Confused by "Power Factor"

[Andy Wade]

They're not all the same.

I think it's been reported here before [by Andrew Gabriel] that the Brennenstuhl one gives quite inaccurate results on low power factor loads.

I have the Maplin one (L61AQ) and that seems to give reasonably accurate results on low power factor loads. Its main limitation is that it's not too great for measuring very low powers and currents (< 5W) due to a lack of resolution and the presence of slight zero offset. But for a cheap thing (£12.50 I think I paid) it's amazing value.

[Andy Wade]

But a moving-coil meter (with rectifier) - i.e. an AVO-type meter on an AC current range - responds to the mean current and is scaled in RMS amps on the assumption that the waveform is sinusoidal. On the peaky current waveform that you get with SMPSs (and other PSUs with a rectifier and capacitor input filter for that matter) the AVO will significantly under-read the current. The same goes for most cheaper DMMs, and even some quite expensive ones. Generally you can assume that if a AC ammeter meter doesn't loudly claim "true RMS" on its front panel, then it isn't.

[Pete C]

Try measuring small inductive loads on top of a larger know resistive load, IME it measures the additional load much more accurately than on it's own.

cheers, Pete.