I am developing hardware and software for a dynamo peak power tracking system. Maximum Power Point Tracking (MPPT) is something I learned about over on a thread at CPF and have subsequently become fascinated about. Widely applied in the solar industry to extract the maximum available power from a photovoltaic array under all conditions of illumination, MPPT is, in essence, a way to tweak the load resistance to reach a peak power point on a current/voltage curve under dynamic conditions. Bicycle dynamos operate primarily as constant current sources. They reach a saturation current at reasonably low speeds and the only extra power to be gained from a 'passive' system is through the increase in voltage that comes at higher speeds. Passively extracting more power is done typically by adding more LEDs in series. If you have 1 LED with a Vf of 3.0V you can get a maximum of 3V x 0.5A = 1.5W. If you add a second 3.0V LED in series you can get 6V x 0.5A = 3.0W, a third gives you 9V x 0.5A = 4.5W, etc. The problem here is that for each LED added there is an increase in the minium speed at which the dynamo reaches Vf of the series chain. Below that you get no light, or, at best, very blinky lights. However, there are some clever passive low speed boost circuits out there to get more power at lower speeds.
Today's power LEDs have maximum currents in the range of 1-3A, so a dynamo that saturates at 0.5 or 0.6A is not able to run these even close to their maximum theoretical outputs. I want to a employ a single LED design for front and rear lamps, so running the LEDs at more than the dynamo saturation current would be useful. In order to pull maximum power out at all times, a switched mode buck converter can be employed. I'm working on a digitally controlled synchronous buck converter that uses LED current and bicycle speed as feedback. Thus far I've developed a peak power tracking system that works with a bench top supply. Essentially, under fixed current conditions (like a hub dynamo), it modifies the pulse width of the switching signal to maximize the power from the available voltage.
|LED Current vs pulse width duty cycle at different input voltages where Vf of the series LEDs is 4.1V|
|Buck converter hardware|
|Hardware is under Arduino µC control|