[General] Current measuring through CT

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Thota Suresh Avionics

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Hi,

I am measuring current through 5 Amps CT,which can measure maximum of 5-Amps, I don't know the number of turns in CT.

I have tried different shunt values to get 1 Volt at maximum CT rating and i have used 860E shunt value , i could find 0.96 V at 5-Amps load on CT.

I am reading 0.96V at 5 amps load through micro controller after removing 2.5V DC offset to applying TRMS algorithm. After I have derived a constant value 'X' and when it is multiplied with TRMS value i could find current proportional to the load and parllely observed with clamp meter.

X = Load in Current / Current in Voltage i.e., X = 5-Amps / 0.96 V =5.1867

the circuit which i have used is attached, is this the correct method to measure the current ? Do we have any better circuits and algorithms?

Rs = 860E

 

I am kind of concerned about the higher shunt resistor you are using which is 860 ohms. May be you need to double check this. Because with higher currents in the secondary, you may risk damaging this resistor and the micro circuit.
The circuit configuration you are using is almost good except for a change i would suggest. Moving of the capacitor from the Ain pin to the CT terminal and 2.5V reference node.
 

how to reduce shunt resistance for the above circuit ? We cannot change reference voltage
 

Thota Suresh Avionics said:
Hi,

Do we have any better circuits and algorithms?
Click to expand...

I can suggest you better circuit which I already used for measuring AC current from 50Amp to 300Amps. The output of CT connected with 0.1R and then feed to True RMS AD736 chip. You can read the output of AD736 with any ADC.
 
ashad said:
I can suggest you better circuit which I already used for measuring AC current from 50Amp to 300Amps. The output of CT connected with 0.1R and then feed to True RMS AD736 chip. You can read the output of AD736 with any ADC.
Click to expand...

Hi,Thanq for the reply..

Can you please post your circuit so that i can check
thanking you.
 
ashad said:
You may use amplification circuit as per your ADC input specifications.

View attachment 114153
Click to expand...

I think that the connector CON2 you shown in the schematic is connected to CT with sense resistor in place. Whats the value of that Rs?
Also the circuit you have given is a difference amplifier, so it just acts an amplifier of AC signal at the Rs and feeds it to the TRMS to DC converter. If no amplification required then all the resistors in opamp circuit shall be of same value.
For the Qn asked, if not much of the circuit is required to be implemented then the Rs along with Vref circuit and current limiting into ADC pin should be sufficient. The TRMS to DC converison can be handled by the firmware.
 


hi this method didn't worked for me how did you made it work? coding?
 

Mrunal Ahirrao said:
Can you please post your code too?
Click to expand...

Nothing special in coding I used MSC51 controller with ADC0808. Then special thing is the above circuit which make my life easy. It give equivalent value of AC current in term of DC and I read that value through ADC0808.

- - - Updated - - -

ravindragudi said:
Whats the value of that Rs?
Click to expand...
0.1R I used.

ravindragudi said:
Also the circuit you have given is a difference amplifier, so it just acts an amplifier of AC signal at the Rs and feeds it to the TRMS to DC converter. If no amplification required then all the resistors in opamp circuit shall be of same value.
Click to expand...

Yes, it is difference amplifier which measures the AC voltage across the shunt resistance then it is feed to RMS to DC converter then as per your ADC requirement you can set ur amplifier gain.

ravindragudi said:
For the Qn asked, if not much of the circuit is required to be implemented then the Rs along with Vref circuit and current limiting into ADC pin should be sufficient. The TRMS to DC converison can be handled by the firmware.
Click to expand...

sURE, it is upto you. If you donot want to use RMS to DC converter than you may used ur own algo. In any case you have to convert the AC signal into DC or insert offset to shift the AC variation in one quadrant.
 

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