"RogerN" wrote in message
m...
"Tim Wescott" wrote in message
...

On Sat, 01 Feb 2014 17:19:56 -0600, RogerN wrote:
snip
Anyone here mess with signal conditioning for current
transformers?
Seems that I need a "precision rectifier" circuit and a capacitor
to
charge up to the peak, then scale to RMS amps.

Do your A to D conversion at the 3rd or higher harmonic of line
frequency
(higher really is better), square, and average. Then take the
square
root, and you have RMS.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

I did some checking, the Arduino's AnalogRead() function is supposed
to take around 100uS. I'm thinking I can take a A/D reading ~641uS
apart and get 26 readings in 16.666mS. Then do the RMS calculations
from the readings. This should be great because I should be able to
take the voltage from the burden resistor to provide 2V P-P at full
scale. Note I'm putting a 100A CT on a 15 A circuit so I should
have around 1/10 amp resolution per count (10 bit). If this doesn't
read well, I'll get a lower amperage CT.

So thanks for the idea! That saves an Op Amp rectifier and filter!
I never thought of doing the calculations right from the AC signal.

RogerN

This gives a hint of how the Kill-A-Watt measures power:
http://www.ladyada.net/make/tweetawatt/