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I want to interface the following CT to Arduino:

enter image description here

So I have the following data:

  • Primary Current: 600 Arms
  • Secondary current: 5 Arms
  • Burden: 2 VA

Burden = 2 VA means that:

Rmax = P/I^2 = 2/25 = 0.08 Ohms max burden resistor (am I right?)

At 5A, there should be a voltage drop of V = P/I = 2/5 = 0.4 Vrms

Let's say that Rburden = 0.05 Ohms:

Pmax = 5^2 * 0.05 = 1.25W.

0.05 Ohms/3W is available on the market, so I'd go for it.

Given Rburden = 0.05 Ohms, at 5A we have Vdrop = 0.05*5 = 0.25 Vrms or 0.35 Vpeak.

So the output voltage at the burden resistor will range from 0 to 0.35V. I want to map this range from 0 to 4V for the Arduino's ADC.

I have decided to bias the signal at 2.5V as follows:

enter image description here

This means I will have a max differential voltage of Vdiff = 4-2.5 = 1.5V. I would want the Arduino to read 4V at 5A.

A non-inverting amplifier will be suitable for this case. Calculating the gain resistors I get R3 = 10K and R4 = 33k. See output waveform below:

enter image description here

To sum up, these are my questions:

  1. Would this approach work properly? Do I need to add some filtering or adjust something in the current design?
  2. This approach uses the DC bias principle, but another approach is to use a precision rectifier. Which one would perform better?
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  • \$\begingroup\$ If you want to maximise resolution, a precision rectifier along with a comparator to tell you whether it's positive or negative would probably be better, but really, all of this depends on what your requirements are. \$\endgroup\$ – Hearth Dec 17 '19 at 0:31
  • \$\begingroup\$ "I would want the Arduino to read 4V at 5A." - I simulated your circuit in LTspice using all the same numbers, but only got 1.33V to 3.66V. How did you get 1V to 4V? What do you want to use the current measurement for? \$\endgroup\$ – Bruce Abbott Dec 17 '19 at 3:46
  • \$\begingroup\$ I want to implement IoT, so I would send the current measurement via Wifi, for example, to another device. When the secondary current is 5A (7.1 peak), then I get the peak value = 4V at the output of U2. Of course, the signal swings around 2.5V. Are you sure you have the same I have? \$\endgroup\$ – Blue_Electronx Dec 17 '19 at 3:54

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