DIYbanter - View Single Post - Electric power consumption measuring device

wrote:
Andrew Gabriel wrote:

I bought one of the older ones Maplin stocked a few years ago,
and that's hopeless on non-resistive loads (out by a factor of
3 on SMPSU's -- too high, which is a puzzling way for it to be
in error).

If it does not sample fast enough, then it may well overestimate
current consumption.
snip
Isn't it as much to with low power factor as anything. The
cheap/simple power meters simply measure average (and I mean average,
not RMS) voltage and current, then multiply the two together and apply
the right factor to convert to RMS and say the product is power. It
matters not one whit how fast your samples are if you do this, the
answer will always be wrong.

Even the cheap ones do power factor.
I'm fairly confident that most of these devices have a single channel
8 bit A/D (minimum current is typically 40mA = 13000mA/256), sampling at
400Hz or so. (or maybe 50Hz, with undersampling, I'm not sure.)

Add a 4 bit micro, and an LCD, and you've got a power meter.

A 0 power factor device can have a very spiky current draw.
My PC power supply draw looks like a sharks fin, with a very sharp rise, and
a slow decay to a fast fall.

Typical older SMPSs with a bridge rectifier feeding a capacitor (which
read correct) have a current draw that looks like the top of a sine wave.