Hi Chris



 RM> I'm having a go at building a small one with a PIC and some fets at

 RM> the moment. The highest switching freq I've tried so far is 1.2

 RM> 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?



To assemble something vaguely resembling a sinewave by varying the

mark-space of the switcher in a sinusoidal way. For example, the

following is an approx of the drive for one fet for one half cycle,

with a switching freq of 500 Hz:



     _      __   _____   __      _

____| |____|  |_|     |_|  |____| |____





I'd guess that this will work very well with motors, less well for

general electronics, and not at all for phase dependant speed

controllers. I'm doing this for fun, mainly. The final 3 phase

version will probably use a boring squarewave.



The only ways I can see of faking a 240V sinewave are (a) synth it

with fast pwm, as above, (b) do a squarewave and filter it at hi

power with caps and big fat heavy expensive inductors, (c) synth it

as in (a) and filter the output as in (b), and (d) drive the

transformer with a linear amp, and waste a bit of the 240 output to

drive its cooling fans.



Cheers



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