Re: Controlling Holiday Lights

[sylvan butler <ZsdbUse1+noZs_0610@xxxxxxxxxxxxxxxxxxxxx>]

On Mon, 23 Oct 2006 23:32:07 -0400, Marc_F_Hult <MFHult@xxxxxxxxxxxxxxxxxxxxx> wrote:
> A TRIAC dimming household 60 hz AC can be switched on *exactly* twice every
> 1/60th of a second: once during 0-180 degrees and once during 180-360
> degrees. And a TRIAC can _never_ be 'switched off'. So what you say you are

The microprocessor will switch on and off the gate current.  The triac
will cease conducting within 1/120 of a second (at 60hz).  That's rather
faster than your hand can interrupt the A.C. using a mechanical switch.
It is also reasonable compared to typical mechanical power relays.  If
you don't want to call that "switched off" that is your prerogative nit.

> doing (as I understand it) simply won't work.

But I can see it does.

> OFF). This can be done (eg) with a comparator that compares the voltage of a
> simple (eg) 0-10 volt ramp (sawtooth signal) that begins at the zero crossing
> and a 0-10vdc DC control signal. When the ramp reaches the value of the
> control signal, the comparator goes high and causes the TRIAC to go into
> conduction. This is repeated 120 times a second.

Yup.  Now how would your DMX dimmer generate the 0-10vdc control signal
for the comparator?  Nyquist might help you understand how, and also
how/why I can simplify the whole mess with a bit of software and make a
generic controller, just as long as I keep my switching frequency many
times faster than line frequency.  (And 1khz just so happens to be a
typical lower bound in many PWM designs.)

sdb

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