Re: Making an X-10 lamp module immune to dimming

[isw <isw@xxxxxxxxxxx>]

In article <1bOdnbqFcqTijtTVnZ2dnUVZ_qbinZ2d@xxxxxxx>,
 "Robert Green" <ROBERT_GREEN1963@xxxxxxxxx> wrote:

> "Jeff Volp" <JeffVolp@xxxxxxx> wrote in message
> news:08F1k.18467$102.2347@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> > The delay of that pulse with respect to powerline zero crossings determine
> > the brightness.  It is not clear how they adjust the phase, but it is
> probably
> > done by some sort of timer in the IC whose count for each half cycle is a
> > function of the dim level.
>
> If the line driving the triac is not pulsed in any way, but is constantly
> powered, I thought that the output of the triac will always be 100% but
> obviously it's more complex than that.

No; you are correct.

Absent a drive signal to the gate, a triac does not conduct at all. When
a gate signal is applied, and if there is a voltage across the triac, it
will turn on (start to conduct). Once it is conducting, the gate can no
longer control it (it can not be turned off by the gate). At the next
zero crossing, the triac will stop conducting until a signal is applied
to the gate again. There are other weird conditions that can make the
thing turn on, but they don't affect this discussion.

So, by delaying the gate signal with respect to the zero crossings, the
triac can be turned on for any desired percentage of time.

> Not only is the light output chopped
> each A/C cycle, but it seems that from Isaac said that they number of pulse
> occurring during each A/C cycle is chopped into smaller pulses.  More
> pulses, more light.

No. The "earliest" pulse gates the triac; the others don't do anything.
I don't know why the others are there. Maybe it's just an artifact of
how the chip does phase control.

> Might that be to keep the triac cooler than it would be
> with longer pulses?

Triacs "keep cool" automatically, because of the way they work. When
off, there is no current flowing, so no heat. When on, there's very
little voltage drop across them (less than a fully saturated transistor)
and so again, very little energy is turned into heat. The only time they
show much dissipation is during the time they are switching from off to
on, and they are designed to make that happen as fast as possible --
which is precisely why they generate electrical noise.

> Why would they do it that way instead of a variable
> width pulse that lengthed as the brightness was turned up?

That would work; as I said, I don't know why the X-10 unit uses a string
of pulses. Most inexpensive "lamp dimmers" use a very simple circuit
that drive the triac with a varying width pulse. In higher power
circuits, it's important to get the device turned on as fast as
possible, and doing that requires a very fast, high-power drive signal
which, if it were applied all the time, would destroy the gate. Usually
that is provided by a single very short but high-energy pulse.

> I've been reading about triacs all night and it says at Wiki: "Triac is one
> of many power-enhancing Mini-Cons who help the Autobots search the galaxy
> for pure energon ore."  No, wait, that's: "It should be noted that once
> avalanche breakdown has occurred, the thyristor continues to conduct,
> irrespective of the gate voltage, until either: (a) the potential VG is
> removed or (b) the current through the device (anode-cathode) is less than
> the holding current specified by the manufacturer.
>
> So it seems that even keeping the gate high all the time wouldn't work

That would cause it to turn on shortly after each zero crossing, and
stay on until the next. If what you want is a silent on-off switch (like
a relay), that works just fine.

> It
> seems that one of the mods extracted the ZC via an optoisolator and that
> implies that the triac has to be pulsed in phase with the AC cycle for it to
> allow current to pass.

It does need to be synched to the AC phase, but not "to allow current to
pass", but rather to vary the percentage of time the current is passing;
if you pulse that gate, say, at each voltage peak, the triac will
deliver power for the second half of each half cycle. Remember, it turns
itself off at every zero crossing.

> That's not how I thought of a triac, conceptually.
> When the author warns "this circuit only works if line current is fed into
> pin 6 of the optoisolator. If you hook the circuit up wrong, the
> zero-crossing circuit will not detect the voltage zero crossing and will not
> trigger the alternistor" it seems to imply that to pass A/C current, the
> triac must be synched to deal with the constantly reversing polarity of the
> AC current.

Triacs will conduct for both polarities of the AC signal; their cousins
in the thyristor family, SCRs (Silicon Controlled Rectifier), will only
conduct for one polarity, and then only when a gate signal is applied.
Because of the way they are built, SCRs can handle hugely greater
currents than triacs (thousands of amperes or more). When it's necessary
to control that sort of power, two SCRs are connected back-to-back, and
the gates are driven alternately, on alternate half cycles.

Isaac