Matching to the impedance of an alternator.

An hour or two a day for nearly half a month.

At 1.8W (or a little more if I can coax it out) - pretty unlikely.

Highly unlikely - the magnet is crap!

I was thinking voltage regulated - about 7.2V sounds about right.

Should be minimal if I let the Vo expand on light load and step down.

Adding a couple of capacitors to the bridge rectifier making it a doubler avoids the complexity of buck/boost.

Sod that! - I've just posted a wanted ad for another motorcycle.

'Bout the best advice you're likely to get here is to characterize the device and load it accordingly. If you want practical experience, you'll have better luck in a group dedicated to overclocking bicycles.

Google "candlepower forum". It's a lot about LED lighting, but there have been threads on bicycle applications that might be relevant.

Many thanks.

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I know. In the early '70's I designed the electronics for such a bike, based on using an alternator as both the physical load and a source of power for the electronics. ...Jim Thompson

Its just crossed my mind that it might be easier than I think!

Assuming a buck converter draws less current than shunting the output with a rectifier/SLA battery; the voltage will rise.

As the buck converter starts drawing current to maintain the battery, the voltage will fall again.

I'm thinking it might automatically find its own level at the best output power the alternator can provide for the RPM at that time - all I have to do is provide input OVP for the buck converter for when the buck doesn't need current to maintain the SLA.

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So the electricity that does as much work as a reasonably fit person could do, in an 8-hour day, costs a few cents. And a lot of the population of the world doesn't have access to electric power.

John

I'm guessing you'll have lots of voltage if you just rectify and filter. Possibly too much.

But TRY it!

John

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Not to mention that the food needed to supply that electricity would cost a lot more than that few cents and something a lot of the world's population doesn't have (food to spare). Electricity (and oil) are a pretty good deal, all around.

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you'll want to look up solar power circuits, these have a similar problem, pulling the most power from a variable source... the cells have an IV curve that varies with the amount of light, and for each curve, there is a MPP max power point...

Mark

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Yeah. Bodies are a very inefficient way to convert food to work.

If shale gas is as plentiful as it seems to be, it could have a revolutionary effect on poverty. Robert Lestz may wind up being the greatest benefactor in human history.

Oil and natural gas are an astonishing gift to humanity from a benevolent universe. Imagine trying to live on a planet that was, say, pretty much uniform basalt. No oil, no gas, no coal, no separated and concentrated metals and minerals.

John

"Ian Field" wrote in news:mPXNq.87310$ snipped-for-privacy@newsfe22.ams:

a little hint about the 3W Cree XR-E Q5 LED's; they are rated 228 lumens at 1A,but checking the spec sheet,they output ~114 lumens at .3A,so for my homemade bike light,I used two LEDs,each driven at .325A,for a total of .65A,and getting the original 228 lumens but at 2/3 the power draw. Plus,I get a broader beam. I drive the setup from 4 AA NiMH cells,2350maH. It works great.

I got my LEDs,drivers and reflector optics from DealExtreme.

We already know that Ian's hub does rectify, and is "filtered" by his SLA battery. Two lamps load it down to barely charging his battery. That's the issue, _not_ how much open-load voltage it can generate. ...Jim Thompson

Yes!

The input impedance of a buck regulator is negative. An alternator is pretty much a current source. The combination may be unstable.

Interesting.

John

Similar issue. For the alternator, the optimum load current is a function of frequency.

There must be some simple tricks here.

John

I tested my alternator (a Soubitez model) by holding it to a spinning rubber wheel driven by an electric drill. The output was a few hundred Hz, 14 ohms. The impedance depends on the drive (RPM) level. because the loss of efficiency at high revs/second is how the output current is kept from burning out your lamps on a long hill.

I was happy with a voltage-doubler rectifier (two diodes, and a coupling capacitor), into a three-cell SLA (sealed lead-acid) battery. I used a low-dropout regulator circuit (not an IC, those weren't available back in the 1980's), after the battery.

Just to be contrary, I did it in positive-ground fashion. The low-dropout pass transistor was NPN, the design was cleanest with a positive common regulator circuit.

A shorted alternator delivers no power, so a short is not the maximum torque load. Elementary stuff.

John

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Well, then, don't short your alternator.

What was the open-circuit voltage?

The impedance depends on the drive

Not loss of efficiency in the power sense. It's just that the generator is primarily inductive. When the speed increases, the internal source voltage increases but so does the frequency. Into a mostly inductive circuit, with a low impedance external load, that makes the current pretty much constant versus speed.

Maximum power transfer will happen at roughly half the open-circuit voltage, which does increase with speed.

John