John Henderson wrote:
> "Frank Krygowski" wrote:
> > FWIW, I usually use a 2.4 watt bulb without a taillight. I'm sure I'm overvolting it somewhat
> > (but not much, since I've got the zeners for a regulator). Brightness is somewhat better than
> > with the taillight, and bulb life has been fine.
> A properly designed zener-diode regulator should prevent overvoltage. At a regulated 6.0V, your
> 2.4W bulb will draw 0.4A only.
An opposed zener pair does not regulate the voltage as precisely as you seem to think. The zener
spills current it the applied instantaneous voltage exceeds its rated voltage. But remember, bike
generators are AC devices. As a first approximation, the zeners cause the AC sine wave to transform
into something vaguely like a flattened sine wave, with its peak shaved off. But if the sine wave
being input to the zener pair has a higher voltage, the RMS value of the output from the zeners is
still higher. The waveform fills in more of the area below the flattened peaks, and the zeners can't
do anything about that.
IOW, a bike generator with an _ideal_ AC voltage regulator would show RMS voltage rising with speed,
from zero mph to whatever speed gives 6 volts RMS. The voltage would stay precisely 6 V RMS at any
greater speed. But with a zener pair as a regulator, voltage rises as above, but then continues to
rise, only at a lower rate. The voltage-vs-speed curve is sort of a rising dogleg curve, with a high
slope followed by a much lower one.
At least, that's what I recall. My data's in my other office.
> Most bicycle dynamos are more total-power-limited than voltage-limited, capable of delivering 3W
> max. (but with poor voltage regulation, depending on the load to do this). With a "hot" resistance
> of 15 ohms, a 6V, 2.4W bulb being fed with such a 3W source will stabilise the voltage and current
> at about 6.7V and 0.45A (while it lasts).
No, bicycle dynamos are current-limited, not total-power-limited (unless, perhaps, they are
tire-driven units that slip, which would be a design fault). If you present a bike generator with a
higher output resistance, it will develop a higher voltage. It tries to maintain a constant current.
It doesn't succeed precisely, of course (any more than a battery always delivers a constant voltage)
but under reasonable conditions, it comes pretty close.
On one of my bikes, I've got the generator driving two 2.4 watt lamps in series. (The second one can
be switched off at low speeds.) Above roughly 13 mph, the generator puts out enough voltage to run
both lamps at only slightly reduced current. I'm getting about six watts out of that "three watt"
Frank Krygowski [email protected]