Re: Controlling Holiday Lights

[Marc_F_Hult <MFHult@xxxxxxxxxxxxxxxxxxxxx>]

On Mon, 23 Oct 2006 23:32:07 -0400, Marc_F_Hult
<MFHult@xxxxxxxxxxxxxxxxxxxxx> wrote in message
<m82rj2966h57g91kt8pmh6l1va51b0mnt5@xxxxxxx>:

>On Mon, 23 Oct 2006 18:06:07 -0600, sylvan butler
><ZsdbUse1+noZs_0610@xxxxxxxxxxxxxxxxxxxxx> wrote in message
><slrnejqm7f.a3i.ZsdbUse1+noZs_0610@xxxxxxxxxxxxxxx>:
>
>>On Mon, 23 Oct 2006 11:25:18 -0400, Marc_F_Hult
><MFHult@xxxxxxxxxxxxxxxxxxxxx> wrote:
>>> Or are you talking about lamps or LED's  operated on DC?
>>> Is so, there are DMX512->PWM and 0-10vdc analog-> PWM converters that
>>> modulate DC.
>>
>>I'm modulating D.C., or rather, creating modulated D.C.  The technique
>>of modulating the D.C. is PWM.  That D.C. is then used to switch a
>>triac to control A.C. loads.  Phase synchronization is mainly needed to
>>control R.F. and sometimes to control switching transients.  With fairly
>>low power loads (typically under 200 watts) R.F. is minimal and
>>controllable in other ways.  Switching transients aren't significant
>>with primarily resistive or incandescent loads.
>
>ROTFL
>
>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
>doing (as I understand it) simply won't work. Everything that can be done to
>control a TRIAC can be done with no more than 120 low-to-high transitions
per
>second. Having the ability to switch at 1000 hz is intrinsically no more
>useful than  120. It is _when_ during the cycle with respect to the
>zero-crossing that the SCR is turned ON that determines the dim level.
>
>>> There are of course many DMX512--> AC dimmer and 0-10vdc --> AC dimmers
>>
>>You seem to be confusing how the dimmer is controlled (with DMX or
>>0-10vdc or...) with how the dimmer controls the A.C.  I'm doing the
>>latter.  Not real interested in the former at the present time.
>
>No, it's all quite clear to me. DMX can change the 0-255 dim level setting
up
>to 44 times per second. Based on the 0-255 level instruction it receives via
>DMX (which can change up to 44 times per second) the dimmer module (in the
>case of household AC lighting) sets the dim level of the TRIAC/SCRs by
either
>turning the TRIAC into conduction at 0 and 180 degrees (= Full ON), at some
>phase angle 0 < a < 180 and 180< a < 360 (= dimmed"), or not at all (= full
>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.
>
>So being able to switch at 1000hz or 10000hz or a gigahertz has no intrinsic
>advantage over switching at 120hz (at the right times) in controlling
>TRIACs/SCRs. But only being able to turn the TRIAS on at 1000hz/120hz = 8.33
>(dimensionless) different phase angles (your 1000hz case)  means that you
>will only have a maximum of 1000/120 = 8.33  different dimmer settings
>(assuming that you are synchronized with zero crossing, which you don't seem
>to be). In contrast DMX512 has 255 different dim levels (my previous
comments
>about dimmer curves, preheat and un-useful phase angles not withstanding).
>This is the same as I wrote before. What you need to grok is that a TRIAC is
>not turned OFF when the TRIAC gate control signal goes low (= OFF). So
>turning the control signal ON-OFF does NOT turn the TRIAC ON-OFF lock-step.
>
>... Marc
>Marc_F_Hult
>www.EControl.org

To expand and clarify: In order to provide the gate control signal for a
TRIAC/ Back-to-Back SCR  dimmer with the 256 dim levels achieved by DMX512,
your fixed-frequency scheme needs to operate at a frequency of 120*256 =
30720 hz.

So your 1000hz target, which you suggest/imply is itself extravagant, is a
factor of 30 slower than what is needed to accomplish what off-the-shelf,
(relatively ;-)inexpensive, globally accepted, ANSI-standard,
http://www.usitt.org/standards/DMX512_FAQ.html  DMX-512 lighting can do.

You can change the arithmetic co9mpletely by timing the gate control voltage
with the AC line zero crossing, but if you do that with a PC, all you've
really done is create the world's largest (? I dunno ;-) dimmer controller at
a substantial cost in time expended, size, power consumption, complexity, and
difficulty of repair and maintenance by others.

'Course I do things related to HA just as time-consuming and idiosyncratic,
so no criticism is implied ;-)

... Marc
Marc_F_Hult
www.ECOntrol.org