Originally Posted by
Leisesturm
Ok, I'll take that as fact since I don't know any better. What would the frequency of the oscillation be? I thought about this while most of you guys were sleeping. Household AC alternates at 60 cycles per second, and it is maintained with such precision that you can use the alternation to keep clocks on time. How does a generator, dependent on the wildly varying speeds of a bicycle wheel maintain a constant alternating current?! Like I said, I have no idea.
My measurements on my Schmidt SON28 dynamo (in a 700C wheel) show that it has a frequency of 27Hz at 10mph and 43Hz at 16mph.
The dynamo's open-circuit voltage is essentially proportional to the speed, which does present a challenge when trying to deliver a constant power to something as delicate as an incandescent bulb.
This is accomplished by the design of the dynamo and the selection of the bulb's resistance.
The dynamo has a large internal inductance. For my SON28, this is about 0.15 Henries. At 16mph, the dynamo produces an AC waveform with a frequency of 43Hz. The 0.15H inductance has an impedance of 39 ohms at this frequency. The bulb has a resistance of approximately 12 ohms. so we can see that the dynamo's inductance is the major portion of the overall impedance and largely determines how much current will flow.
The neat thing about inductance is that the impedance is proportional to the frequency, just like the dynamo's open-circuit voltage.
Therefore, as the dynamo speeds up, both the voltage and the impedance go up the same amount, resulting in nearly the same current delivered to the 12 ohm bulb. Such a clever design!
Originally Posted by
Leisesturm
What I do know, however, is that LED bulbs (Light Emitting Diodes) must have DC in order to work. .......
So if the generator hub is producing AC then there is a bridge rectifier somewhere, either in the hub itself, or in the housing of the light, but you cannot present an LED light bulb with AC, it simply won't work. ......
well, that's a nice thought, but it ignores that the LED could simply be powered by half of the AC waveform when no bridge rectifier is used.
It also misses the possibility of wiring two LEDs in parallel, but opposing polarity. Each LED could conduct during opposite halves of the AC waveform.
I would suggest that your future posts be phrased in a way that reflects your tentative understanding of electronics. i.e. less "I know", and more "I think that it works this way...."
Steve in Peoria