-=> Roy J. Tellason said to Greg Mayman
 -=> about "VEHICLE LED'S" on 09-03-03  04:06.....

 RJT> Hm.  Perhaps they make lower-voltage chips of that sort?

Hmmm... there are some low dropout regulators, but IIRC they all
have a higher reference voltage than 1.25v, so in the current
regulator configuration they would still give too much drop.

 GM> Personnally I think a constant voltage regulator would be better.
 GM> You would only have to supply about 2 volts above the lamp voltage.

 RJT> The common 7800 series parts wants more like a 3 volt difference, 

I was looking at the data sheet for the LM317 that was mentioned
previously. And that 2v figure was the lowest point on the curve.

 RJT> I wasn't aware of that.  Ok,  a schottky diode (lower voltage drop)

Or four diodes for a bridge which still gives a minimum of 2
diode drops on each half cycle. Thats about 2 x 0.5v = 1 volt. In
a total of 6v, that's a lot!

 GM> Even with a capacitor input filter, the impedance of the usual 
 GM> cycle dynamo is too high to charge the cap to the peak voltage, as
 GM> you can do with a mains supply to offset the diode drops.

 RJT> How have you determined that?

By the fact that the inductive reactance of the generator is
usually used to drop the voltage as the frequency goes up, to
keep the voltage across the lamp more or less constant.

From Greg Mayman, in beautiful Adelaide, South Australia
   "Queen City of The South"    34:55 S  138:36 E

... Too much of a good thing can be wonderful. - Mae West
___ Blue Wave/386 v2.30

--- FLAME v2.0/b