Re: Motion Sensor Light for Front Entrance

["Jeff Volp" <JeffVolp@xxxxxxx>]

"Dave Houston" <nobody@xxxxxxxxxxxx> wrote in message
news:4ca0ad4c.19629062@xxxxxxxxxxxxxxxxxxxx
> Rather than further spaghetti-ize things by trying to respond inline to
> your
> inline comments, I'll just p*ss off all the anti-top-posters and try to
> respond to your major points here.
>
> Inline ammeters only work for purely resistive loads where voltage and
> current always remain in phase with each other. With reactive loads
> (inductive or capacitive), voltage and current are 90° out of phase,
> requiring multiple, simultaneous measurements. Power=Voltage*Current so
> averaging each doesn't work. You have to average the products of the many
> readings.
>
> Non-linear loads are usually those related to power suplies with diodes
> that
> tend to draw current only at certain points in the voltage sine wave.
> These
> also require multiple, simultaneous measurements.
>
> I'll leave it to Jeff Volt to explain why you had the problem with the
> XTB.

Most inexpensive X10 devices have transformerless power supplies, which use
a capacitor to drop the line voltage to the low level needed by their
electronic circuitry.  The charging current is reactive, which is not
in-phase with the applied voltage.  The Kill-a-Watt has two ways to measure
power consumption - watts and VA.

Watts is the "real" power that you pay the electric company for.  VA (volt
amperes) is the average of the real-time multiplication of voltage and
current over the entire AC cycle.  Purely reactive loads will draw current
charging up at one point in the AC cycle, and dump that energy back to the
poweline elsewhere in the cycle, resulting in no "net" power consumption.
However significant current can be drawn during the charge and discharge.

The amount that VA differs from watts is a function of the power factor of
the device.  When the power factor is 1.0, the VA and watts will be
essentially the same.  At lower power factors, the numbers can differ by
large amount.  For a Maxi Controller, the Kill-a-Watt reads 1.0 and 10 for
watts and VA respectively.  Even though the Maxi Controller label says it
only consumes 2.5W, the low pass filter in the XTB must deliver the same
current as if a 10W load was plugged in.

You can think about this in another way.  If you stick a big capacitor - say
2.2uF - into an AC socket, it will pull about the same current as a 1200 ohm
resistor.  If that were "real" power (watts), the capacitor would dissipate
about 12 watts.  However, the capacitor won't even warm up because the
current is "imaginary" (90 degrees out of phase with the applied voltage).
I just verified this with my own Kill-a-Watt.  It measured 0 for watts, and
11 for VA with a 2.2uF capacitor.

The power supply in a X10 transmitter functions a lot like that capacitor.
The only "real power" that the Kill-a-Watt measures is that actually
consumed by the module electronics.

It the case cited earlier wherein the XTB low pass filter was overloading by
removing one of the 5 loads, that one load had a power transformer.  So, its
"imaginary" inductive current was opposite that of the other X10
transmitters, partially canceling out their effect.  When that load was
removed, the current pulled by the 4 remaining capacitive loads (about 40VA)
exceeded the rating on the XTB low-pass filter inductors.  This is similar
to the issue that the power company deals with by placing capacitors in
their distribution network to balance out the inductive reactance from the
various motor loads.

I realize that this may be a difficult concept for some.  Hopefully, the
capacitor example above will help you understand the issue.

Jeff