Arfa Daily wrote:
"John Williamson" wrote in message
...
Arfa Daily wrote:

Sorry to butt in here, but when I was studying such things, the RMS value
of a current or voltage waveform was calculated by working out the area
inside the curve plotted over a full cycle, which then allowed you to
calculate an equivalent DC value. This involved counting squares on graph
paper of the plotted waveform or similarly counting squares on a
calibrated oscilloscope tube face. The earlier & most of the current cheap
meters that give an alleged RMS reading take a peak reading & apply a
correction factor of 0.707 to it (1 divided by the (near enough) square
root of 2), as that gives the right answer with a clean sine wave, which
is what most of these meters are used to measure. (Mains power round here
is near enough a pure sine wave that you can ignore the error, as it's
less than the accuracy of the meter)
The RMS value of a (theoretical) pure square wave is exactly the same as
the average of the absolute values of the positive & negative peaks, as
the value is either fully positive or fully negative, with, in theory, no
other value being present.

The most (theoretically) accurate way to measure RMS values is to use a
hot wire meter, which doesn't care what the waveform is, it just measures
the heating effect which is more or less frequency independent & includes
any DC offset automatically.


Tciao for Now!

John.

Yes, all agreed, but the shape of the mains waveform is immaterial, unless
you are talking a purely resistive load like a light bulb. The point that I
have been trying to make all along is that when you are trying to measure
power, it's a function of both voltage and current draw, and in the case of
a SMPS, especially one that's in standby mode, the current draw from the
mains supply voltage, is very likely to be anything *but* sinusoidal.

Arfa


It's also likely not to be a simple product of the RMS current drawn &
the RMS voltage of the supply, as the input circuitry contains reactive
and regulatory elements that alter the phase relationships between the
current & the voltage in a manner that can't necessarily be predicted
easily.
Most of the ones I've looked at have a rectifier across the mains feed,
with inductive & capacitive elements in the circuit before the
rectifier, with a SMPS pulling power from the rectifier after a
smoothing filter. This gives a power factor that varies with load,
possibly cyclically even at steady load if the oscillator of the SMPS
isn't locked to the incoming mains frequency.
Then the invertor on the output just runs off the DC from the battery
pack/ SMPS combination.

As you say, though, the shape of the mains waveform is immaterial, apart
from harmonics altering the power factor by altering the relative
impedances of the inductors & capacitors.

Tciao for Now!

John.