> The circuit as you have shown it will not work in your application.
> There's nothing wrong with the circuit per se - just that they have
implied
> something in it which is not necessarily obvious.
> ie the  sine wave they show is a true bipolar signal where first it has
the
> polarity +/- and then -/+
> . A PIC PWM output etc is single sided going +/0  and then 0/0.

Note that the AC input voltage in Jinx's circuit is completely AC coupled to
everything else.  Any DC componenet on it is irrelevant except for the
capacitor voltage requirements.

The problem with driving it from a square wave instead of a sine is
effeciency.  If the input voltage goes thru a sudden step, then something
will suddenly have a voltage drop accross it.  It's not going to be a
capacitor, so that leaves the diode or the switching element in the square
wave voltage source.  This will waste power until the capacitors catch up to
the new input voltage.

One fix is a small inductor in series with the square wave voltage source.
The inductor takes the sudden voltage jump, which decreases as the current
builds up and the caps charge up.  You will need both positive and negative
flyback diodes from the square wave voltage source to its rails.  Make the
inductor as small as you can to limit the peak currents to tolerable levels.
You then have to make sure the switching frequency is low enough to make
sure the inductor is fully discharged before the next square wave edge.  I
would do it like this:

1  -  Decide the maximum tolerable instantaneous current from the voltage
source and thru the diodes.

2  -  Pick the smallest inductance to guarantee the maximum current from #1
is not exceeded.

3  -  Find the longest possible inductor charge then discharge time per
square wave edge.

4  -  Set the total square wave period somewhat longer than twice the time
from #3.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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