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

[isw <isw@xxxxxxxxxxx>]

In article <F7udnWa4cqAYo9fVnZ2dnUVZ_rCtnZ2d@xxxxxxx>,
 "Robert Green" <ROBERT_GREEN1963@xxxxxxxxx> wrote:


--snip--

> I am still a bit confused about one thing.  When the AC waveform crosses
> over, does current through the device drop to zero and the triac stops
> conducting simply because there's no current at that point on the sine wave?

Precisely.

> Does the triac have to be "retriggered" at that point and after each
> subsequent recrossing or does it start firing again as soon as current
> begins to flow again in the opposite direction?

It will not conduct again until the proper amount of gate current is
applied, while there is a voltage across the device.

> I have another question.  When taking apart the lamp module, I noticed that
> the tiny triac has an enormous aluminum heat sink.  What process generates
> the high heat developed?

Mostly, it's the voltage drop across the chunk of silicon that forms the
triac, although some of it may be due to the tiny wires that connect the
chip to the external leads.

A transistor is a three-layer device (NPN, say); a SCR is a four-layer
device (NPNP), and a triac comprises five (NPNPN or the other way
around). Interestingly, because of the way thyristors work, even though
they have more layers than transistors, the voltage drop across one when
conducting is lower.

But still, whatever that drop is, multiplied by RMS current that is
flowing, creates heat.

If you think about it, the worst case is when the device is "half on",
because the voltage drop is much larger. One reason to use a triac or
SCR instead of a transistor is because they sort of "automatically" go
through that halfway state very rapidly. Another is that the voltage
drop when conducting is lower.

Isaac