piclist 1996\04\09\014720a >
Thread: simple D/A for 500W AC?
face BY : John Payson email (remove spam text)

> Tom Sgouros wrote:
> > Does anyone know of an easy way to control the voltage on a 120VAC line? I
> > need to vary the light from some lightbulbs, and I don't want to use the
> > standard SCR solutions because they produce artifacts that seem to wear
> > the bulbs out faster.
> I can give you this much of a really good design my brother & I did
> years ago:
> Use a cheap optically isolated scr or triac AC relay. Engineer a signal
> to the micro at the AC line zero crossing. Maintain a number that
> corresponds to the desired intensity level: say, the number of cycles
> out of 128 that you expect to deliver power. Construct an algorithm that
> turns the relay on or off at each zero crossing such the the following
> two requirements are met: 1) the average voltage is zero and 2) the
> number of on cycles are evenly distributed throughout the 128 cycle
> period.

While I acknowlege that your approach would probably work, its output would
have a significant 1Hz flicker component which could induce flicker and also
reduce bulb life.  I would think it would be preferable to modulate the bulb
at a higher frequency [perhaps from a filtered DC supply, to eliminate 60Hz
flicker/wear], with a little LC filtering to minimize RF junk.

In reply to: <m0u5PQo-000BXpC@mailbox.mcs.com> from "terogers" at Apr 5, 96 10:58:44 pm
See also: www.piclist.com/techref/io/d2a.htm?key=d%2Fa
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Subject (change) simple D/A for 500W AC?

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