Second post here - please be gentle with me.
I'm running a Shimano DH3N80 and a Philips dyno light - which is great apart from an epic fail on the mounting bracket.
What I'm looking for is some DIY circuitry that will enable to me to switch between:
Running the light
Powering my GPS (mini USB)
Charging up NiMh AA batteries
- or run all three together when I'm going fast enough,
- or run light +GPS at night
- or run GPS + charge batteries during day.
I saw some circuitry a while ago that did nearly all of that - bridge rectifier circuit and using the impedance of the batteries to control the output voltage to the USB.
I think the user who posted that was called n4zou but I tried searching without success on here and some links to diagrams other forums came up with Photobucket - picture moved or removed.
The links I did have were pretty old, so I was wondering what the current thinking on this might be - whether it has evolved at all ?
good question.... I know that I've brought up the subject of maximizing the power out of a dynamo and managing the different loads. A search for "peak power" or "max power" might bring up something.
The whole matter is an interesting problem. The power that a bike dynamo can produce varies with speed and the load applied. Higher speeds permit higher output voltages and therefore higher power. Loads with higher impedances are able to take advantage of the higher dynamo voltages, but will be less able to get power from the dynamo at low speeds.
To my knowledge, this can be solved in a DIY approach if you have a fair amount of experience and a good familiarity with circuit analysis. Having a degree in electrical engineering is also useful.
Even the comparitively simple task of charging batteries is not that simple. You need to be able to limit the charge, or the batteries will be damaged.
I don't recall seeing any designs that were built up and were successfully operating.
For a simple DIY approach, the best bet might be to just build up a switch box to allow you to manually add the different loads... AA NiMH charging, GPS power, headlight, and maybe a low power mode to the headlight or a separate small "be seen" headlight. You would start by calculating how much current each load can have. Assume you have 500mA to work with. How much current does the GPS need (you'll need to apply a fudge factor to account for the fact that the GPS may be running while the bike is stopped)? This lets you calculate how much current is left for NiMH charging and the low power headlight.
Charging NiMH is a separate problem. How fast do you need to recharge your NiMH's? If you use them a lot, you'll have to run the numbers to see if you can possibly get them recharged. If you pull out 2 amp-hours out of them each day, you'll need to put a bit more than 2 amp-hours back into them each day. If you only have 100mA left from your dynamo to charge the NiMH's, then you'll need to ride 20 hours a day.
Anyway... not a trivial problem, and I haven't seen a good solution yet.