Woops, sorry for my previous post.
It looks you need a DC-DC power supply. 350mA is a quite large value
to use a linear device. A simple isolated supply using a transformer
driven by a local oscillator (supplied directly from 90Vdc) and one power
mosfet for transformer driving. Fast recovery diodes in transformer
secondary and filtering. With 75-80% efficiency it seems you'll need
maximum 2W of switching power for one led module. (assumind the module it
consumes indeed 1W)

top 10 wishes,
Vasile
http://surducan.netfirms.com


On Thu, 4 Dec 2003, Andrew K Lindsey wrote:

> I have a light string driving module which produces a 0-90VDC output,
 generatind a sawtooth wave output, rising and falling with a period of
several seconds.  It's normally used to drive a large string of lights in
 series.  I have a 1W LED module from www.luxeonstar.com I want to drive
off the supply.  The module wants 3.42V at up to 350mA.
  How can I scale the 0-90V from the supply down to the 0-3.42V the LED assembly wants, reasonably cheaply and efficiently?
>
> The quick and easy method would be a resistor array, but some quick calculations shows I'd need a 30W resistor to handle the nearly 90V voltage drop at that current level.  I'd prefer to avoid that - it would be huge and inefficient.  Second thought is to divide the 0-90VDC down to 0-5VDC or thereabouts with a high-resistance, low-wattage voltage divider, then use a 12C672 or similar to read the analog voltage and drive a PWM output through a FET to drive the low-voltage LED assembly.  Unfortunatly, this requires an independant power supply for the lamp and drive electronics, and is probably more complex than needed.
>
> So - anyone have any ideas on how to scale a 0-90V source to 0-3.42V at about a third of an amp current, efficiently and compactly, and without requiring a seperate power supply?
>
>
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