Measuring difference current between live and neutral?

[Flyback]

Hi,
We have live and neutral wires going through a torroid aperture, each carrying 16A maximum
The torroid is wound with a 100 turn coil.

Whenever live and neutral currents are not the same, there is obviously an output on the 100 turn coil.
We wish to tell when the live and neutral conductors are >30mA (RMS) different. (ie, its an RCD function)

Whats the best/smallest circuit to allow this measurement? (measuement will be by ADC of micro)?

We obviously cant diode rectify the coil output it as the diode drop would give inaccuracy.
Obviously when the live and neutral are 30mA in difference, we will get an output of 300uA out of the coil.

 

[rjenkinsgb]

A load somewhere around 4K7 to 10K, and an opamp precision rectifier, then into the ADC or a comparator.

See Fig. 4 on this page for a suitable full-wave precision rec.

Precision Rectifiers

ESP Circuit Ideas - Precision Rectifiers

sound-au.com

Note that in practice, a bridge rectifier _before_ the load should not have any significant effect on the voltage across the load resistor, as it is a current-based circuit.

 

[crutschow]

We obviously cant diode rectify the coil output it as the diode drop would give inaccuracy.

That's true for a voltage source.
But as rjenkinsgb noted, the output of a current transformer looks like a high-impedance current source, so rectifying the output with a diode will have little effect on the resistor load voltage, since it won't change the current.