reh: burden resistor sizing

This post was moved from the old forum:



I have a circuit board to take inputs from 4 CT sensors - it is basically 4 copies of the single CT sensor circuit. I want to have different inputs sensitive to different current ranges, but am having some trouble choosing the burden resistors for low current values (i.e. the most sensitive input).

using the same CT sensor and a test 8.6A purely resistive load, i get the following results (the first column is the burden resistor in Ohm as directly measured on the board; the second is the minimum-maximum reading from the Arduino when using an RMS 8.6A current; the third is the consequent calculated maximum RMS current that can be read; the fourth is this range time the burden resistor value):

burden		min-max A/D	max current range	range x burden
30.2		106		83			2509
51.4		157		56			2883
102.7		259		34			3492
997		340		26			25823

According to my reading of the detailed system design page, the last column should be a constant (related to the number of turns in the CT sensor and the bias voltage). The agreement is pretty good for the first couple of lines, poor for the third, and useless (out by a factor of 10) for the last.

Is there a more accurate way of calculating the burden resistor value, or should i just use trial and error?