Latest EmonTx 3-phase firmware questions


  1. I can see that the emontx-3phase repo was last updated 8 months ago and the default emonTx v3 firmware was updated a number of times since then. Are the two code bases pretty similar and is there important features that are missing from the 3phase version?

  2. The README for the 3phase firmware says that it uses the same voltage value for all 3 phases and the inaccuracy isn’t too bad. Does it do anything like what the IOTAWatt does to generate a derived voltage value?

  3. I’m guessing there’s no way to order an EmonTx preloaded with the 3 phase firmware?

1. It’s horses for courses. The two are totally different internally - as you can see if you look. The most important feature is both measure power continuously.

2. The emonTx has only one voltage input - this is the reason for that. Robin Emley’s 3-phase diverter does measure all three voltages, and you might like to consider that, if you need better accuracy. I have no knowledge of ioatwatt, except that the accuracy claimed (when I last looked) is better than the accuracy of the sensors it uses.

3. I suggest you email The Shop and ask. The sketch is capable of very many adjustments to suit your needs (for which version of emonTx, 3-wire or 4-wire system, which inputs are allocated to which phase, what format is required for the output, etc) which require recompilation, so if the shop is prepared to pre-load it, you need to specify exactly what you need.

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Hi Robert,

Thanks for the quick response.

Is there supposed to be a link attached?

There was, and Discourse had tangled it. It works now.

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Follow up question around updating the EmonESP firmware to work with an EmonTx running the 3phase firmware. I’ve seen you’ve gone through this before, but I want to make sure I haven’t missed something.

Is the only change I have to make to the EmonESP code is reduce the Baudrate to 9600?

As far as I’m aware, yes.

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Apologies, but one final question.

In you’re detailed emontx-3-phase-userguide.pdf you talk about 4 wire and 3 wire systems. If I have a 4 wire system, do I need to have a 4th CT around the neutral wire? Will it affect energy readings if I dont?

The neutral current is the vector sum of the three phase currents. So unless you particularly need to try to measure leakage to earth or somewhere else (and for that you’d get a far more accurate result with one c.t around all 4 wires) then there’s no point in measuring the neutral current.

Ideally, though probably never in practice, the neutral current should be zero.

For the purpose of measuring the energy in a multi-phase system, you need one fewer wattmeter than the number of wires. This is standard theory. Translated to the emonTx, it means 2 c.t’s for a 3-wire (no neutral) system and 3 for a 4-wire system.


I feel I’m in good company with Ivan Kravets.

  1. I suggest you email The Shop and ask.

FYI Robert, I emailed them and they said they’re happy to do it, I just have to leave a comment with my order.

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I thought they probably would, but I couldn’t commit them. Remember to look at the source code for the sketch and specify exactly which options you need. (The #define... choices like #define SERIALPRINT down to the comment “Users should not need to change anything below here”)

If you’re fitting an ESP8266 to the emonTx, you must specify 9600 baud for that (it doesn’t matter if you are using the RFM 433 MHz ISM band radio).

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