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200A and 100A CT Sensor detection

Tags: #<Tag:0x00007fe7b2ff7188>

Hi everyone,

I’m currently using the EmonTX with ESP module on my projects.
I use also different Current sensors as 100 and 200 Amps.

But I am not sure of the fiability of the measure with them.
I wanted to know, how the software can detect that the sensor we use has a maximum of 100 or 200 A ?
Because, as I know, in the calibration part of the program, the current is calculated using the analog input you read and the maximum value of the CT sensor (100 or 200), am I right ?

Thanks in advance for your clarifications.
Best Regards,
Antoine Niceron

Welcome, Antoine, to OEM.

It cannot know that. You are correct, all the emonTx does is measure the voltage generated across its burden resistor, then multiply that by a number that you tell it, called the calibration constant, and it is only then that you can call that number “amps”.

Which current transformer do you have?

Okey I understand,
The value generated by the sensors are not “calibrated” inside the emonTX.

We use this two ones :
SCT-019-000
SCT-013-000

Thnaks for your reply !

The emonTx’s calibration for the SCT-013-000 should be correct in the default sketch for your emonTx.

It will be wrong for the SCT-019-000. The calibration constant in the sketch is the current that would give 1.0 V at the burden resistor. The resistor is 22 Ω, therefore the current to give you 1 V is 45.45 mA.
The transformer ratio is 200 A : 33 mA, you would require 275.48 A. (My maths does not understand that you cannot do that in the real world!). Your calibration constant will be 275.48.

If you have a programmer, you can disconnect the ESP Module and use the ‘on-line’ calibration to adjust that CT input. If you do not have the programmer, then the easy change is to multiply the power you read by 275.48 ÷ 90.91 (that is the default calibration that you have for the SCT-013-000) = 3.03

It’s sounds great to me,
Thanks a lot for your help !