Will running an LED designed for batteries on a NEXUS dynamo hub damage the LED? I have a question about LEDs on 6V Shimano dynamo hubs. I built up an old Schwinn with a Nexus dynamo hub, and the headlight I'm using is based on an LED 6V headlamp. The LED is a Chip-on-board design, and I've wired it directly to the two terminals on the hub without using the frame as a ground.  I have the pulses set up to alternate between the headlight and the tail lights. The light works great, but I wasn't aware that the Shimano dynamo is putting out reverse-polarity pulses as well as the ones properly oriented to light the LED. Is this bad for the LED bulb? Will it burn out prematurely because of the reverse pulses?
Reverse pulses at low voltage probably won't damage the LED, but the COB (chip on board) that gives the flashing modes might not be able to cope with the full reverse voltage.
The problem is that the hub dynamo is more accurately a constant current source, and will supply about 500mA. One common trick is to run two 6V lights in series off a hub dynamo, and most dynamos will give full brightness from both... they put out 12V. The actual voltage you get out of it might be clamped by a zener, but it's unlikely to be a 5W zener that can take the full power of the hub forever. Not that you want that anyway, it's extra drag.
I have run bare 5W star LEDs straight off a SON hub dynamo (well, with a diode and capacitor to reduce flickering) because they're designed to take 500mA and that's what the dynamo supplies. It worked really well, just using a chunk of aluminium channel as a heat sink/weather sheild/reflector. I don't really know about longevity, after ~10,000km/a year or so I got a proper light cheap so switched to that.
So if your 6V LED is actually a 1W light it might be getting 12V or more. Whether it survives that is anyone's guess. Worse, even a 5W light is likely to be getting +6V on that half of the wave, then -12V on the other half when it's not drawing any power. I suggest a basic diode or bridge rectifier in series, capacitor in parallel to give you DC.