 # question on measuring ac current

from your experiences what is the easiest wy to calculate the rms current@60hz? Could I just meausre the current at the peak of the cycle and divide by 1.41 and get an accurate number? Or should I have to measure it at multiple points and do a little more math? The only reason I imagine the first wouldn't work is error due to the power factor, could I perhaps figure out the phase angle differance and get a really accurate number? But I guess the second method would be the same ?

"Could I just meausre the current at the peak of the cycle " how are going to measure the current ?

winner10920: from your experiences what is the easiest wy to calculate the rms current@60hz? Could I just meausre the current at the peak of the cycle and divide by 1.41 and get an accurate number? Or should I have to measure it at multiple points and do a little more math? The only reason I imagine the first wouldn't work is error due to the power factor, could I perhaps figure out the phase angle differance and get a really accurate number? But I guess the second method would be the same ?

AC measurements are not DC measurements. One difficulty with this method is that unless you are assuming the load will be a pure resistance and can therefore use simple ohms law derived calculations, it's much more difficult then it may seem. In reality you must consider the load impedance which may very well find that current and voltage are not in phase (and either leading or lagging current flow) and therefore one has to deal with the fact that there is an 'apparent' current and a power factor correction need for accurate general purpose AC power consumption measurements. One only has to observer once the AC current draw waveform from a typical PC switching power supply to see the complexity one may need to deal with. I will not try and provide a 'solution' for your use but rather suggest you fully define the measurement application you are aiming for and to then determine if simple ohms law calculations are going to work to met that application requirement. A robust AC power measurement system requires either pretty complex external component/module help or pretty sophisticated DSP type signal analysis looking of both the voltage and current wave forms.

Lefty

Could I just meausre the current at the peak of the cycle and divide by 1.41 and get an accurate number?

That approach works if the current is perfectly sinuous. Otherwise, you are out of luck.

I would adc a full cycle and calculated the square root of the sum of the current squared. The resulting figure is perfectly proportional to the current's rms.

edn has a nice analog multiplier using optocoulers. Very nicely done.

I've got an analog device that can measure the current up to 20khz, so 60hz should be no problem profiling the current along the cycle, but that's a good idea I am kinda curious now on what the actual current looks like, How many samples would you suggest from the cycle to get an accurate enough reading?