Power Meter accuracy

"*" indicates a correction to an error (usually tpyographical).

Like that.

An error of 2W is obviously wrong, and was hopefully only a typo.

Tim

Line voltage is usually 230 +/- 20 or around ~10% variation.

So the dynamic range is limited by the current measurement. This tends to suggest that they actually use a 12 bit ADC in the common ones over here since they will measure 1W and max out at ~4kW.

Clamp on ones are much cruder and good to about 10-20W but with a much higher maximum to measure the entire domestic consumption.

Regards, Martin Brown

He was apparently assuming that 10-bit quantization of the current waveform limits power resolution to about 1 part in 1000. We are talking about an AC line power meter, where the voltage doesn't change much, and a 10-bit ADC is used to sample both the voltage and current waveforms.

(Funny, I designed a 6-channel "power strip" meter recently, 10 bit mux'd ADC, with ZigBee communications. I charged $1 for the design.)

A power meter needs extra headroom for the 1.414 peak thing, and more for non-ideal waveforms, so the quantization thing is even worse than it looks. On the other hand, if you dither the current signal before you digitize it, the quantization disappears it's easy to resolve one watt out of 20KW.

John

What exactly are you trying to measure, real power or apparent power ? Is it assumed that the current (and/or voltage) looks like a sine wave?

While a constant amplitude clean sinusoid voltage connected to a pure resistive load will produce an in-phase sinusoid current, measuring the peak current (and assuming nominal voltage) could be used to give some real power indication. Measuring the current with 10 bit ADC could give a 1000:1 power range (such as 2 kW:2 W).

Any load with some reactive components would cause a phase shift between voltage and current and the simplistic system would only produce the apparent power. Any rectifiers would create a large peak to average current ratio, so the 1.41:1 peak/rms ratio for a sinusoid is no longer valid.

For real power, you must multiply the instantaneous voltage with the instantaneous current and average out. This can be done by an analog multiplier followed by analog averaging before the ADC or use separate ADCs for current and voltages sampled at a high frequency and do the multiplying and averaging in digital domain.

With rectifier loads, the peak current can be quite high, often much higher than the fuse ratings and the harmonics on the voltage can cause long duration peaks higher than the fundamental frequency. Thus the signal conditioning, multiplication and averaging require a quite large dynamic range, regardless if the most processing is done in analog or digital domain.