Re: Water heater eating X-10 signal

[ddl@danlan.*com (Dan Lanciani)]

In article <9iqVh.322422$5j1.206155@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>, JeffVolp@xxxxxxx (Jeff Volp) writes:

| I thought a bit more about this since my earlier post.  A 240V load should
| be on its own circuit. There shouldn't be any "signal suckers" on that
| circuit other than potentially the 240V load itself.  So the starting signal
| will be the differential level at the panel.  In an installation with
| balanced loads, a repeater would produce essentially the same signal level
| on both legs.  So, that could present a problem for 240V loads if both
| phases were driven in phase.

Yes, I suspect this is why they decided to give the advantage to 240V
modules.  The "difference of large signals" argument applies just as
well from the point of view of 120V modules with the legs driven out of
phase, but it is probably even more unlikely that the drop on one leg
will be exactly matched by the net coupling from the other leg through
the various 240V loads.  As I've often said, the analysis here is more
complicated than it might at first appear. :)

| The differential line pair only refers to driving a 240V load, which you
| agree is rare.

I agree that it's rare in my house.  If I had a few 240V modules that
weren't behaving I might sing a different tune.  Having a switch at
least gives you a choice.

| I think the better approach is to provide best performance
| for normal 120V loads, which are probably over 99% of X10 devices.  I don't
| like the possiblity of a 240V resistive load possbily causing havoc with
| signal levels.

It is not at all clear that this is common occurrence, though.  If (as
I suspect) it turns out that all commonly available repeaters drive the
legs out of phase we would probably have to do a lot more investigation
to determine why it (may have) happened in this case.  Also, keep in mind
that driving the legs out of phase does not make 240V loads a special
problem wrt reducing the signal; they will have approximately the same
effect as two 120V loads of similar power.  If a 240V load causes a
problem it may simply mean that the signal levels were already on the
edge.  Driving the legs in phase mostly takes the 240V loads out of the
picture, but not necessarily without a cost:

Consider that driving the legs in phase means that every 120V load will
be coupling the carrier in-phase to the neutral, reducing the apparent
leg-to-neutral signal available to all 120V modules.  In a distribution
system where the 120V loads are reasonably well distributed between the
legs the loss of the ability to cancel the carrier current in the neutral
may dominate any advantage gained by taking 240V loads out of the picture.
It would take a lot more analysis to convince me that driving the legs
in-phase is in fact (at least on average) the way to provide the best
performance for 120V loads.

| If the installaton does include a 240V load that has a
| problem with signal levels, it could be addressed by placing a .1uF
| capacitor to neutral on one of the hot leads to imbalance the signal levels.

You'd probably have to put the capacitor at the panel since pure 240V
loads don't normally have a neutral available.  In addition to the code
issues you wouldn't have the advantage of the series impedance of the
line feeding the device to help drop the level, so you would be trying
to unbalance the whole system.

| That was an easy test to do - just confirmed an old Leviton 6299 does flip
| the phases.

That certainly confirms the behavior I saw.

| Interesting is that its peak coupling is at 134KHz.

Maybe just poor QC?

				Dan Lanciani
				ddl@danlan.*com