I have a device that measures the current in the mains through a 0.01 Ohm 20W shunt resistor. The voltage across the shunt is amplified by a op amp and feed into a uC's ADC for farther calculations.

But to measure the voltage drop across the shunt the the mains needs to be ground coupled with the op amp and the rest of the components, which means that the D_GND and the A_GND will be 220 V above earth, and I feel very uncomfortable about that,

So what can be that to avoid raising the GND to such a high voltage? I know one solution would be opto-coupling the signal, but it doesn't really fit in my project. Also a capacitor between the GND and the mains might work. Any thoughts?

Here is the schematics:

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    \$\begingroup\$ Mains current measurement is usually done with a current transformer, which avoids this problem. \$\endgroup\$ – pjc50 Jul 11 '13 at 11:11
  • \$\begingroup\$ Isn't one side of your mains connected to earth ground at some distribution point? \$\endgroup\$ – Joe Hass Jul 11 '13 at 11:17
  • \$\begingroup\$ No, in EU a mains plug can be plugged both ways, which means you can end up having the GND connected to NULL or the live 220V line \$\endgroup\$ – Chris Jul 11 '13 at 11:21
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    \$\begingroup\$ How does opto-coupling "not really fit" in your project? It seems you have set up some impossible constraints. \$\endgroup\$ – Phil Frost Jul 11 '13 at 11:33
  • \$\begingroup\$ Well, I could use an opto-coupler but I was looking for smaller, in size, alternatives \$\endgroup\$ – Chris Jul 11 '13 at 11:39

Optocoupler could be quite small, but a magnetic current sensor could be an alternative to provide isolation. Having a flowting transformer powersupply for the complete electronic would allow also to connect both grounds.


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