Re: LED Headlight and generator?

Jim Adney wrote: 
On Thu, 20 Oct 2005 09:18:31 +0200 Andreas Oehler
<andreas.oehler@xxxxxx> wrote:


Wed, 19 Oct 2005 21:36:33 -0500, Jim Adney:


Do you know the internal impedance of these hubs? With that voltage
output, I'm gonna guess that it's rather high.

Resistance of the coil is in the 2-4 Ohm range. 


If I take the numbers you folks have been feeding me, here's what we
get:

Lets say I'm riding 20 km/hr, that's 20 volts, peak.

I run this thru a full wave bridge, which I'm gonna say is normal
silicon PN junctions, so that takes 1.5V off the peak, leaving me with
18.5V.

The LED drops 4 V, so this leaves 14 V to be dropped across the
internal impedance of the generator. For the sake of argument, lets be
generous and assume that the AC impedance at this frequency is twice
the DC resistance, or 8 Ohms.

That means that the current is 14 V/8 Ohms = 1.9A. The RMS value of
this will be about 1.4 A.

This means that the power dissipated in the LED will be 1.4 A x 4 V or
just under 6 W. The 3 W LED is toast.

If I put 2 LEDs in series, the current would be 10 V/8 Ohms, or 1.25
A, and each LED would see 5 Watts.

I'm hoping that the generator has a higher internal resistance than
this, but the 2-4 value may be correct.

My ideal lighting system would carry some storage batteries along with
it, to assure continuous lighting. It would also use a PWM output to
hold a constant output voltage for an uncontrolled input voltage.

-
-----------------------------------------------
    Jim Adney            jadney@xxxxxxxxxxx
           Madison, WI 53711 USA
-----------------------------------------------


Maybe you need to look at the inductance of the generator. If you connect a large solenoid to the mains power, even though it has a resistance of a few ohms, it won't go bang. That's because the inductance impedes current flow. From my calculations the output of the Shimano generator (26 pole) will be about 65 hz at 20 km/h on a 700c wheel. (Someone verify this please). This is a higher frequency than mains power, and so we can concede that the inductive reactance will be quite significant.

Marty
.

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