AC Current measurement

[moisture]

I am making a overcurrent relay using ATMega16 ucontroller and 10A/1A Current Transformer.
To convert current to equivalent voltage, I have designed a circuit that converts DC current into DC voltage, but I have difficulties rectifying AC current from CT to DC.

I have attached my ckt, in which I have used a burden resistor, and then clamp ckt to limit voltage to 5.1V under any circumstances please guide me how can I modify my circuit to include the rectification thing while maintaing the linearity between current and voltage.

 

[crutschow]

You need a precision rectifier such as discussed here.

 

[tpetar]

I am making a overcurrent relay using ATMega16 ucontroller and 10A/1A Current Transformer.
To convert current to equivalent voltage, I have designed a circuit that converts DC current into DC voltage, but I have difficulties rectifying AC current from CT to DC.

I have attached my ckt, in which I have used a burden resistor, and then clamp ckt to limit voltage to 5.1V under any circumstances please guide me how can I modify my circuit to include the rectification thing while maintaing the linearity between current and voltage.

Use Hall sensor. You will get voltage according to current.


Best regards,
Peter

 

[moisture]

Thanks for sharing, I have designed the below circuit. Now I am using 20A/5A CT.
It gives following voltage equivalent values from Multisim simulation:
For full 20A---->4.88V
For 10A---->2.48V

Please point out if there is some mistake in this circuit, I am short on time, therefore I want to solder it directly without checking it on breadboard.
Again many thanks.

Regards
Moiz

 

[crutschow]

You can't use a 741 op amp in the circuit with only a single supply. You need a single supply type op amp such as an LM324 or LM358.

If your simulation showed proper operation with a 741 then your 741 model is incorrect.

 

[FvM]

There are several issues with this circuit.
- The first stage needs a dual power supply. uA741 would need it anyway for a ground referenced signal, and you also need it because the signal itself is bipolar.
- The second stage implements a peak detector, which is not suitable for a basic AC voltage measurement. The present peak detector would also expose stability problems, if operated with the necessary dual supply.

There are circuits that implement halfwave and fullwave precison rectifiers with single supply, using rail-to-rail OPs. The rectified output should be averaged by a low-pass rather than processed by a peak detector.

P.S.: I have to correct myself in one regard: The first stage can work as a fullwave rectifier, when using a suitable OP.

 

[crutschow]

You don't need dual supplies if you use a single-supply op amp. The negative input voltage swing is converted to a positive output by the inversion of the op amp. The input thus never goes negative due to the pseudo-ground voltage at the inverting summing junction.