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variable gain

Anandarup Das
Prodigy 40 points

Hi,

In my application, I am sensing a high value of 50 Hz AC current using a current transformer, which after some signal conditioning through OPAMPS, goes to an ADC. The circuit works well at normal current levels, however at fault conditions, the current increases to a great extent (atleast 15-20 times). This saturates all the OPAMPS. I was thinking of using a variable resistance before the OPAMPS, which will limit the voltage at high values of short circuit current. It is not important to maintain accuracy at high values of current, but the AC waveshape should be preserved.

I was thinking if it is possible to use the MOSFET 'on state resistance', controlled by variable Vgs, in a voltage divider circuit. Or, is there a variable gain OPAMP that can be used ? The bandwidth requirement is not high, and a 0-3V output is desired.   

Any ideas/suggestions is greatly appreciated.

Regards,

Anandarup Das