Emontx V3 to monitor house and Geothermal in the US

As the new owner of a Geothermal heating/cooling installation located in the US (Massachusetts), I am hoping to leverage the OEM project to monitor my whole house usage, along with my heatpump compressor and other circuits if possible.  This is of interest to me as I want to get a good handle on what my heating and cooling costs are and I also want to monitor the effect of improvements to my building's envelope.  Monitoring the heatpump should allow me to assess those improvements.  In addition, I'd like to monitor a few circuits in an effort to better understand certain loads (ie hotwater heater - and the effect of a Desuperheater) and how I may be able to further optimize them.  

I've read up on the posts by Dan Woodie, and in general the difference the US power system present for the OEM project.  I have in hand an emontx V3, but I'm concerned about it's utility in the US.  

1) I have 200A service.  In Dan's system, he uses two Magnelab SCT-075-000 CTs in combination with changing the burden resistors to get the output compatible with the emontx.  As the owner of a V3 emontx (preassembled), am I in trouble using that approach?  From the docs it looks like the burden resistors are 22R 0.25W.  My electrical engineering competency is limited, so I'm not sure how to interpret this...

I've also seen a system in AZ described here in this post which uses a much more affordable 200A CT sensor:

http://www.desert-home.com/p/test-html-code.html .  In this solution, two SCH-019 CT sensors where used in series.  I've also seen a few examples that weren't too detailed showing SCH-019s used much like Dan did, acompanied by different burden resistors.  Once again, is it possible to do this on an emontx V3?  From the looks of it, the SCH-019 comes in either a 33mA output or a 0.33V.  From what I've read, the current output is the preferred way for the emontx to receive this data?

2) The emontx V3 has 4 inputs, so if I can get around the burden resistor problem, I hope to use a couple of normal CT sensors to monitor two additional circuits.  Am I thinking about this correctly?

3)  I plan to to use an AC to AC adapter for more accurate measurements.  I have found plenty of US 9VAC adapters.  Most output between 300-1000 mA.  Is there a reason to choose a specific output?


Any help is greatly appreciated...

Robert Wall's picture

Re: Emontx V3 to monitor house and Geothermal in the US

My personal preference in your situation would be to start with an emonTx V2, rather than the V3, as the V2 comes in kit form and uses wire-ended components that can be changed very easily especially if that is done on assembly. If that's not an option, then you are most probably (given that I haven't done the sums) faced with changing the burden resistors, and that could be very tricky.

Starting then with the c.t., it is inherently a current source. Like any other source, it has a maximum power output that must be observed. The emonTx needs a little over 1 V rms for maximum resolution, therefore if you are using a c.t. that is rated to give (say) 33 mA out at 0.33 V for 200 A primary current, you can use it at 11 mA and 1 V out by having a higher burden resistance (3 times the original value) but the maximum primary current is now limited to only 66.6 A. (This is why I recommended very recently in another thread using a 600 A ct. derated to 200 A.)

Ideally then, you need to choose a c.t. that will give you 1 V into the existing burden resistor at your desired maximum current. If you can use a lower resistance burden, then it's easy to put another resistor externally in parallel with the fitted burden to achieve the correct value. Whether you can buy a c.t. that fits this requirement I don't know.

If you want to add one more current input (using the one presently committed to the DS18B20, 5 inputs are already used - 1 voltage and 4 current), then you need to replicate the input circuitry and connect to ADC 5 which is conveniently brought out to a connector, but you'll need to remove a resistor used to power the temperature sensor.

The current drawn by the emonTx V3 from the AC adapter is fairly small, so the VA rating is unlikely to present a problem. For best accuracy, you want the one with the least phase error and the most faithful reproduction of the mains waveform.

glyn.hudson's picture

Re: Emontx V3 to monitor house and Geothermal in the US

The emonTx V3 has the option for using thru-hole burden resistor in place of the smt components. Removing the smt burden resistors should not be too difficult. 

sbeausol's picture

Re: Emontx V3 to monitor house and Geothermal in the US

Glyn, that is good news.  I'm assuming removing the resistor is as simple as this video?

http://www.youtube.com/watch?v=6XyrNjNXSo8 - any tips would be appreciated....

I am guessing the holes adjacent to the smt resistors would be where the through holes would go?  Finally, 22r would mean 22Ohms correct? 

I need to sit down and figure out what kind of resistor would work best with a 600A CT derated to 200A as Robert suggests....



Robert Wall's picture

Re: Emontx V3 to monitor house and Geothermal in the US

Simple: You take the secondary current at 200 A (or whatever maximum current want to measure) and use Ohm's law with a voltage of 1.1 V. That will give you a few percent in hand. R = V / I. Then choose the nearest preferred value resistor below, unless the nearest preferred value above is very close. The power rating of the resistor shouldn't be a problem, but if you want to calculate it, P = V2/R = I2R = VI,  where I is the secondary current and V = 1.1 V.

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