Re: Motion Sensor Light for Front Entrance

["Robert Green" <robert_green1963@xxxxxxxxx>]

"Jeff Volp" <JeffVolp@xxxxxxx> wrote in message
news:aPsoo.184659$Bh.13615@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> "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

Thanks for the detailed explanation.  It's actually quite suprising how much
I've learned about the X-10 transmission process just following this group.

--
Bobby G.