emonTX3CM Output


Have just hooked up an emonTx v3.2 with emonTX3CM and configured it with w0 to turn off the radio and turn on serial.

Am getting the following output:


A few things stand out here:

The voltage is really high. When I measure it with a multimeter, it jumps between 242V and 243V. I guess I need to calibrate it? - is it normal it’s that far out?

I have 2x clamps connected, but one is reporting only 22/23W - is there an explanation for that, or is it just a dodgy one?

Watching the output for a bit, there is the odd lines like this:


Is that normal?



re the 2nd CT clamp, I swapped them round and fiddled with the clamp and it seemed to fix it:

RF off
POST.....wait 10s
'+++' then [Enter] for config mode
CT1 detected, i1Cal:90.90
CT2 detected, i2Cal:90.90
AC present

Quite a big difference between the two, but I guess that’s calibration. I’ll have to read up on that!

Interesting that the voltage is now reading lower. Though not as low as i’d expect (unless my multimeter is way out! - but that reading 242/243 is what i’d expect, so that seems about right).


EmonLibCM is of course fully documented, if you need further information.

So am I reading this correctly…?

I need to measure the voltage in a mains socket with a multimeter and tweak this with trial and error until the emonTx reports the same value:

float vCal  = 268.97;

I then need to measure the current inline with a load like a kettle or fan heater and modify this line (or set it on the serial config interface), again using trial and error until the power reported matches:

float i1Cal = 90.9; 

Is that right?

I don’t quite get what this does - do I need to touch that? -

float i1Lead = 4.2;



Have you read the calibration instructions in the ‘Learn’ section? In there, the procedure is written out very carefully.

You write “trial and error” but in fact it’s usually possible to get inside 1% (compared against your meter - and that will have its own tolerance limits, but there’s nothing you can do about that) with just one reading. Compare what you get (the emonTx) with what you want (your meter) and scale the calibration coefficient accordingly.

You should check it - the value will vary according to the individual c.t. that you’re using, and with the current, so you aim to set it at a current that represents your “typical” usage.
You can see how the phase errors of the two transformers change - and this phase lead setting is an attempt to compensate the combined effect of those - in the test reports in ‘Learn’. It’s referred to in the calibration instructions as “phasecal” - but it’s essentially the same adjustment.

I did read it, but didn’t fully understand it. I obviously need to read it a few more times.

Obviously it refers to old code in the discrete firmware, so then I was trying to cross reference it with the emonTX3CM document and trying to derive what to change.



Is anything else reading the serial input (like emonhub)?

Yeah, I think it was emonhub - oops!

Have just been working on the calibration.

I was measuring 3.26v on the 3.3 point on the screw terminals, so I set:


After a few goes, i’ve got it giving roughly the same voltage as my multimeter (I say roughly because it bounces around a bit, and emonTx only samples every 10s):

float vCal  = 261.907; 

and then after a lot of trial and error, I ended up with:

float i1Cal = 88.3;         // (2000 turns / 22 Ohm burden) = 90.9
float i1Lead = 4.2;

My multimeter is giving values between 7.99 and 8.02 and emonTx is now giving:


Sound like i’ve done it sanely?
(no guarantee my multimeter is bang on of course!)

Playing around with i1Lead doesn’t really seem to change much - is a PF of 0.9996 close enough?



It’s probably as close as you’ll get. Remember that the phase error of the transformers changes with current and voltage, so it will only be that at the current you used and the voltage you had at the time.

Great, thanks.

Having connected it back up to the meter, I can see it’s reading a little low compared to the meter display and the pulse count.

I’ll have to tweak it in place and see if I can get closer.

Is there any way of getting the emonTX3CM firmware in to configuration mode (or reboot it) via serial - or is that only possible on boot?



Do you mean Calibration mode - the set of commands you can issue when it is running? It should be available - if it responds to ‘l’ (list) or ‘?’ then you can issue calibration commands.

Ah that does work! - I didn’t realise you could do that!

Is there any way to reset the accumulated power & pulse counts?



I have been playing with calibrating against a pulse count by multiplying the input power in emoncms. I’ve got it pretty good. 0.1kWh difference over about 45 days. Looking in detail and at shorter periods, I can see where the difference increases / decreases presumably as the voltage changes. Overall I’m pretty happy calibrating this way.

Had a play with the iXCal values and have got it close - within a watt or so of the smart meter, and between 3 clamps on the same cable of each other, at base usage (~400W).

However, when I turn on the tumble dryer and it goes up to ~3.2kW, the 3 clamps and the meter all differ with each other by as much as 100w or so.

Is that the iXLead value, or just the way it is?

At low usage, this isn’t bad over an hour or so though! - the 3rd clamp is dead on with the pulses and the other two are within a watt or two.




You should probably set the calibration at the high current, then adjust for the changed phase error at the lower currents. The “cure” will be to spend a lot more money on the voltage and current transformers to get lower errors.

I did calibrate it using a 2kW fan heater - then plugged it back in and tweaked it to get it right at the base usage, so possibly it would have been nearer at high current if i’d have left it what I ended up with calibrating against the multimeter!

But then you could argue it’s better to have it nearer at low usage as that’s what. you are doing the majority of the time.

I guess the main thing is - it’s in the ballpark…

A few hours in and the accumulated usage is still impressively close to the pulse count.



8 A? The SCT-013-000 c.t. doesn’t have a value for accuracy quoted below 10 A.

That’s a shame when for residential use, you’re generally doing more low current than high.

Check this out after almost 24hr!


The accumulated usage for channel 1 is spot on with the pulse count! - amazing!

Looking at the graphs - the reason the other two are slightly out is they are identical at low current but give slightly higher usage at high current - so obviously the curve isn’t quite right.

1 Like

Most UK dwellings have a supplier’s fuse that has a rating of 60 A, 80 A or 100 A, and it is normally necessary to size the c.t. to the rating of the circuit it is measuring, especially as the shop cannot know the individual circumstances of each customer.

As I pointed out earlier, you can substitute a different c.t. if you wish. But you might need to change the burden to suit.

Within the limits of accuracy that I can measure, the c.t. is good amplitude-wise to a value of current that is far lower than the 3% non-linearity from 10% - 120% of rated current that the data sheet claims, it’s the phase error that becomes increasingly large as the current falls - which is inevitable when you consider the way the losses in the c.t. change with the operating flux in the core and the currents.