Community
OpenEnergyMonitor

OpenEnergyMonitor Community

200A and 100A CT Sensor detection

Emoncms Development
Tags: #<Tag:0x00007fe7b2ff7188>
#1

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

#2

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?

#3

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 !

#4

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

#5

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