AVR-DB: emonTx V4, new hardware in progress

Fair enough but you did say 1.5mm² originally :slight_smile:

I would advise reading this piece with a degree of caution. Even though it is written with good intentions, there’s one glaring mistake throughout: what is referred to as the “input impedance” in most cases isn’t that at all, it’s the combination of the output impedance of the voltage source (the “source impedance”) and the input impedance of the ADC in series. And again in most cases, the former will be dominant because the manufacturer of the ADC will strive to make the true input resistance along the charging path to the sample-and-hold capacitor as low as possible, while making the impedance of any shunt paths for bias or protection as high an impedance as possible.

The demarkation point between the source impedance and the input impedance is usually the ADC input pin.

Sorry, about that I just double-checked, and they are 1mm^2. It would be nice if we could use 0.5mm^2, I looked into getting cables custom-made, but they were more expensive than using off-the-shelf moulded plugs. For the next batch I’m looking into purchasing them custom-made in bulk which should reduce the cost and allow us to use slightly thinner flex.

The best option for a tidy installation is to choose the emonVS without any plug and hardwire to an FCU, there’s not getting away from the fact UK plugs are bulky! I

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If it comes back in stock, this might be a good one too https://cpc.farnell.com/pro-elec/pel01285/uk-plug-to-open-end-0-75mm-white/dp/PL16107?st=pel01285

Or PEL01284

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Just curious: why does the emonVS use an earth connection as a voltage reference? What does it provide that the line and neutral alone cannot?

2 posts were split to a new topic: New emonTX4 - data loss

It doesn’t appear to. The earth connects to the ground plane under the ZMPT101B transformers and the 5 V output, hence to the emonTx V4 and GND and the c.t. input sockets sleeve connection.

So it should be providing some earthed screening to the low-level signals.

The voltage input is tapped off Line(s) and Neutral.

We made the change to include the earthed screening before a later change to the way the bias is applied on the input circuitry (with one side of the CT and Voltage sensor now being tied to emonTx4 GND), adding the earth made a noticeable difference previously but actually retesting here today, I cant notice a difference with or without the earth connected. So it may be something we can revisit now that the other changes have mitigated the original observed noise. I will have to do some more indepth testing, we had to move quite quickly and not change too many things through the design process for the emonTx4 so it may be that this earth connection is something to revisit again.

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Postman finally turned up with my EmonTX4 yesterday (it shipped last Friday, ironically an IP camera I was waiting for shipped from Hong Kong Monday and arrived first - Royal Mail living up to its reputation!).

First impression was it was a large cable in a box with some other stuff :rofl: as others have commented the cable is huge, mine was 2m not 1m - I assume the moulded plug is for regs?

I would consider looking at IEC leads, they are cheap and plentiful and you can just chop the IEC and wire it in? I need to change the lead somehow as this one is massive and I have no where to hide it in the WC where the builder helpfully installed our CU. I’ll have to look at adding a fused plate maybe and wiring it into that directly to neaten it all up - low WAF as shipped (Wife Acceptance Factor).

I will update my EmonPi first, I am on 11.0.9, reading the release notes can’t see much change to 11.2.3 but might as well be current.

My EmonTX are all CM FW now, can the EmonPi run continuous monitoring by default, or is it still for experimenting only? At the moment my solar is on the EmonPi so any increase in accuracy is welcome.

-5c here in the south of England this morning, but the sun is out and the skies are clear so off the grid at least!

In case it helps anyone, it looks like EmonTX4 is Node17, its confusing as I already had an EmonTX4 which seems to be my 2nd EmonTX3 with Node ID 16.

I clearly need to do more reading as I am not sure if you can rename nodes already in use? As long as the feeds are not touched it seems you can delete and add nodes and inputs freely?

Is there any news on the 6 CT Expansion Board yet ?
Also will it fit into the existing case ?

Hello @Tank, the firmware to support 12 CT sensors is almost ready, going through testing at the moment. I cant say exactly when it will be available in the shop yet. The Expansion board itself does fit in the existing case, it just needs a different fascia. Are you interested to add this to an emonTx4 that you have already or is this a new system?

Thanks @TrystanLea , It’s to add to my existing Tx4.
That’s good to hear that it will fit in the existing case as well…

The 3 phase compatible firmware library called emonLibDB and associated firmware examples has now been released, thanks to @Robert.Wall. We are looking for folk happy to test and give feedback on the process of using the firmware.

Please see the forum thread here for more information, it also links to the original emonLibDB library release that makes this functionality possible:

EmonTx4 3-phase support with emonLibDB

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The 6 CT Expansion board is now available in the shop

and the installation and firmware guide can be found in the docs here:
6 CT Expansion board — OpenEnergyMonitor 0.0.1 documentation

The expansion board is currently listed in the development boards section of the shop as it’s really for those comfortable with soldering the header and manually configuring, compiling and uploading the firmware using the Arduino IDE. We can do these steps for you if required when ordering the expansion board alongside an EmonTx4 in the same order, please email us first at [email protected] to discuss how you would like us to configure the unit.

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We are also gearing up here for the emonPi2 release! Probably around a month away.

The emonPi2 shares a mostly identical PCB design to the emonTx4. The only difference is the layout of the board, RaspberryPi GPIO header, OLED display connector and RJ45 rather than RJ11 emonVs connector for more power carrying capability needed for the RaspberryPi. This allows for a single enclosure design that integrates the RaspberryPi base station.


  • 6x clip-on CT current sensor inputs (suitable for a range of 333mV voltage output CT sensors).
  • emonVs precision voltage sensor and power supply input
  • Full Real/Active power measurement & continuous sampling.
  • Both single and 3 phase support.
  • 2 pluggable terminal block inputs for DS18B20 temperature sensing & pulse counting connected directly to the RaspberryPi GPIO header.
  • 1 pluggable terminal block analogue voltage input.
  • On-board USB to UART converter for easier programming and serial output.
  • 433 MHz RFM69 radio transceiver
  • Standard RaspberryPi GPIO header connection for direct connection to a RaspberryPi
  • Wall-mount aluminium enclosure
  • OLED 128x64 Display
  • Microcontroller: Microchip AVR128DB48
  • Arduino core support DxCore by SpenceKonde
  • ADC Resolution: 12-bit
  • Precision voltage reference: MCP1502
  • 6 CT Expansion board option as above to increase number of CT inputs to 12. Though only when used with the Pi Zero rather than full Pi 3/4 due to enclosure space constraints.

I’ve added an overview page to the documentation and will be adding to this over the coming weeks in preparation for the release:



With regard to the 6 CT expansion board, Do you plan to make a firmware available pre compiled that will connect via USB, as the documentation seems to only show source with RF included. Please correct me if I am wrong (quite possible).

Hello @Tank there is a firmware example here that prints out in the standard format via serial https://github.com/openenergymonitor/emontx4/tree/main/firmware/EmonTx4_DB_12CT_WiFi
It prints all power values but not energy values as it was customised for use with the EmonESP WiFi boards and I wasnt 100% sure on stability if the string length doubled again.

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Looking forward to Emonpi2… I am ready for a hardware refresh…

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Probably a little late in the design cycle,… but has any consideration been given to some forced cooling,.My ‘emonpi v1’ can get a little warm,… particularly when the ambient temperature reaches 40C… …although chips may be running within their rated temperature spec,… its always nice for their extended life to run them as cool as possible,…