someone is normally doing about 60 rpm on a bike
(apparently the figure is 60-90)

2. Take the distance that a foot pedal gets pushed as being 15 cm

3. Work done = Force x Distance

4. Dynamo as being 50% efficient (apparently an alternator can be up
to 60% efficient http://en.wikipedia.org/wiki/Electrical_generator)

5.

P = 0.15F / 2

P = Power in Watts out
F = Force in Newtons

(as at 60rpm the output in Joules will be for 1 second, which is
therefor the power rating in Watts)

So the force required to generate 9W (e.g. for charging a little and
running the laptop):

9 = 0.15F / 2
F = 120N = 12.23kg (given gravity = 9.81N per kg)

Which does not seem to be an unreasonable amount...  Our idea is to
see if it is possible to generate continuous power during usage in a
manner that is both quiet enough and reliable enough.  We can get
Dynamos from the local market for about $0.80 each.

I know that calculation does not include the required loss to a bridge
rectifier... but it all just seems a little too simple - can anyone
spot the mistake here?

If that's correct then when the chopper takes us back to Kabul we can
try to connect up the rectifier to the machine and see how we might be
able to charge the XO...

Would also be interested to calculate if one assumes that most of or
all of the calories burned by the human body come from a certain
source, then how much will this add to the food bill?  Given that I am
told a human normally produces 60W of just heat... hopefully not too
much.

Regards,

-Mike



2009/1/8 Steve Holton <sph0lt0n at gmail.com>:
> On Thu, Jan 8, 2009 at 5:49 AM, Mike Dawson <mikeofmanchester at gmail.com>
> wrote:
>>
>> I think if we plug in two of these dynamos in series it should do the
>> trick at the right speed...
>
> You will find it much simpler, I think, to convert the power to DC  before
> attempting to put things in series. That is to say, to have a seperate
> bridge rectifier dedicated to each dynamo, rather than adding the AC domain
> voltages together and rectifying the result.
>
> If the AC voltages  provided by the dynamos are not kept perfectly in-phase,
> then to some extent they are cancelling each other out. In the extreme (180
> degree out of phase) case the serial sum of the two would produce no net
> voltage at all at any speed.
>
> Best of luck.
>
>>
>>
>> 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
>> >
>
>
>
> --
> Steve Holton
> sph0lt0n at gmail.com
>