Originally Posted by
Salamandrine
Since a light bulb will run happily on AC, it's unlikely your headlight has any sort of rectifier built into it. You'd be best off doing something a little fancier than a basic bridge rectifier. Some capacitance will smooth things out. Here's one example, there's lots of others on the web:
Dynamo Powered LED Bike Lights: 7 Steps
I'd imagine that someone must sell some sort of LED unit with a built in regulator that could be shoehorned into your old light housing. That would be the easiest way to go.
It would be a good idea to determine your dynamo's voltage and current output before proceeding. Got a multimeter?
The web says 6v, 3w, but nothing more that I can find. I'm reticent to connect it directly to my $80 B & M light, but I do have some spare bulbs around. I'll see if they can be shoehorned and check the results.