In uk.d-i-y, wrote:
Mike Barnes wrote:
In uk.d-i-y, Ian Stirling wrote:
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.

Presumably if you measured total energy consumption over a long period,
things would even themselves out. But how long, I wonder? My Maplin
meter stops at 9999 kWh.

But *how* do you measure "total energy consumption over a long
period", that's the original problem! If the meter's view of
instantaneous power consumption says 1kW thenit's going to consume
(according to the meter) 9999kWh in 9999 hours (unless the
instantaneous reading changes of course).

If the draw looks like a sharks fin, then the instantaneous reading
*will* vary won't it? Unless the sample times are synchronised to the
wave form.

Never mind. I'm quite happy to believe that the readings are unreliable
for PC power supplies etc.

--
Mike Barnes