Leo,

I am not certain if you and Jon are using the '5 Amp' rating in the same way. Usually it is an RMS rating, with the corresponding Peak-to-Peak value being 2.8x larger. So, I would expect you to need a load resister of 0.2 Ohms. That could be 5 resistors in parallel, each of 1 Ohm value with a dissipation rating in the range of 2 to 5 Watts (per resistor). That, in turn, would produce 1.0 volts RMS, or 2.8 volts P-P. If a single load resistor failed in the 'open' condition, it would create 3.5 volts P-P (which would saturate the mbed ADC pin, but not over-load it). If you need to measure larger (transient) currents (from start-up loads, lightning strikes, etc.) the load resistor must be correspondingly smaller. It is also true that you should plan on 'snubbing' out-of-range transient signals at the ADC input with a buffer circuit (such as Jon's) or with a limiting resistor and catch diodes. Such snubbing was also discussed earlier in a thread on using a mbed in an automotive application.

Jon's circuit has a 0.45 Ohm load resistor a 10x gain, so the current levels he can accurately measure might be just a fraction of the 5 Amp (RMS?) rating.

If you measure over a 100 mS interval the RMS calculation will avoid 'partial cycle' effects from both 50 Hz and 60 Hz sources. That advantage fades as the line frequency strays further from either nominal. Measuring at a 1280 sample-per-second rate gives 128 samples each 100 mS, which will allow the RMS calculation to accurately incorporate (out to at least) the 9th harmonic of either 50 Hz or 60 Hz.

The easy and straight-forward way to calculate RMS is to first calculate the average of all the samples in an interval. Next, sum the squares of the differences between each sample and that average. Finally, divide that sum by the number of samples in the interval and take the square-root of the quotient.

The effort to get RMS via a FFT is probably only needed when you also want include or reject certain harmonics from the resulting RMS value. For 'true' RMS the straight-forward method includes all harmonics (which is what is needed).

Note that it is possible to re-arrange the straight-forward calculation so that most of the work can be spread over all the samples, instead of all-at-once at the end of the interval but before the next sample.

I'm looking to measure the power output from a 1600amp CT. according to the spec, the secondary output of the CT, when at full 1600A load, is 5A. Now I can convert that output current to a voltage that the MBED can read through analogin by adding an appropriate burden resistor to the CT output with, say, a 0.5ohm resistor.

My question is, is it safe to connect that input to the MBED's analogin. I'm still learning about this stuff and it sounds to me like 5A would be far too much current!

Thanks....