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I am working on an electronic project where I have to measure AC current. While searching, I often encountered a term called "isolation", especially when using the shunt resistor. What is this problem and how do I avoid it?

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  • \$\begingroup\$ Here "Isolation" means much the same as "Insulation". For safety, you would generally want your measurement circuit to be insulated from the 120/240 V AC circuit you are measuring. \$\endgroup\$ Aug 20, 2022 at 21:30

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The best way to measure AC current is by means of an isolated current sensor. One popular and inexpensive component is the ACS712, which is a Hall effect transducer. For more details, see:

https://www.seeedstudio.com/blog/2020/02/15/acs712-current-sensor-features-how-it-works-arduino-guide/

Depending on the magnitude of current you are measuring, a standard current transformer (CT) may be what you need. Many of them are designed for high current up to thousands of amperes, with a standard output of 5A. You can wrap multiple turns on the primary to read smaller currents. And there are some that have a turns ratio of about 500:1 so a 5 ampere current will produce 10 mA, which can be used with a 100 ohm resistor to get 1 volt.

https://www.digikey.nl/en/product-highlight/a/amgis/ct-series-current-transformers

You can also use a high side current sensor like HV7800:

https://ww1.microchip.com/downloads/en/DeviceDoc/HV7800%20A062813.pdf

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Using a shunt resistor generally requires that the sensing circuit is connected (with wires) to the AC circuit being measured. If this is high voltage (say 110 VAC or 220 VAC), this is likely dangerous; requires special care, and limits how the results can be delivered to a computer etc. If the sensing (readout) circuit was battery powered, or doesn't have exposed connections that a person could touch, it might be OK.

Alternatively, a clamp-type current meter can be used. This is basically a specialized transformer, and since the primary (the AC line) and secondary are isolated, the secondary can be connected to instruments or touched by a person without risk.

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