Re: Controlling Holiday Lights

[Marc_F_Hult <MFHult@xxxxxxxxxxxxxxxxxxxxx>]

On 28 Oct 2006 20:33:23 GMT, ddl@danlan.*com (Dan Lanciani) wrote in message
<1334494@xxxxxxxxxxxxxxxxxxx>:

>In article <slrnek5vme.vse.ZsdbUse1+noZs_0610@xxxxxxxxxxxxxxx>,
ZsdbUse1+noZs_0610@xxxxxxxxxxxxxxxxxxxxx (sylvan butler) writes:

>| Assuming 60hz.  Typical phase control the triac must be triggered 120
>| times per second at a TBD offset after the zero crossing.  If the triac
>| is instead triggered exactly twice as often, it will function exactly
>| the same -- the output A.C. waveform will be identical.  Same if the
>| triac is triggered exactly 4x as often.  Or 8x as often.  8x is 960/sec.
>| If I instead trigger at 1000/sec, that is a max of 4% error when it
>| happens at random instead of synchronized to zero crossings.  This error
>| has imperceptible effect (to my eye when using incandescent lighting).
>
>What (not synchronized to the zero crossing) triac trigger waveform
>will you synthesize to obtain an approximately 50% dim level?

Now ya dunnit, Dan ;-) This may be the last we hear from sylvan on the issue.

As you know, the independent variable controlling output in a TRIAC-based
dimmer is phase or time offset with respect to the zero crossing. But sylvan
does not (knowingly) synchronize the trigger with the AC waveform. So sylvan
has no adequate answer to your question.

There are at least three possible explanations for his claim that he can
control TRIAC dimming with a 1000hz signal not synchronized to the zero
crossing but none that I can divine that support the claim of actually
deterministically providing many different dim levels without flicker.

1) His actual trigger (control) signal has a significant component of 60hz AC
noise ("hum") and so is in fact inadvertently synchronized to the AC line

2) His dimmer circuit skips entire half-cycles. This would happen if his
trigger repeated at less than 120 hz. One could, for example, make an
approximately  50% dimmed by repeated cycles of a 8.2 millisecond burst of
1khz square wave followed by a low signal 8.4 millisecond in duration, thus
only triggering on (approximately) alternate 1/2 cycles. This is why I pressed
him on the flicker issue. (We have been assured that his TRIAC-based dimmer
system meets rigorous theoretical and empirical tests for flicker.)

3) The dimmer is in his head and hasn't been instantiated.

... Marc
Marc_F_Hult
www.ECOntrol.org