EmonTX Shield, SCT-019-000, used in North America

Hello All. I have researched this myself from various sources, but I need some clarifications on my setup here in the US.
I am using the EmonxTX Arduino Shield with 2 SCT-019-000 CT connected. I will be using CT1 and CT2 along with a 9V AC-AC adapter. I am using the firmware here:

I have a few questions:

  1. Do I need to make the burden resistor 24 ohm in place of the 33 ohm that comes on the CT inputs?
  2. In changing the code, do I merely change this line?
    from scales = 1,1,1,1,0.01 to scales = 3,3,1,1,0.01

I also see these lines on down, which I was wondering if I should change in place of changing the scale at the top of the sketch

if (CT1) ct1.current(1, 60.606); // Setup emonTX CT channel (ADC input, calibration)
if (CT2) ct2.current(2, 60.606); // Calibration factor = CT ratio / burden resistance
if (CT3) ct3.current(3, 60.606); // emonTx Shield Calibration factor = (100A / 0.05A) / 33 Ohms
if (CT4) ct4.current(4, 60.606);

I am definitely aware that I need to change my voltage to 130 for use in U.S.
Any input would be greatly appreciated.


Welcome, Eric to the OEM forum.

Because you are using the emonTx Shield rather than the emonTx or the emonPi, the optimal values are going to be a little different. This is because the emonTx and emonPi work off 3.3 V, whilst the emonTx Shield runs of 5 V.

The SCT-019-000 generates 33 mA at its rated input of 200 A, my ‘rule-of-thumb’ is you need the burden resistor to generate 1.6 V rms to fully load the input, whilst allowing a small margin for component tolerances and headroom. Therefore, your ideal burden value will be 48.48 Ω. You should use a 47 Ω component in place of the fitted SMT resistor. This presumes you will not be using the c.t. above the rating of 200 A. If you need to go up to 300 A (and I very much doubt that you will), then you could keep the 33 Ω resistor.
You might decide that the resolution you will lose is acceptable and you want to not change the burden resistor. I can understand that.

The current calibration constant (“60.606” above) is 128.95 with a 47 Ω burden and 183.65 for the 33 Ω burden. (In each case it’s the current that gives you 1 V across the burden resistor, i.e. 1 V alternating at the ADC input.)

The voltage calibration constant (the “130” you mention) is similarly the line voltage that gives you 1 V alternating at the ADC input, and depends on the a.c. adapter you’re using. 130 is correct for the 77DA-10-09.

If you’ve changed the voltage and current calibration constants in the sketch, you must not change the scales in emonHub. The best place by far to get the calibration correct is in the sketch.

Thanks for the very thorough response! This is exactly the information I needed. I have no reservations about changing the 33 ohm resistors to 47 ohm. Also, I definitely will not be above 200A on the CTs, the only reason I even went with those was the wire size coming into my breaker box. (Its like 16 mm)

The AC adapter I will be using is this:

I may end up replacing it with 77DA-10-09 if my readings aren’t accurate enough

Thanks again for the information

I can almost guarantee that the voltage calibration constant for that will be different. If you can tell me the no-load voltage when the input is exactly 120 V, I (or you) can calculate the calibration constant - because there’s a 13:1 voltage divider between the a.c adapter and the ADC input. (The no-load voltage is not 9 V - that’s the voltage on full load. It will be more like 10½ - 11 V.)

So I just measured. My line voltage at the AC outlet was exactly 121.0 volts and this AC adapter was outputting 10.61 VAC with no load.

That makes your voltage calibration constant 136.79

Thanks Robert! I really appreciate your expertise and help with this. I’ll be sure to post back when I get everything going.

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