One way to change the load on the dynamo is through the use of a switching power supply. In a manner of speaking, you can think of it as a transformer, where the power into it is the same as the power out, minus some losses that are due to the conversion itself. The input voltage and current are transformed to a different output voltage and current. Of course, resistance can be calculated by dividing voltage by current, so the resistance that the dynamo sees can be adjusted with the switching power supply.

One of the interesting parts of designing a circuit to extract the maximum power from the dynamo is that the circuit has to evaluate how much power is being drawn. A common method is to continually experiment with small changes to the transformation ratio (or the resistance that it presents to the dynamo). If the power extracted goes up, then that change will be kept. If not, it'll keep making the small changes. Essentially, it should end up at the max power point on the dynamo's voltage-current curve, while continuing to make small excursions to each side of that point as a way of verifying that it is still at the max power point.

This is pretty impressive all by itself, but.... you also have to consider that the dynamo's max power point changes as the speed changes! This means that the circuit will have to chase the max power point constantly. Modern microcontrollers make this much easier, but it still adds cost, size, etc. It also adds drag, because that extra power extracted from the dynamo is coming from the cyclist.
Of course, the same circuit could be used to draw 3 watts from the dynamo, but do it at a different point on the dynamo's V-I curve that would have fewer losses in the dynamo, resulting in less effort required by the cyclist. I wonder if people would pay extra for this feature??

For all of these reasons, it seems relatively uncommon to see circuits that track the max power point on bike lights and battery chargers. It still seems like a good idea to me, and it's nice to see Andreas write up an article to shed some light on their performance.


Steve in Peoria