Mains ammeter circuit

I'm planning a circuit to measure the current in mains. all tips and feedback is welcome. The circuit makes the stepped down current into a voltage drop with a burden resistor. If i havent messed up this circuit should give the ADC a voltage between 0 and 3.3 volt where 3.3 volt would indicate 20 amps running through the mains wire.

The 3.3v voltage follower is there to take waveform from ac to alternating dc

In the final design I'll be using a different microcontroller so the attiny is just a placeholder.

Have you seen this;


Tom... :slight_smile:

I have indeed. i partually based my circuit on a guide for that circuit. i also looked at this guide: Alternating Current AC Measurement using Pic Micocontroller with CT

In Tom circuit the blue voltage is depends to current, in your cricut depends to voltage of power supply.

The circuits is wrong, all you get is rectangular pulses. Read more on using OPA as buffer amplifier topic.

You've connected Rf to the non-inverting input instead of the inverting input.

Your circuit is confusing as you appear to connect the CT secondary to both inputs of the opamp,
when in fact you are connecting one side to virtual ground and have drawn it oddly.

The 100 ohm resistor on the output of the follower serves no purpose as you aren't decoupling
the virtual ground rail with any capacitance - such a resistor serves to preserve stability for an
opamp with a large capacitive load.

"2K2pf" is the most contorted description of a capacitance I've ever seen. 2.2nF or 2n2 is the usual
way to notate this. the symbol for farad is F, not f, f is the femto prefix.

C4 is doing nothing. C1 is also irrelevant. I think you might be trying to add EMI suppression, but that would be best done with a few 100pF ceramic on the input to the opamp.

I overlook - mains and, so my answer was for amp meter which is on Tom's drawing, than for your circuit - you are correct except the name of B1.
Burden resistor = the load oo mains.

google - differential amplifier, there is no C1 or C1, you can keep them because they have the same value, change R3 to microfarads, 100 picofarads or 10 for 50 Hz is like 10 mega ohm resistor.