Originally Posted by
roashru
yes, you will fry it. go to the cree web site look for the product page and read the "data sheet". white for that model says 3.2 volts.
Correct me if I'm wrong, but I thought current was the prime factor with LEDs, not voltage. The LED can take the 500-600 mA my hub can put out, so that was the big concern. Normally if the voltage increases, the current will as well, but the Dynohub can only do so much.
Originally Posted by
christ0ph
Dynamos are a constant current source, so paradoxically, adding another LED in series will in many cases get you more power without the penalty you would expect, or so I just read. That said, I think you would be wise to invest in a constant current driver, given that they are so cheap. You'll end up with a more efficient setup that way. One thing you really need to worry about is the hub putting out a much higher peak voltage which because of the rectifier, wont have anywhere to go except charging that cap to a high voltage.. (BEFORE you connect the LED, why not perform an experiment to see how high it will go with your bike on a stand or upside down..with you turning the pedals by hand.. that would be interesting.) In other words, nomatter what the RMS voltage of the generator, I think that with no load, the cap will charge to the PEAK voltage... So then, unless you leave both lights on all the time (which makes some sense if the drag isn't high, but which will slow you down if it isn't)
How old is the hub, (many newer hubs have bult in zener diodes but older ones dont, Ive read, and they can put out very high peak voltages which will then get stored by that cap) and what are the current ratings on the two LEDs?
In the meantime, why not pick the lower of the two LED's maximum current value and then, using Ohms law, pick a low value resistor to throw in series, also put an extremely high value - in (CRT) TVs, bleeder caps are usually megohms- but that might be too high, use a multimeter if you have one to see how long the cap holds a charge under different "road conditions" (you spinning the wheel by hand) ) to put across the cap to bleed off the high voltage should your light get disconnected at some point, to prevent it from being burned out by the charge on the cap.. which could "potentially" get high.. For the series resistor (MAKE SURE THATS ITS *BETWEEN* THE CAP and the LEDS...not before it.. I'd also start with a value that would limit the voltage if the LED was not there to the peak current for the smaller of the two LEDs at some arbitrarily higher voltage- the value I would pick is 20volts.. and then work down from there (because they are both probably way too high)
The thing you dont want is the cap charging up high while riding with the headlight not connected, i.e. no load, and then, connecting it only to have it burn out because unknowingly you gave it a 30 or 40 volt zap there..which ended up pushing much more current through the LED than the peak design value . Even a good heat sink wouldnt help you in that situation.
In other words, smooth out the potential for voltage and current peaks..
I'm feeling a bit frazzled today so maybe I am not making sense.. Does that make sense?
You make some sense, but it's limited by my understanding. I may just throw another LED in series for a rear light and be done with it. I just leave them on all the time, so I'm not worried about the capacitor. The X-FDD is a newer hub, so I'm pretty sure it limits output.