"David" wrote in message
. net...

"Arfa Daily" wrote in message
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"David" wrote in message
et...

But be aware that in general, these cheap 'power' meters are expecting
'traditional' sine-draw loads. I have seen wildly inaccurate standby
figures being given for equipment, by eco-campaigners that have been
let loose with one. A lot of modern equipment that makes use of switch
mode power supplies, handles standby mode by brief bursts of full draw
operation. This can confuse a simple power calculating algorithm that's
expecting continuous draw. Also, the draw by many cheapo switchers is
very asymmetric and 'dirty', and may also not produce a true reading.

I wouldn't connect a UPS to a fridge. These things are notorious for
pulling a short-term startup current of tens of amps, as they get the
compressor turning over. The UPS would probably fall over before being
able to supply this, and might, as someone else suggested, even sustain
damage.

Arfa
I have a "Kill-A-Watt" and it appears to display true RMS for voltage
and current readings. The voltage reading for a modified sine wave from
a standard UPS displays the actual RMS voltage. I do not know what crest
factor it can handle but displays power factors of considerably less
than unity for most consumer electronics with DC rectifiers off of the
line voltage. It is a very nice unit for the price.

David


But that's my point. You can't have a 'true' sine-based RMS figure for
power consumers that draw a pulsed current. Just because it is a sine
wave that's *available*, it doesn't mean that the load will draw anything
like a sinusoidal current, from it. The only items that will are those
that are totally 'passive' in nature, such as light bulbs. Even power
supplies that are transformer based, are likely to draw current in pulses
from the available sinusoidal supply, and switch mode power supplies, on
which most modern consumer electronics are based, most certainly won't
draw a sinusoidal current from the supply.

Arfa
Arfa,
I know that the current draw on these things is anything but sinusoidal.
My point was that the "Kill-A-Watt" seems to actually compute the RMS
value for that complex current waveform as well as non-sinusoidal voltage
waveforms.

David


OK. I'm not sure that 'RMS' is the right term to attach to any value derived
from a ragged-arsed waveform, as it is a mathematical function normally
associated with symetrical waveforms, which the draw by a SMPS may very well
not be, but I see what you're saying.

What I am trying to say is that a chip which is designed to produce an RMS
reading from a sine wave, may well produce a meaningful figure from a
non-sinusoidal waveform also, but *only* if it is still symetrical.

Arfa