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 <[email protected]>: > > > On Wed, Jan 7, 2009 at 8:57 AM, Mike Dawson <[email protected]> > 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 <[email protected]>: >> > On Wed, Jan 7, 2009 at 7:36 AM, Mike Dawson <[email protected]> >> > 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 >> > [email protected] >> > > > > > -- > Steve Holton > [email protected] > _______________________________________________ Olpc-open mailing list [email protected] http://lists.laptop.org/listinfo/olpc-open
