On 13 Sep 2000, at 16:16, Montaigne, Mike - NRC wrote:


> > I understand phase angle control - in fact I have even built a few, for
> > example to control lighting in our church using a 555 starting its delay
> > from zero crossover, but it seems to me that a power control that would
> > drop out individual cycles of the power - the more percentage of complete
> > cycles you drop out, the less power you transfer to the load - would be
> > better because then you could always use zero crossover switching.  It
> > wouldn't be useful for lighting, because you would see the flicker, but
> > should work fine for heating and work O.K. in our case of going through a
> > variac/transformer.

That technique is called 'burst fire' and is indeed used for heating control.
Low EMF and low transients all round.
I have seen it done with a PIC and another with a 555 generating PWM
at a rate of 13Hz varying from complete cycles to missing complete cycles
to alternate half cycles at the lower settings.
The 555 article is in Silicon Chip July1998 "Heat controller"
www.siliconchip.com.au
and the PIC project was in a book "PIC Cookbook Volume 1"
by Nigel Gardner and Peter Birnie, sold by Farnell 654991 (www.farnell.com)
ISBN 1-899013-02-4.

_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand

www.psych.auckland.ac.nz

_____________________________

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