Microcontroller + Current Transformer (+Opamp)

[mdvrajkot]

Hello,
My goal is to measure the AC current ranging from 0 - 20 A (with 8-10% accuracy) with microcontroller. For that I have developed one small circuit. The schematic of the circuit is attached. 30/5 CT is used. I am getting the desired output from this circuit, which i need to solve.
Still, I have some doubts.
1. What is the reliability of this circuit.
2. I have seen various Op-amp based circuit. And still I don't understand why many circuits have op-amp for current measurement?

 

[andre_luis]

Assuming that CT connector is connected to the current transformer solely, it is missing the rectifier diode/bridge on this circuit.

 

[mdvrajkot]

CT in : CT connector. I am measuring peak voltage. Rectifier diode is not used in this circuit.

 

[FvM]

You didn't tell the resistor values, we have to believe that they are chosen appropriately.

Provided they are, the microcontroller will be driven by an AC voltage (positive and negative cycles). Negative input voltage and current might exceed the µCs maximum ratings. You also loose the capability to observe the instantaneous input voltage over the full cycle.

I understand that "measuring peak voltage" means to sample the input voltage continuously and assume the highest value as peak value. That's not particularly accurate and sensitive to non-sinusoidal waveforms. If the current waveform is a sine, you should be able to achieve the intended 10 % accuracy.

 

[mdvrajkot]

You didn't tell the resistor values, we have to believe that they are chosen appropriately.

Provided they are, the microcontroller will be driven by an AC voltage (positive and negative cycles). Negative input voltage and current might exceed the µCs maximum ratings. You also loose the capability to observe the instantaneous input voltage over the full cycle.

I understand that "measuring peak voltage" means to sample the input voltage continuously and assume the highest value as peak value. That's not particularly accurate and sensitive to non-sinusoidal waveforms. If the current waveform is a sine, you should be able to achieve the intended 10 % accuracy.

yes, resistor values are chosen appropriately.

 

[c_mitra]

Remove the ground connection; use a differential mode to measure the voltage. Use a diode clamp to limit the max voltage (just to save the microcontroller). Use a proper 4-terminal current shunt for reliable results (20A is not really a small current).