Andrew Gabriel wrote:
snip
Averaging a SMPSU current pulse or failing to take into account
it only happens when mains voltage is around 340V would give
an under reading. I can't imagine what mistaken assumption
they've made which results in such a load 3 times over reading.


To quote from an earlier post I made on this.
"

A quick patch involving a 1.8 ohm resistor in the neutral lead and
resistive
dividers protected by zeners indicates that the current has a really
odd shape.

The couple of switchers I checked exhibited the expected short
half-sine
looking pulses at maximum voltage.
The power indicated by the meter was within 20% of my eyeballing the
scope.

On trying it on my PC, I found a rather odder shape sort of sharks-fin
like shape.
The current rises to a peak of 2.2A in .4ms, then decays over the next
couple
of milliseconds more or less linearly to 0.6A, then drops in 0.5ms to
0.

That's a total of 2.25Ams per half-cycle, at 320V, that's .721Ws per
half-cycle, or 72W when idle.

The meter however reads 138W.

And in suspend, it's around 30W, around 10W 'off'.

"

It may well be assuming that the current will be either a 'normal' sine
wave, with a power factor of whatever or a sharp chopped-sine pulse near the
wave peak.

This worked fine, until various regulations about harmonic content on power
supplies came in.
Then the PSU makers started working out the cheapest way to meet these
directives.
This ended up as very odd waveshapes.

As to why.
Well, let's assume that it samples at the peak of this pulse, 2.2A.

Let's try 2.2A * 2ms = 4.4mAs.
4.4mAs * 320V = 1.4J
* 100 cycles per second = 140J/s, or 140W.

Surprisingly this works quite well.

However, I suspect the real answer is that it's trying to make a nice smooth
wave out of it.
To get an accurate number, you're going to need to sample at better than
1Khz.

Alternatively, if it's sampling at 500Hz or so, and trying to assume that
the waveshapes may be symmetrical (be it intentionally, or through filters)
then it may get vastly misleading answers.

Especially as we're not talking about a 2000 quid scope that's had hundreds
of man-years spent on the design, and verifying it's correct, but maybe a few
hundred hours and checking on a few loads around the office.

Anyway, hypothesising on how it fails is kind of pointless, in the face of
the fact that it can't be repaired (practically)