Michel Gagnon said:
"stu" <[email hidden]> wrote in message news:<[email hidden]>...
Quoted message said:if it is a 6V system and you want to bulbs in series, you want two 3V bulbs(each 1.2 watts) or
you could run two 6Vbulbs in parallel(each 1.2 watts) don't know if you can get either bulb
sorry, but parallel is that way to go you can switch then on/off separately in parallel if one
bulb blows the other one will still work in series if one blows you lose them both
pretty sure this is right and am sure someone will yell at me if it isn't
Right in theory, but wrong in practice.
The Schmidt Dynohub (as well as its cheaper "cousin" the Shimano Nexus) delivers voltage that
increases with speed. At 10 km/h, you get 6 V, and at 20 km/h you get 12 V, and so on... Bottle
generators have similar patterns, BTW.
This doesn't tell the story very well.
First, voltage generally rises with speed, but it's not a linear function. Decent generators are
designed to self-regulate. The voltage tops out at about six volts for all but a few (rare) 12
volt models...
... ASSUMING you've got the standard bulbs fitted! That would be a 2.4 watt headlight and a 0.6 watt
taillight, or just a 3 watt headlight.
Nearly all bike generators actually attempt to supply a constant current of 0.5 amp. If you put in a
bulb with higher resistance, the voltage output rises to the amount necessary to pass the 0.5 amp.
Obviously, if you have no bulb attached, the voltage will get quite high (and will be pretty much
linear with speed). But this has little to do with actually powering a headlight.
Quoted message said:
If you plug your own headlight directly into the hub, you will have to ride very slowly or you
will burn lights in a jiffy. That's why the Lumotec and Bisi headlights have a
voltage-limiting diode..
Such diodes aren't always necessary. Again, most generators self-regulate pretty well, based on the
inductance of the coils. The diodes help somewhat, but they're not always necessary.
Quoted message said:If you want symmetry, you will have to design your own electronic limiting device or you will have
to find some way to install both bulbs (6 W 1,2 W in parallel) _after_ the diode.
If you want symmetry, you can install two ordinary generator lights in series, with
suitable switches.
Putting lights in parallel reduces the resistance the generator sees. This will cause the generator
to put out less voltage. It probably won't regulate itself as well, either.
I've got two headlights mounted in series, with a switch to short out the second one, thus taking it
out of the circuit. My generator (an old Soubitez) drives one light up to about 14 mph. At that
speed, I can open the second one's switch, putting that lamp in series with the first. The generator
responds by raising the voltage to about 12 volts, and I've got two lights. Total output is about
six watts, which is _way_ plenty for me.
If the first lamp did blow out, I could short across its terminals and use just the second light.
But generators are soft-start by nature. My bulbs seem to last forever.
Quoted message said:
BTW, on Peter White's site peterwhitecycles.competerwhitecycles.comOpen ↗ , you'll find a lot of theoretical info
on these headlights.
Regarding performance, installing the headlight heside the wheel (somewhere along the fork) shows
lots of shade from the tire, and if you ride in rain, snow or mud, it will get dirty very quickly.
I would suggest you install the headlight on the fork crown. Best compromise, very good results,
still allows for handlebar bag, unless the rider is only 5' tall.
My handlebar bag is pretty huge. It would block any lamp mounted at the head tube. I made a
lightweight rack to hold the two headlights out ahead of the front of the bag, just above the front
tip of the front fender. I've tried several different mounting points, and this is by far my
favorite. It does away with that problem of wheel shadow - which was a real annoyance, once.
--
Frank Krygowski [email hidden]
