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Implications of using different CT's with Open Energy Monitor Hardware

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This thread is to discuss the implications of using the new 20A and 200A CT’s, that are available for the IotaWatt, with other hardware systems provided by Open Energy Monitor.

My first, very basic, question relates to the burden resistors. It wasn’t clear to me if the 20A and 200A CT’s have their own burden resistors. Could someone please comment on this?

If I can get my head around the formulae and CT saturation etc then I believe it should be possible to use the new CT’s with at least some of the other hardware systems, in addition to IotaWatt.

We are concentrating on the emonTx and ESP8266 systems at the moment in combination with our own Smartphone app. The benefit of the Smartphone app means it’s a simple task of changing any of the variables without flashing new firmware to the hardware. That is, when I know which variables to change :slight_smile:

did you make any head way with this, it would be great if we had some information on how to modify the EmonTX to support the 20A (SCT006) CT. is it just a case of adding a burden resistor

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SCT006 - 24ohm burden

Y axis primary current
X axis voltage across burden
As you can see it saturates at 20A. I haven’t tried it with smaller burden. Maybe thats’s where the 30A range works.

Here’s the phase shift.
Y axis - phase lead
X axis - primary current
Reasonably steady below 18-20A, especially at the low end, but it goes off the reservation pretty quickly near 20A. Once again, that upper range may be better with a lower burden.

SCT019-000 - 24 ohm burden.
Didn’t test above 50A primary, but this matches the YHDC specs and no reason to believe it won’t handle 200A.

Phase lead:

I’m using the SCT019 with IotaWatt to measure US mains. Results are typically within 1% of the meter.
STC006 matches the SCT013 when I run them on the same circuit. Once again, against revenue grade (EKM) meters they run within 1%.

I’m not saying they will produce that kind of accuracy on any equipment, only that they have the potential for pretty good accuracy.

Speaking about the SCT006, I don’t see why it would be any less accurate than the SCT013 given the same burden. The SCT013 produces 10ma at 20A primary. The SCT006 produces 25ma at the same 20A primary. So substituting for an SCT013-000, I calculate it would have 2.5 times the resolution.

Not real familiar offhand about what has to be done to set the CAL, but if measuring a circuit with less than 20A (4600 watts in the 230V world), I think they are good to go.