Hello,

I am testing the EVAL-ADE7880EBZ and I have some questions about the use of current transformers:

1. First of all, for the calculation of R1/R2 resistors (burden resistors), when we talk about "Ifs", we are talking about the current max measured at the primary level? Correct?

2. For example, I would like to measure signals up to 5Ams, I currently only have current transformers with me that convert 100/5A. Are they too big, so they won't be good for my application, or can they still be used? (Model: ASK 31.4, 100/50A, 2.5VA, 0.2 accuracy class). If it is not good, can you tell me some current transformers to use for my application (with relatively good accuracy)

3. On my current transformer it says 2.5VA, does this mean that the secondaryside is already 0.1 Ohm (R*i'^2) or is it something else?

Thank you in advance and best regards,

Maxime

• 1. you want to select the burden resistor to include and surge currents. If you make a meter that is 100A(10A test current) you want to use >100A to cover any surge. say 150A

If you use 100:5A ct the 150A might not be linear you might need a 150:5 or 200:5 ct

but as an example calculation assuming 100A      .3535(full-scale adc rms) / 100A/100/5 = .07ohm burden  va = 5^2*.07 = 1.75v so the 2.5va can drive this burden.  The VA rating is how much power the CT can provide and be in spec.

Do you need to have a 5A secondary?

If you use 2000:1     .3535(adc fullscale) / (100A/2000) = 7.07ohm burden

https://www.crmagnetics.com/ansi-metering-class-current-transformers

https://www.vacuumschmelze.com/products/Inductive-Components-and-Cores/Current-Transformers

Dave

• Thank you for answering my 3rd question. On the other hand, your message does not really answer my first two questions?

I'm not talking about having a 5A on the secondary side, but on the primary side.

Maxime

• Is your max current into the 100:5 CT 100A or 5 amps? If you expect your max current into the ct 5A your really should use a different ct. 1000:1 to 2000:1 with the appropriate burden.

If 100A is you max current the secondary will be 5A and this is how I explained it before.

• I am talking about a maximum current of 5 amps.

I think there is something I don't understand then.

With the CTs I currently have in stock (Model: ASK 31.4, 100/50A, 2.5VA, 0.2 accuracy class), if I do the calculation of the burden resistance:
0.3535 (adc fullscale) /5A/20 = 1.41 burden
VA = (5/20)^2 * 1.41 = 0.8838V < 2.5VA

So what's the problem? Why use a CT with a higher ratio?

If I do the calculation with another CT that I found on Crmagnetics that I find interesting (CR8410-1000-A), the calculation becomes like this :
0.3535 (adc fullscale) /5A/1000 = 70.71 burden
VA = (5/1000)^2 * 70.71 = 0.001767V < ... (no info)

I don't see the difference, hence my question in the answer is still not completely clear to me:

Is my CT 100/5A only good for a maximum current of 100A and should not be used if my maximum current is only 5A?

=> This is what I'm not sure about and don't quite understand

If yes I could for example use the CR8410-1000-A from Crmagnetics (1000:1, Ir = 7, Vmax RMS =2.5, DCR = 39) which would be more suitable?

Thank you for your time, best regards