Jinx wrote:

> So if I understand correctly, if there's sufficient energy present to
> cause RFI at the switching point, it's irrelevant whether the triac
> is turned off (if that were possible) or on.

Correct, it's the dv/dt that causes RMI. No matter if it's
positiv or negative. Also no matter of it's an traic, mosfet, IGBT
or whatever.

> The practical difference
> being that the triac "turns itself off" at the next z-c point, which
> keeps switching and s/w simpler

Yes, you just have to "count" from the last z-c point ant "trig"
the triac, possibly with just a short puls. The shutoff is handled
by the triac itself at the next z-c. Note that for inductive loads
this gets a little more complicated...

> From what I can gather, series inductors, shielding and good layout
> are fairly effective at minimising RFI

I also thinks that you can get pretty low RMI by using suitable
shokes on the line (mains) terminals. The large part of RMI is
transitted from the mains cords attached to the switching device.

> To keep it simple, it seems that
> something other than a triac is needed for PWM if you wish to use
> the leading z-c and turn the juice off mid-wave

Yes, that's why people are looking at MOSFETs, IGBT's and other
devices. But note, the reason these gives less RMI are that they are
switching off slower (at mid cycle) then the triac turns on
(also at mid cycle). Slower switching also gives higher loss (heat)
in the swithing device. If you are switching the MOSFET hard (as
you normaly want to do) up to the same speed as the Triac, you stll
get the same level of RMI. There's no free lunch, right ? :-)

Jan-Erik.

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics