Special low input CT4 info

Hi all, I am interested in finding out the reason for the special low input CT4 on the EmonTX devices. Why are they there and can you point me in the right direction to to a guide to change to be 100A inputs as per the other 3.


The intention is to offer a greater sensitivity so that Input 4 can be used for solar arrays limited to 16 A.

There’s no written guide to desensitise Input no.4, you need a 27 Ω wire-ended resistor which you solder into the pair of holes immediately behind the socket. You must edit the sketch to change the calibration to be the same as channels 1-3.

I’ve written this here a few times - a search should have found it.

Thanks @Robert.Wall . Sorry searched and was unsuccessful. What is meant by edit the sketch. Sorry for the rookie question

There are two ways to do that - edit to change the calibration constant for CT4, and recompile and reload, or use the on-line calibration. Both need a computer set up with the Arduino IDE and a serial programmer (from the Shop). The same goes for your parallel thread about the 3rd emonTx.

Ah OK, now I see, I have tried the 27ohm in parallel with the 121Ohm. But didn’t work so explains the sketch editing. I think we will have to invest in a serial programmer. Is this the one?:

Programmer - USB to serial UART - Shop | OpenEnergyMonitor

RE: the thread about a 3rd EmonTX unit - as you know every install is different and our project is gaining traction I am trying to streamline installs and thus different unit setups for individual installs it is nice to have different options available - again the serial programmer will be useful

Thanks again.

That is the one. I wrote a page in ‘Learn’ for it over Christmas, so that’s where all the details are.

It did work - the input voltage to the ADC was correct, but the scaling for the numbers was wrong. That’s what the calibration constant puts right.

It’s not 121 Ω, it’s 120 Ω - 12 × 10¹

Reviving thread here with some more info…

Thanks to @Robert.Wall, I have now successfully modified the CT4 input to de-sensitise the burden resistor. As instructed, I have added the 27ohm wire-ended resistor in parallel with the existing 121ohm one and changed the sketch accordingly.

Lets say, however, I go to site and perform the modification… once I unplug the emonTX unit from the setup, and upload the sketch onto it, it does not seem to register on the emonBASE as the same unit again - its as if uploading a modified sketch changes the units ID, even though I have not changed the node ID in the sketch. i.e., lets say the current emonTX has node ID of 15, then I take it and modify with uploaded sketch, when I plug the unit back in, the emonBASE registers it as a different unit.

Is there something I am doing wrong here.

For some clarity on the situation - I wish to keep the nodeID of the emonTX unit as I need to keep the information posted to emonCMS.org uninterrupted i.e., the data needs to be fed into the same feed ID.

Hope all makes sense, but happy to clarify anything…


There must be. I can switch an emonTx off for days, power it up again and it reappears with the same NodeID. What is the emonBase – a RPi with the RFM69Pi attached, and your emonTx is sending its data via the built-in RFM 433 MHz ISM Band radio – which you seem to imply?

And it’s not 121 Ω, the figures printed on it are 121, that’s 12 followed by one zero, or if you prefer, 12 × 10¹, so 120 Ω.

Yes, this is the setup I use.

Yes that’s it.

Yes, this is normal for me too - once the emonTX is plugged in, it can be out for days or weeks and once plugged in again, registers the same as before BUT…

This is where uploading a new sketch comes in to play. Once I upload a modified sketch onto the emonTX, only then does the change in identity occur.

Its as if re-flashing the arduino changes identity of the emonTX unit.

Deleted duplicate post. BT - Moderator

That’s confusing me - You surely have an emonTx - the emonTx Shield that goes with an Arduino has all four inputs rated the same - or are you now talking about something different?

Assuming you’re confused and you really do have an emonTx after all, then you must have a different NodeID in the new sketch, or it’s picking up a different NodeID from the EEPROM.

The node ID it uses can come from several places:

  1. It will use the value set in the top of the sketch.
  2. If the DIP switch is ON, it will add 1 to that.
  3. If the setup has been stored in EEPROM, the stored value will overwrite the value after step 2.

If you leave your programmer connected and displaying in the Arduino IDE serial monitor as you power up the emonTx, you’ll see the set-up menu. Set the NodeID to the value you want using that, then save the settings. They should be used forever after.