Hi Everyone. I'm trying to build a 3-phase energy meter based on the ADE 7880 & Arduino. I bought a 50A/5A current transformer. How do I calculate the burden resistor for the CT interface ?

I found this equation in the ADE7880 eval board user manual :

R1 = R2 = 1/2 × 0.5/√2 × N/IFS

However I'm not sure if the N is the turns ratio or the CT current ratio which in this case is 10. The turns ratio is not specified in the CT name plate.

If I use the N as 10 & Ifs as 5A (secondary full scale), I get a value of 0.35ohms. Is this correct ? or am I missing something? Also please let me know if the burden resistor can be a quarter watt component.

I'm looking forward to your suggestions. Thanks.

Hi vignesh

You have two options:

1. You can change your burden resistor everytime you change the CT current ratio.

2. If you want to keep the same burden resistor, just scale the register readings accordingly when you read values.For eg: If you design a burden resistor value based on a 50A/5A CT and use it along with a 500A/5A CT, you will have to scale the register readings by 10 to compensate for the CT ratio change.

However, option 2 has a drawback in itself that if the current level after the CT is inaccurate( i.e., if the current ratio change is 10.5 instead of 10), then the readings will be inaccurate as well.

Regards

hmani