EmonPi datasheet

Hi everyone,

I recently got a EmonPi and I’d like to get some information about it.
I need to know what are the errors associated with the power, voltage and current measurement?

Can anyone help me with this? Where can I find this type of information (datasheet)?

Also, the emonpi can read current rigth? How can I see that informantion on emon

Thank you.


The analogue front-end of the emonPi is exactly the same as an emonTx. There is a full analysis of the possible sources of error for that in Resources > Building Blocks.

The emonPi Atmega328 Discrete Sampling sketch does calculate rms current, but it’s not used and not sent to emonHub as part of the data structure. I’m afraid you will need to extract the currents and add them after the call to calcVI( ):
etc, recompile and reload the sketch and edit your emonhub.conf to accept the new values. How to recompile and re-load is in the Wiki, I believe.

Thank you so much for your help Robert. I’ll try to do that to see the current.

I have here another question. I want to measure a very low current value, in the order of 0,5A. The 100A max clip-on current sensor CT that i have has sensitivity for that or I need to buy another sensor? And if I need to buy another one which should I buy?

Thank you.

The 100 A CT will not read down to 0.5 A with any degree of precision. Which to use instead is a difficult question. I assume that as you want to measure such a small current, you will be able to disconnect the wire? If you can, then that does make things a little easier, because you have the option to use a ring-core CT or the split-core one. (Ring-core are inherently more accurate.)

The smallest CT that I know to be satisfactory are the ones that Robin Emley uses in his diverter:
the 20A:10mA version here: http://www.yhdc.com/en/product/425/ or this 20A:10mA one http://eppep.com/?action-viewnews-itemid-140364 from http://www.ebay.co.uk/itm/281436762683?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
or Continental Control Systems do some rated down to 5 A - the ACT-0750-005 is a 5 A split-core, and I know you can get that in the 1 V output option, which you need. There are other ring-core ones, but you would need to enquire about a 1 V output option. The standard 0.333 V output version is not suitable for the emonPi (or emonTx), as it only uses one third of the input range.

Whichever you choose, you can lower the current rating by winding a multi-turn primary winding. If you can get 10 turns through the 5 A version, you have a 0.5 A CT. (5 passes through will give you a 1 A rating etc.)

Also, you will need to remove or change the burden resistor inside your emonPi. If you tell me which CT you will be using, I can calculate what you need to do.


I realy appreciate your help, thank you so much.

For now we ordered this one: DL-CT08CL5-20A/10mA 2000/1 0~120A Micro Current Transformer.
Hopefully arrive quickly… :slight_smile:

About the burden resistor, how do I do that?

Unfortunately, I cannot find sufficient data about that (i.e. the VA rating or something that allows me to infer the VA rating), so I cannot be certain that it will be suitable. However…

The ratio is 20 A : 10 mA, but we do not know even the size of the window (the centre hole). However, it is rated at 20 A and a PVC/PVC 600 V cable rated at 20 A is approx 3.7 mm diameter, so you might be able to get a thinner wire that is still good for 0.5 A through it several times. I will do the maths for a 3-turn primary winding. If you can get more through, that would be better.

Your current is 0.5 A, but by having three turns, it effectively increases to 1.5 A.
At that current, you will have a secondary current of 1.5 A × 10 mA ÷ 20 A = 0.75 mA.
You require 1.1 V across the burden resistor at maximum current, therefore your burden resistor value should be 1.1 V ÷ 0.75 mA = 1.467 kΩ.
I suggest a 1.3 kΩ resistor if available, otherwise a 1.2 kΩ.

You will need to remove the existing burden resistor in your emonPi, and fit the new burden resistor in two holes provided. Look at the Atmel 328 circuit board (picture: EmonPi - OpenEnergyMonitor Wiki) and immediately behind the current input jack socket you can see a surface mount resistor with 2 holes for a wire-ended resistor at each end. That is the burden resistor. Unsolder it, and solder your new resistor in using the holes.

When you have done that, you must change the number in “scales = …” in emonhub.conf so that your calibration is correct. The number will need to be about 200 (not the default value 1 - because your input is about 200 times more sensitive) and will depend on the number of turns for the primary winding and the resistor that you used.
Alternatively, you might want to follow the calibration procedure in Resources > Building Blocks and adjust the calibration coefficient in the sketch instead.

According to this, the window dimension is 8.2 mm.

Yet it doesn’t appear on what appears to be the manufacturer’s website…

If that’s so, then @Wi-Green can presumably have a few more primary turns than I anticipated, and a lower value burden, and the errors get smaller. The more primary turns, the better.


could you clarify if the the CT inputs on the EmonPi are equal to the CT1-3 and/or the CT4 on the EmonTX(v4), i.e rated for 100A/24kW or the lower 19A/4.6kW?


They are the same as Inputs 1 & 2 of an emonTx.

Thank you.