This weekend I got my new EmonTX Shield connected. I can’t say enough good things about the OpenEnergy project. Between the documentation, explanations, and hardware, this group/project is terrific.
I’m in the US with a split phase 100amp service so two 50a mains coming into the building although we never exceed 20amps on either leg. I’m mostly concerned with accurate low wattage (0-400w) measurement and tracking. I attached the EmonTX shield to a Arduino Due for better analog resolution. In the firmware I set the output be a json string over serial so that it can be inserted into emoncms with a few lines of python to a Pi attached via USB – this works great.
I have a few questions and please excuse the long post:
I’m confused about the correct burdens. I got two SCT-013-030 (30amp) attachments which have an internal 62 ohm burden so I removed two of the 33 ohm burdens on the emontx shield and calibrated. 62 ohm matches the numbers produced by the burden calculator linked to in the Learn section but I’ve read elsewhere on this forum that 100ohm burdens would be preferable for 30amp CTs using the shield. What am I missing?
Aside: One of the CT attachments had a bad internal 62ohm burden and it drove me nuts for a bit trying to find out why that channel on the emontx was giving bad readings. I switched to using a SCT-013-000 on another input which still had the shield’s 33ohm burden.
It seems all the CT transformers in the SCT-013-0x0 series are identical except the for the burden resistor and slight reduction in secondary turns. The circuit boards in these have solder pads for the resistor, even if it isn’t used in that model. For safety, isn’t it better to solder the burden resistor directly in the CT? (Also, is there a preferable wattage for this resistor?)
When calibrating the Emontx Shield I used a “1500w” (1045w actual) space heater and a plug-in meter. I found the AC transformer shipped from the OEM store needed to calibrated to 150.1 to give the correct voltage reading. (Using the 130 figure in the code comment had it 10v too low.)
I wasn’t able to get the phase calibration correct, although I didn’t spend that long at it. It seemed realPower and apparentPower where always within 4-5w of one another. The documentation says phase calibration should be 0-2.0 – is that correct?
Finally, is it possible to flip the 1/8" surface mount connectors to the top of the shield? This would help with mounting. (I haven’t followed the traces.)