Hello Roy,

 RM>> I'm having a go at building a small one with a PIC and some fets
 RM>> at the moment. The highest switching freq I've tried so far is
 RM>> 1.2 kHz, with fair results.

 CB>> I haven't seen an under 20Khz switcher or DC converter for ages.
 CB>> Just curious - what's the reasons for using a PIC instead of a
 CB>> purpose built switchmode controller IC like a 494?

 RM> To assemble something vaguely resembling a sinewave by varying the
 RM> mark-space of the switcher in a sinusoidal way.

Oh.  I assumed you wanted the usual DC output - it's all clear now.

 RM> I'd guess that this will work very well with motors, less
 RM> well for general electronics, and not at all for phase
 RM> dependant speed controllers.

Or split-phase (capacitor start) motors - some variations on 'em
anyway.  Saw someone a while back use a "modified sinewave" 24 to
240 inverter on one of those.. it ran for a while, then was hurridly
shut down when he realised the hissing sound from the pump was
actually the innards of the motor's starting cap escaping in the
form of steam :-)

 RM> I'm doing this for fun, mainly. The final 3 phase version will
 RM> probably use a boring squarewave.

Use a faster CPU.  Had a look at a nice piece of gear this week..
a guitar amp which contained a fancy DSP circuit and a CPU rated
at 48 MIPS!  The DSP part was used to make it mimic the distortion
characteristics of well known valve amps (Marshall, Boogie, and
about a dozen others) and other tricks used by guitar players, like
using variacs in the 240V supply line...

God only knows what speed the digitals were running at - but I
doubt a Pentium would have been up to the task.

 RM> The only ways I can see of faking a 240V sinewave are (a) synth it
 RM> with fast pwm, as above, (b) do a squarewave and filter it at hi
 RM> power with caps and big fat heavy expensive inductors, (c) synth it
 RM> as in (a) and filter the output as in (b), and (d) drive the
 RM> transformer with a linear amp, and waste a bit of the 240 output to
 RM> drive its cooling fans.

It depends on the intended load, I guess.  Option (d) would be
probably the "cleanest", but also the least efficient...

Regards,
Chris.

--- GoldED