Gary, I have done this twice.  It takes a little thevenin analysis.  The
center pin on the 431 reference wants to see 2.50VDC.  So the supply
raises the output until this pin is 2.5.  If you inject a bit of current
into this node, you will satisfy the circuit sooner, and the output
voltage will be lower.  You should apply 0-2.5V to this node through a
resistor, the value of which is determined through circuit analysis.
When the command voltage is 0, the new resistor is in parallel with
another to ground.  You might get a notepad out and do some equations.

The easiest way to get 0-2.5VDC is to use a PWM output through an RC
pair.  Then add a resistor across the capacitor equal to the R.  You
will combine a filter and a divider in one.  Done right, you will have
0-2.5VDC.  You might choose to buffer it, or use a salen key to filter
it, it is up to the designer.

I also suggest that you only adjust over a specific range, say 25% to
100%.  Switchers are notorious for not working well outside of a
designed range of operation.  Test it well.

Have fun. Chris~

Gary Faria wrote:
>
> Sorry, I posted the file.
> http://gffaial.homestead.com/power.html
>
> I want to control the out voltage of this power supply using a pic12c508.
> The out put voltage needs to vary from 5-12V @ 7A.
> I attached the schematic power.jpg
> Can someone help me!
>
> Thanks,
> Gary
>
> --
> http://www.piclist.com hint: The list server can filter out subtopics
> (like ads or off topics) for you. See http://www.piclist.com/#topics

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics