Measuring current on 230V line with pecision of 1mA

[epekaric]

I would like to enquire about possible methods of measuring current precisely with accuracy u to 1mA from a 230V power line.

I am working on a Solid State relay board that should control devices supplied by 230V. The problem is that I need to monitor (and measure with good precision) the current flowing on those devices in the range from less than 10mA to 0.7A of AC 50Hz current.

Normally I would simply use a current sense amplifier with a shunt resistor, but this time I am measuring at common mode voltages that reach 230*√2 volts so I need some form of isolation. Also the price should be as little as possible.

So far I have been looking into the following measurement methods.

1. Current sense transformers:
Bulky and fairly expensive, also innacruate. Designed for large currents of more than 1A. I doubt I could discern currents of several mA.

2. Hall effect current sense:
Easy to use, but expensive and also have limited accuracy, cca 50 mA or less.

3. Isolation amplifier with shunt resistor
Fairly expensive and complex, but seems to be a way to obtain desired accuracy. Many problems with offset, gain error...

4. Simply using a transformer to get the shunt resistor voltage from 230VAC to lower levels and then using current sense amplifier.
Seems like the best soultion, but i have big trouble finding the right transformer. Something fairly cheap in industrial temp. range that can work at 50 Hz frequency.

Please, I would like to ask those that know to recommend how to solve this measurement problem. Am I missing something?

 

[mikersia]

If you don't need to have an instant current value and average or RMS value is enough, try to use ICs for power measuring. In that case you have a choice with different accuracy, programmability, etc. A sensor could be shunt, IC can be supplied from line with simple transformerless regulator, optoisolation is done at digital output (pulse or serial interface). Total cost of IC from say Analog Deveces and auxiliary parts will be less 5...10$, plus detailed application notes how to implement.

 

[epekaric]

Thank you, I have came across these IC's, but touched them very briefly. RMS will be enough and I will study them in greater detail.