the thing is..lets say we looking to cut off the relay, wen the current in the line reaches 9 amps so an alternating voltage signal corresponding to the current is generated using a simple resistor, then we rectify it using full bridge diode rectifier, now the problem is that the ripples produced in the rectified DC signal is too much to be directly given to the PLC so as to make a comparison with a pre set value. Even if we add a RC filter circuit after rectification, then also the level of precision is not per our satisfaction. As far as operating relays is concerned and rise time of current, we r dealing in range upto 100milli seconds. so the rate of rise of current is not a problem. the problem is if the current above a certain value persists for 100ms or more, then only relay should operate, and that kind of time precision can be obtained using PLC driven relays. The main problem concerning us is to reduce the ripple in rectified DC signal. Or if you have any other suggestion for conversion of AC signal to DC signal, do suggest. the precision we lookin for is say for 5V, the maximum peak to peak ripple should be 0.3 or 0.4 Volts.. so around 5%.
plz do reply.