Re: Controlling Holiday Lights

[Marc_F_Hult <MFHult@xxxxxxxxxxxxxxxxxxxxx>]

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