n4zou

Standard bicycle dynamo lights use a filament just like any other kind of filament type bulb. They just happen to be designed for 3 watts of AC current. This takes the form of a headlight bulb with a 2.4 watt filament and a taillight bulb with a .6 watt filament. This provides a balanced load so the dynamo generally will not produce more than about 6 volts at "normal" bicycle speed. 3 watt bulbs are used in the headlight when no taillight will be used. If you try to use a 2.4 watt 6 volt bulb without a .6 watt taillight the dynamo will produce more than 6 volts so the bulbs life will be shortened. Just any old 6 volt light will not work. The reason LED's have become so popular for use with dynamo light systems is due to there use of current for light production other than voltage like that for a filament bulb. A typical dynamo will produce about 500mA and due to it's design can't produce much more even with higher speeds. Voltage however is variable increasing with speed. Most bicycle dynamos can produce 12 volts when using a 6 watt bulb but those have become almost impossible to find. LED's produce light from the current flowing through them and not voltage like filament bulbs. When using LED's in a dynamo circuit voltage is no longer a problem as current determines light output and not voltage. Most power LED's are rated for voltage but only when used with batteries. The dynamo is limited in current production so as long as the current is below the maximum for the LED being used it can't be burned out even if voltage is higher than stated in the specifications. If light producing LED's could stand reverse current as they do forward current a bridge rectifier would not be needed.
You can add a battery to the circuit so the dynamo light remains lit at stops but this adds complication. Now you must use a voltage regulation circuit with the lights or LED's as voltage become a problem once again. It's so much easier to simply allow the LED headlight to quit at stop's and add a blinkie so that others can see you when your stopped than go through all the harassment of batteries and regulation circuits for your headlight and you'll have a lot more light when you do start moving again. My dynamo LED headlight provides enough light at walking speed as I'm pushing it up a steep hill to allow me to see where I'm going so a battery is never needed. Of course I always carry a small flashlight just in case I ever need to fix a flat or other repair when it's dark.