RE: Re: Variac vs. Fan speed control

Subject:       RE: Re: Variac vs. Fan speed control
       Date:   Mon, 21 Apr 1997 20:31:37 GMT
       From:   robert.michaels-at-online.sme-dot-org (Robert Michaels)
Organization:  Society of Manufacturing Engineers
         To:   tesla-at-pupman-dot-com

[ ... ]

-> quite simple. Original sine-wave (ascii art:) :

->              /-\
->             /   \
->   -\     /-/     \-\     /- etc.
->     \   /           \   /
->      \_/             \_/

-> now, start the dimmer..

->              /|
->             / |
->   -\  |----/  |----\  |--- etc..
->     \ |             \ |
->      \|              \|

-> now, if you turn the dimmer to a lower position :

 [ ... ]

        Excellent explaination and ASCII art!  You reall must have put
        some work into that.

        For the sake of completeness - and truth in posting - a foot
        note needs to be added to this:

        Older triac controllers worked exactly as you described. Cheaper
        ones today probably still do in many places:  The phase angle
        of the ac is varied in proportion to the setting of the control

        In the US however, it is no longer permissible to sell such
        devices.  The sharp cut-off of the ac wave generates rf which
        causes considerable interference with communications.

        So - modern solid-state controllers use zero-crossing control.
        The switching performed by the controller is done only at the
        split instant when the sine wave crosses the X-axis.  Since the
        switching is done at or near zero volts, rf radiation is

        When viewed with a 'scope, the waveform of such controllers
        appears to be whole sine wave cycles which occur sporadically.
        The intervals between such whole sine wave cycles become shorter
        and shorter as the control knob setting is increased.  At full
        power the intervals are essentially zero and the output is a
        continuous sine wave.

        ... But they =still= shouldn't be used anywhere near Tesla
        coils as I believe the original poster has established to
        to his (and most anyone else's) complete satisfaction.

                                        Sine-ing off in - Detroit, USA

                                        Robert Michaels