i want to make an anti power theft system to a digital meter,for that purpose i am first trying to make digital meter to measure how much power is being used in our house.

So i want to measure current in the line being drawn when any devices are on ( i.e instantaneous current), I have planned to use current transformer but, before going to the question i would like to ask that same job of the current transformer can be done using a power resistor then why not to use that??

ok if that is being answered then :i am actually going to measure the voltage across c/t or the resistor but i am stuck at this point how to find the voltage??.

Actually i wanted to use differential op-amp circuit which could measure exactly the voltage difference across the resistor/ c/t and could be fed to the adc. But the input is ac if i opt for rectification & various other procedures then there will be lot of stuff to be included and the signal will attenuate.. so can any one suggest whether that idea of using differential opamp is right??...and how should signal be maintained?

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    \$\begingroup\$ Please edit the question to break the block of text into paragraphs, for readability. An empty line between paragraphs shows up as a paragraph break. \$\endgroup\$ Commented Aug 28, 2013 at 6:15
  • \$\begingroup\$ i have edited it can u read now? & give me an answer to the question? \$\endgroup\$ Commented Aug 28, 2013 at 14:35

1 Answer 1


You may use a resistor + an OPA to measure the AC current. However, there are a few things that make this less desirable:

  • a resistor + an OPA is not galvanically isolated (the current transformer is),
  • the power lost on the resistor will limit how large signals you can get, and the offset voltage of the OPA will limit how small signals you can detect - these two together will limit the usable range of your measurements (the current transformer can amplify the signal level, and still represent only a very little load to the circuit to be measured).

If I were you, I'd use a current transformer, and some specialized IC (like the ADE7753 and its relatives). See this project: http://www.delorie.com/electronics/powermeter/

  • \$\begingroup\$ please can u tell me detailed information about current transformer.i searched it on website but didn't find good enough information \$\endgroup\$ Commented Aug 28, 2013 at 14:32
  • \$\begingroup\$ Here is a complete design for you: AN-679 from Analog Devices \$\endgroup\$
    – HL-SDK
    Commented Aug 28, 2013 at 14:52

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