Originally Posted by Pedaleur

It can actually generate quite a lot of voltage, because what SweetLou says is accurate. The dynamo is "trying" to push a certain current, so if there is a high resistance load, voltage will keep building up until the current is pushed. In my tests, I reached hundreds of volts, if I remember correctly (and burnt my finger on the resistor).

Also, my calculations for a 3W/6V Shimano showed the limit to be about 1000mA, at which point the inductance limits the current. This is at odds with SweetLou's statement of 500mA -- I could certainly be off in my measurements. Nevertheless, I use two CREE's rated to 1000mA just to be safe.

If you understand the math (second-order differential equation), I can dig up the numbers. If you don't care for math, we can just talk in generalities.

LEDs are very sensitive to voltage, if powered with a constant voltage source (battery). If you look at the brightness curves for an LED, the difference between 3V and 3.5V can be huge. By limiting the voltage with your regulator, you _are_ limiting the current through the LEDs. The math gets a little complicated (or not).